The test shot, dubbed Trinity by Oppenheimer, was the most violent man–made explosion in history to that date. 

Test planners chose a flat, desert scrub region in the northwest corner of the isolated Alamogordo Bombing Range in southern New Mexico for the test. The site was several hundred miles from Los Alamos, and the nearest offsite habitation was twenty miles away.  

On July 16, 1945, the Trinity device detonated over the New Mexico desert and released approximately 21 kilotons of explosive yield. The predawn blast, which temporarily blinded the nearest observers 10,000 yards away, created an orange and yellow fireball about 2,000 feet in diameter from which emerged a narrow column that rose and flattened into a mushroom shape. The blast scoured the desert floor, leaving a shallow crater, 10 feet deep and some 400 yards across. 

In 1943 the situation was that the development of the gun-type uranium weapon, which was to become “Little Boy”, moved confidently ahead. However, work on implosion (the method in which a subcritical mass of plutonium is compressed to super-criticality by high explosives) was slow, frustrating and often seemingly hopeless. By late 1943 it was evident that there was no alternative: the implosion device would have to be tested.

The starting point for this page is the entire text, word-for-word, of Part II: Trinity of a report entitled Los Alamos: Beginning of an era. I have also add much of the text that is in the Wikipedia article Trinity (nuclear test). All the parts in italics have been added from a variety of other sources. 

In many ways this Webpage is designed to be read after the introductory page on the Manhattan Project.    

By Aug. 1944 the high velocity uranium gun had been thoroughly proved in principle, but the plutonium gun had been abandoned. The main effort was now on overcoming the difficulties with the implosion assembly. Moving from theory to experiment to production were "enormous steps”, and a test of the device was considered necessary. 

In fact the longer the plutonium remained irradiated inside a reactor, necessary for high yields of the metal, the greater the content of the plutonium-240 isotope, which undergoes spontaneous fission at thousands of times the rate of plutonium-239. The extra neutrons it released meant that there was an unacceptably high probability that plutonium in a gun-type fission weapon would detonate too soon after a critical mass was formed, producing a ‘fizzle', i.e. a nuclear explosion many times smaller than a full explosion. This meant that the “Thin Man” bomb design that the laboratory had developed would not work properly as a plutonium bomb. 

The Los Alamos Laboratory turned to an alternative, albeit more technically difficult design, an implosion-type nuclear weapon. In Sept. 1943, mathematician John von Neumann had proposed a design in which a fissile core would be surrounded by two different high explosives that produced shock waves of different speeds. Alternating the faster- and slower-burning explosives in a carefully calculated configuration would produce a compressive wave upon their simultaneous detonation. This so-called "explosive lens" focused the shock waves inward with enough force to rapidly compress the plutonium core to several times its original density. This reduced the size of a critical mass, making it supercritical. It also activated a small neutron source at the centre of the core, which assured that the chain reaction began in earnest at the right moment. Such a complicated process required research and experimentation in engineering and hydrodynamics before a practical design could be developed. The entire Los Alamos Laboratory was reorganised in Aug. 1944 to focus on the design of a workable implosion bomb.

Why test?

It was thought that if the implosion device were not tested, too many questions would be left unanswered. A nuclear explosion was so entirely new, the implosion method so far removed from any existing practice, the construction of the atomic bomb so entirely dependent on dead reckoning, that no one was willing to risk the first trial of such a device over enemy territory, or even in demonstration for the Japanese, as had been suggested. A failure would wipe out the crucial psychological effects of so monumental a weapon.

What we are talking about here is a kind of fat bomb, 5 feet round, weighing 5 tons, and containing 13.6 pounds (6.2 kg) of plutonium

The neutron initiator was about 2.0 cm in diameter, and was held in an initiator cavity of about 2.5 cm in diameter (all diameters are 'outside' diameters). This 'urchin' consisted of a hollow beryllium shell, with a solid beryllium pellet inside, all weighing about 7 gm. The out shell was 2 cm wide and 0.6 cm thick, the solid inner sphere was 0.8 cm wide. The ‘urchin’ had 15 concentric latitudinal grooves cut into the inner surface of the shell. Each groove was wedge-shaped and 2.09 mm deep. The shell was formed in two halves by hot pressing in a nickel carbonyl atmosphere. The surfaces of the shell and central sphere were coated by a layer of nickel, plated with 0.1 mm of gold and 50 curies of polonium-210 (11 mg) were deposited on the grooves inside the shell and on the central sphere. The gold and nickel layers protected the beryllium from alpha particles emitted by the polonium and surrounding plutonium. The plutonium core or ‘Pit' (6.2 kg of delta-phase plutonium-3% gallium alloy) was about 9.20 cm in diameter, and the uranium tamper shell was 22.225 cm in diameter. The plutonium sphere consisted of 2 half-spheres (hot pressed from ingots) and had a hole and plug to allow the insertion of the initiator after assembly. Originally the plutonium was silver-plated, but it blistered and had to be ground and layered with gold leaf (silver was replaced by nickel in later cores). There was also a 0.1 mm thick gold gasket placed between the two half-spheres. The natural uranium tamper weighted 108 kg, and provided both an inertial confinement, and added about 20% to the yield. This core with the tamper represented about 70% of a critical mass before implosion. As an added safety precaution, a cadmium wire was left in place, and only removed when the initiator was inserted. The implosion doubled the density of the 'pit' lifting it to above 3 times critical mass. The boron-plastic shell had a diameter of 22.86 cm, and made up the ‘pit’. This last layer was a neutron-absorbing thermoplastic enriched with boron-10, thus acting to reduce the neutron background (from fission neutrons scattered back towards the 'pit' by the hydrogen-rich explosive layer). The “Implosion Assembly” consisted of an aluminum pusher shell of 46.99 cm surrounded by an inner high-explosive booster shell of 92.075 cm, and the explosive lens of 137.80 cm diameter. These layers each consisted of 32 explosive blocks (20 hexagonal and 12 pentagonal blocks) which fitted together in the same pattern as a soccer ball. Each lens block had two components, the body made of high velocity explosive, and a parabolic low velocity explosive focusing element on the inner surface. These pieces formed the lens that shaped a convex, expanding shock wave into a convex converging one. The complete lens weighed about 1,800 kg. The explosive sphere casing consisted of a cork liner (diameter 140.30 cm) and a Duraluminum case (diameter 145.40 cm) consisting of two polar caps and five equatorial segments. The ballistic case had an outside diameter of 153.00 cm. Both the Trinity device and “Fat Man” were assembled on site from separate pieces, only later was a trap door added to allow assembly and the addition of the “pit” and initiator at a later time. 

Uranium enrichment was carried out at the Clinton Engineer Works near Oak Ridge, Tennessee. Whereas plutonium is a synthetic element with complicated physical, chemical and metallurgical properties. Until mid-1944, the only plutonium that had been isolated had been produced in cyclotrons in microgram amounts, whereas weapons required kilograms. In April 1944, physicist Emilio Segrè, the head of the Los Alamos Laboratory's P-5 (Radioactivity) Group, received the first sample of reactor-bred plutonium from the X-10 Graphite Reactor at Oak Ridge. He discovered that, in addition to the plutonium-239 isotope, it also contained significant amounts of plutonium-240.


The Manhattan Project later produced plutonium in nuclear reactors at the Hanford Engineer Works near Hanford, Washington.

Furthermore, it was essential to obtain detailed and quantitative information on the various effects of the new weapon which would serve as basic technical data for tactical planning in the future. Little of this could be obtained if the explosion were first observed under combat conditions.

One important question, about which there was substantial disagreement, concerned the explosive force to be expected (see nuclear weapon yield). Only an actual nuclear detonation could settle that question, and then only if meaningful measurements (requiring many new techniques) could be made.

Nuclear weapon yield is the energy discharged, and is expressed in TNT equivalent (often kilotons or megatons). The yield-to-weight ratio is a way to compare yield to the mass of a weapon. For a thermonuclear weapon you can obtain between 5-6 megatons per metric ton of bomb mass. This is equivalent to about 25 TJ/kg, as compared to 25 GJ/t for the full combustion of coal and 42 GJ/t for oil. 

Other questions concerned the performance of the implosion system inside the device; the destructive effects of heat, blast, and earth shock; radiation intensities; fallout; and general phenomena (fireball, cloud, etc.) associated with the explosion.

And so the decision was made (at that time) to sacrifice what was to amount to one third of the America’s stockpile of atomic weapons and its entire supply of plutonium on a secret test on American soil.

The planning starts

The first formal arrangements for the test were made in March 1944 with the formation (in the Explosives Division of George Kistiakowsky) of group X-2 under the leadership of Kenneth T. Bainbridge, whose duties were “to make preparations for a field test in which blast, earth shock, neutron and gamma radiation would be studied and complete photographic records made of the explosion and any atmospheric phenomena connected with the explosion”.

George Kistiakowsky (1900-1982) was responsible for developing new explosives, RDX and HMX, and he helped develop shaped charges. In the Manhattan Project he was responsible for developing the explosive lenses for the implosion-type nuclear weapon (the type used for the Trinity test and in the “Fat Man” dropped on Nagasaki). Between April 1941 and Nov. 1941 he was a member of the National Academy of Science review committee. From early 1944 he was one of the deputy leaders of the Ordnance and Engineering Division in Los Alamos. From Aug. 1944 he was leader of the Explosives Division, and from March 1945 he was responsible, with Norris Bradbury, for “Assembly” on the Trinity Project.

But before Kistiakowsky agreed to join Los Alamos, he had several conditions. “In my case, there were three exceptions made,” he recalls. “[The first was that] my salary was fixed properly, that is, commensurate to a Harvard salary and not the civil service salary which I was getting at the time which was much lower”. The second condition was that he be given his own house on Bathtub Row, “because as a chemist, I would have been given very low quality housing”. The third condition probably stretched Grovess patience. “The third condition,” says Kistiakowsky, “was that my daughter could spend the summers with me, and yet go to college, which was completely an unusual arrangement. She was the only teenager who was allowed to go in and out that way”. 


Kenneth Bainbridge (1904-1996) was director of the Trinity Project, but in his academic life he had also developed a very precise mass spectrometer with which he was able to verify Einstein’s mass-energy equivalence. Between June 1943 and early 1944 Bainbridge was leader of the Group E-2 (Instrumentation) in the Ordnance and Engineering Division in Los Alamos. And from early 1944 he was leader of Group E-9 (High-Explosive Assemblies) in the same Division. From Aug. 1944 he was leader of Group X-2 (Development, Engineering, and Tests) in the Explosives Division in Los Alamos. The group was dissolved when he took command of the Trinity Project.   

Bainbridge's group was known as the E-9 (Explosives Development) Group. Stanley Kershaw, formerly from the National Safety Council, was made responsible for safety.  Captain Samuel P. Davalos, the assistant post engineer at Los Alamos, was placed in charge of construction. First Lieutenant Harold C. Bush became commander of the Base Camp at Trinity. Scientists William Penney, Victor Weisskopf and Philip Moon were consultants. Eventually seven subgroups were formed:

TR-1 (Services) under John H. Williams - construction, procurement, timing,...

TR-2 (Shock and Blast) under John H. Manley - measurement of air blast and earth shock

TR-3 (Measurements) under Robert R. Wilson - alpha, neutron and gamma measurements 

TR-4 (Meteorology) under J. M. Hubbard

TR-5 (Spectrographic and Photographic) under Julian E. Mack

TR-6 (Airborne Measurements) under Bernard Waldman - air blast measurements

TR-7 (Medical) under Louis H. Hempelmann - monitoring, first aid

In fact, the E-9 group was only renamed the X-2 (Development, Engineering and Tests) Group in the August 1944 reorganisation.

First Lieutenant Harold C. Bush often also appears in different texts as Howard Bush, Lt. Bush, H. C. Bush, etc.

In March 1945 William George Penney (1909-1991) was a consultant on 'blast and shock' in the Trinity Project, but there is also information suggesting that he was, from March 1945, also part of Project Alberta, for the preparation of the atomic weapon for combat (possible stationed on Tinian Island). He was also the head of the British delegation working in the Manhattan Project., and later he directed scientific research at the Atomic Energy Research Establishment which resulted in the first British bomb (codename Operation Hurricane) in 1952.

Victor Frederick Weisskopf (1908-2002) was leader of Group T-3 (Experiments, Efficiency Calculations, and Radiation Hydrodynamics) in Theoretical Division in Los Alamos (March 1943 to Aug. 1944). Between Aug. 1944 and Aug. 1945 he was leader of Group T-3 (Efficiency Theory) with a focus on the implosion bomb design. From March 1945 he was also a consultant on ‘physics’ in the Trinity Project. Weisskopf was a co-founder of the Union of Concerned Scientists, and he served as director-general of CERN from 1961 to 1966.

Philip Burton Moon (1907-1994) was part of the British Delegation, and was a consultant on the Trinity Project.

John Harry Williams (1908-1966) was leader of Group P-2 (Electrostatic Generator) in the Experimental Physics Division. This later became Group R-2 (Electrostatic Generator) in the Research Division of Robert R. Wilson. In 1945 he was deputy director of the Trinity Project.  

John Henry Manley (1907-1990) was leader of Group P-3 (D-D Source) that later became Group R-3 (D-D) in the Los Alamos Research Division of Robert R. Wilson. He was a friend and colleague of Oppenheimer, and in 1942 at the Chicago Metallurgical Laboratory he was the experimental assistant to Oppenheimer in the Physics Group (under Enrico Fermi). Later he was leader of Group TR-2 (Blast and Shock) in the Trinity Project. 

Robert Rathbun Wilson (1914-2000) worked with Ernest Lawrence on the development of the cyclotron. In Los Alamos he was leader of the Research Division, and leader of Group R-1 (Cyclotron). He later went on to be director of the Fermilab.

J.M. Hubbard was responsible for TR-4 (Meteorology) on the Trinity Project.

Julian E. Mack, from Aug. 1944, was leader of the Group G-11 (Optics) in the Weapon Physics Division in Los Alamos. In March 1944 Mack was leader of a Trinity Project sub-group called TR-5 (Spectrographic and Photographic), and from March 1945 he was leader of Group TR-5 (Spectrographic and Photographic Measurements) in the Trinity Project.

Bernard Waldman was leader of the Group TR-6 (Airborne Measurements, and later renamed Air Blast) in Project Trinity, and he was, with Luis Alvarez one of the observation team leaders in Project Alberta. 

Louis H. Hempelmann was leader of the Group TR-7 (Medical) in the Trinity Project.

Once the decision had been made, in the spring of 1944, to conduct the test, the search began for a suitable test site. Los Alamos was ruled out immediately for both space and security reasons and the search spread to eight possible areas in the western United States. To please the scientists, security and safety people alike, the site requirements were numerous. It had to be flat to minimize extraneous effects of the blast. Weather had to be good on the average with small and infrequent amounts of haze and dust and relatively light winds for the benefit of the large amounts of optical information desired. For safety and security reasons, ranches and settlements had to be few and far away. The site had to be fairly near Los Alamos to minimize the loss of time in travel by personnel and transportation of equipment, yet far enough removed to eliminate any apparent connection between the test site and Los Alamos activities. Convenience in constructing camp facilities had to be considered. And there was the ever present question: Could Jumbo be readily delivered there? See later for a description of Jumbo. 

Throughout the spring a committee, composed of Oppenheimer, Bainbridge, Major Peer de Silva (project intelligence officer), and Major W. A. Stevens, in charge of maintenance and construction for the implosion project, set out by plane or car to investigate the site possibilities. They considered the Tularosa Basin near Alamogordo; a desert area near Rice, California (an Army training area north of Blythe); San Nicholas Island off Southern California; the lava region south of Grants (now El Malpais National Monument); an area southwest of Cuba and north of Thoreau, both in New Mexico; sand bars off the coast of South Texas (Padre Island south of Corpus Christi); and the San Luis Valley region near the Great Sand Dunes National Monument in Colorado. By late summer the choice was pretty well narrowed down to part of the Alamogordo Bombing Range in the bleak and barren Jornada del Muerto (Journey of Death). The area had the advantage of being already in the possession of the government and it was flat and dry although almost constantly windy. The nearest inhabitant lived 12 miles away, the nearest town, Carrizozo, was 27 miles away. It was about 230 miles from Los Alamos. The Jornada del Muerto derives its grim name from its barren, arid landscape. Old Spanish wagon trains heading north would be left to die in the desert if they ran into trouble since they could not depend on finding settlements nor water for 90 miles or so.

One report suggested that Groves did not want to use Indian land, because he would then have to deal with Secretary of the Interior Harold Ickes, who might cause difficulties. It has also been written that the final decision was made by General Groves, but other reports say that Bainbridge picked the site. The best site was the Army training area in California, but Groves did not want to ask permission of the “flamboyant” Patten, so the decision fell on the second choice. 

The history of the Jornada is in itself quite fascinating, since it was given its name by the Spanish conquerors of New Mexico. The Jornada was a short cut on El Camino Real de Tierra Adentro, the King's Highway that linked old Mexico to Santa Fe, the capital of New Mexico (it actually went to San Juan Pueblo, New Mexico). The Camino Real went north from Mexico City till it joined the Rio Grande near present day El Paso, Texas. Then the trail followed the river valley further north to a point where the river curved to the West, and its valley narrowed and became impassable for the supply wagons. To avoid this obstacle, the wagons took the dubious detour north across the Jornada del Muerto. Sixty miles of desert, very little water, and numerous hostile Apaches. Hence the name Jornada del Muerto, which is often translated as the journey of death or as the route of the dead man. It is also interesting to note that in the late 16th C, the Spanish considered their province of New Mexico to include most of North America west of the Mississippi!

Today the 3,200 square mile Alamogordo Bombing and Gunnery Range, partly located in the desolate Jornada del Muerto, is named White Sands Missile Range, and is actively used for non-nuclear weapon testing. Only a small portion (18 by 24 miles) of the north-east corner was set aside for the Trinity Project. It has been said that the White Sands Missile Range was initially used for testing the V-2 rockets after WW II. Today it is one of six U.S. ranges, and it is the largest all-land military reservation in the U.S.  

Peer de Silva (1917-1978) worked for the CIA and was the army officer providing security for the Manhattan Engineering District (i.e. Head of Security). He signed out “the plutonium sphere” from Los Alamos, and carried it to the Tinian airbase. He was later based in Seoul, and later still was CIA station chief in Saigon. 

Major W. A. (Lex) Stevens (in charge of construction activities in Los Alamos) is said to have given Trinity its name. According to Robert W. Henderson, he and Stevens were at the test site discussing the best way to haul Jumbo the thirty miles from the closest railway siding to the test site. "A devout Roman Catholic, Stevens observed that the railroad siding was called 'Pope's Siding.' He [then] remarked that the Pope had special access to the Trinity, and that the scientists would need all the help they could get to move the 214-ton Jumbo to its proper spot". From June 1943 Stevens was leader of Group E-10 (Maintenance and Construction for Implosion Project & S-Site Operation) in the Ordnance and Engineering Division in Los Alamos. 

Robert W. Henderson was a specialist in photographic processes and special effects. In fact he received a 1942 Academy Award for developing lighting equipment used in film making. He worked first in the Y-12 plant in Oak Ridge, before leading the Group X-2A (Engineering) in the Explosives Division (renamed X-2 in March 1945). He helped set up the laboratory that would eventually become Sandia National Laboratories.

On Aug. 14, 1944, Oppenheimer wired Groves in Washington that he thought there would be no problem in obtaining the land for their purposes but, he was concerned as usual about Jumbo, specified that “the northern part will be satisfactory to us provided the El Paso-Albuquerque line of the Santa Fe can carry a 200-ton load either from El Paso north or from Albuquerque south to the neighborhood of Carthage”. 

The final decision was made on Sept. 7, 1944 and arrangements were made at a meeting with the commander in chief of the Second Air Force for acquisition of an 18-by-24-mile section of the northwest corner of the bombing range. Not long afterward, when it became necessary to choose a code name for the test, it was Oppenheimer who made the selection. Many people have tried to interpret the meaning of the name but Oppenheimer has never indicated what he had in mind when he chose Trinity. In any case, it did create some confusion, and still does today.

Here the “official” stories differ. We know that Oppenheimer assigned the code name Trinity to the site and test (Trinity Project). It is often considered as a reference to a poem of John Donne (a 16th C English poet and sermon writer), which in turn references the Christian notion of the Trinity (three-fold nature of God). In 1962, Groves wrote to Oppenheimer about the origin of the name, asking if he had chosen it because it was a name common to rivers and peaks in the West and would not attract attention, and elicited this reply: “I did suggest it, but not on that ground ... Why I chose the name is not clear, but I know what thoughts were in my mind. There is a poem of John Donne, written just before his death, which I know and love. From it a quotation:

“As West and East

In all flatt Maps — and I am one — are one

So death doth touch the Resurrection”

That still does not make a Trinity, but in another, better known 14th Holy Sonnet of Donne, it opens, “Batter my heart, three person'd God; for, you as yet but knock, breathe, shine, and see to mend...

The reading given to the first extract from Donne suggests that through the death of the speaker “West” shall meet “East”, at which point the flat map becomes spherical, or a globe made by the hand of God. The second extract from Donne is all about the speaker asking God (along with Jesus and the Holy Ghost) to “batter” his heart, rather than just knock, etc. Other experts see this as a description of a glass blower “blowing” life into an object (the terms are used by glass blowers, as are break, blow, and burn used in lines 3-4 of the same Holy Sonnet).

Yet another suggestion is that Oppenheimer was inspired by the divine Hindu trinity of Brahma (the Creator), Vishnu (the Preserver) and Shiva (the Destroyer). 

Other reported options were that Robert Jungk believed that the name was borrowed from a nearby abandoned turquoise mine. According to another account, an Army Colonel stated that the project team would need help from the Holy Trinity to move Jumbo. The White Sands Public Affairs office suggested that the Trinity might represent the culmination of the work of the Manhattan Project’s three main sites (Los Alamos, Oak Ridge, and Hanford).  

Bainbridge asked Oppenheimer for clarification in a memo written March 15, 1945: “I would greatly appreciate it if the Trinity Project could be designated Project T. At present there are too many different designations. Dwayne Muncy’s (Business) office calls it A; Mitchell’s (Procurement) office calls it Project T but ships things to S-45; and A portion of the Alamogordo Bombing Range was chosen as the site for the Trinity test. Last week in this section it was christened Project J. By actual usage, people are talking of Project T, our passes are stamped T and I would like to see the project, for simplicity, called Project T rather than Project J. I do not believe this will bring any confusion with building T or Site T”.

Nothing was simple in preparations for the test and the securing of maps of the test site was no exception. Lest Los Alamos appear involved, the job was handled by the Project’s security office which managed to avoid pinpointing the area of interest by ordering, through devious channels, all geodetic survey maps for New Mexico and southern California, all coastal charts for the United States, and most of the grazing service and county maps of New Mexico. There was considerable delay while the maps were collected and sorted.

Despite the many complicated steps taken to avoid any breech of security there were a few snafus. As soon as construction began on the test site it became necessary to have radio communication within the site so that radio-equipped cars could maintain contact with the guards and with people at the various parts of the area. Later, communication would be essential between the ground and the B-29’s participating in the test. A request went out to Washington for a special, exclusive wave length for each operation so that they could not be monitored. Months went by and at last the assignments came back. But alas, the short wave system for the ground test site was located at McDonald Ranch which served as assembly headquarters for the atomic device. It was on the same wave length as a railroad freight yard in San Antonio, Texas; the ground to air system had the same frequency as the Voice of America. “We could hear them (in San Antonio) doing their car shifting and I assume they could hear us”, Bainbridge reported later. “Anyone listening to the Voice of America from 6 a.m. on could also hear our conversations with the planes”.

The Base Camp and the McDonald Ranch

In many texts there is a confusion (or lack of clarity), about the Base Camp and the so-called McDonald Ranch. The Base Camp was located about 9 miles southwest of Ground Zero at the former McDonald brothers ranch. This not to be confused with the George McDonald Ranch two miles southeast of Ground Zero, where the bomb's plutonium core was assembled (see below).

The use of “Ground Zero” may have its origins in the catastrophic damage inflicted upon the U.S. fleet and facilities during Pearl Harbor. As the location of a nuclear explosion, it was first used for the Trinity test. 

When the Army took possession of the McDonald brothers ranch in 1944, it was a working ranch and possessed a number of structures, including two ranch houses, several outbuildings, an earthen reservoir, two or more windmills, and a water tank.

Of the two ranch houses, the larger was an H-shaped one-story adobe structure. Facing east, it had a central section about 20 by 30 feet, flanked on the North and South by two wings. The North wing was about 35 feet long and 20 feet wide, and the South wing about 50 feet long and 25 feet wide. A west-facing porch stood between the wings on the West side of the central section, and a north-facing porch shielded the North, extended side of the South wing. A pitched roof covered the centre section of the building and the West porch, while both wings and the North porch had shed roofs. Exterior and interior walls were adobe, and all roofs were corrugated metal.

Test personnel first occupied this larger ranch house in December 1944, when a detachment of military police arrived on site. Referred to as "ranch house number 2", it served both as laboratory space and as Base Camp's headquarters. It was in this ranch house (Base Camp) that the telephone switchboard and electric power were installed. By the time of the test seven months later, its population had grown to about 325 people, 250 scientists and support staff, 30 SED personnel and 45 military police (reports differ on these figure). Little has been recorded about the actual activities at Base Camp, but it was from a bank on the camp's earthen reservoir that most of the Trinity personnel observed the test on July 16, 1945. 

Already in Nov. 1944, contracts were let for the construction of new buildings at Base Camp. Ten Civilian Conservation Corps (CCC) portable buildings were dismantled and shipped in from Albuquerque, including mess hall and kitchen and a 150-man latrine. These were erected south of the ranch houses. Four units were used for barracks. The remaining unit served as the headquarters office, a laboratory building, an orderly room at the infirmary, a supply room, an electronics shop, a post engineer's office, a fire station, and the motor pool. In addition, approximately 20 standard Army hutments were set up in and around the Base Camp complex between March and June, 1945. Like the CCC buildings, they were used for a variety of office, supply, and shop functions Water was brought up from the wells at Base Camp by two electric pumps and stored in the tanks atop the reservoir. A non-potable water distribution system linked the storage tanks to the mess hall kitchen and the latrines, and a sewage system carried waste water from these two facilities to a cesspool located west of camp. 

The other ranch house ("ranch building number 1"), often called the McDonald Ranch, stood west of the first and faced north. It was a simple, rectangular wood frame structure that measured approximately 25 by 40 feet, with an addition about 20 feet wide that ran the full length of the building's north (long) side. Exterior walls were sheathed in tongue-and-groove horizontal siding and the gabled roof was covered with corrugated metal. Other reports tell us that the ranch house built with adobe (mud bricks), and there was an ice house on the west side along with a deep underground cistern. A stone addition, which included a modern bathroom, was added onto the North side in the 30‘s. East of the ranch there is a large, divided concrete water storage tank and a windmill. South of the windmill were the remains of a bunkhouse, and a barn which also served as a garage. The Army poured a concrete floor in the garage to hold a 50 kW motor generator plant. Other records describe instances when test personnel used the ranch's concrete reservoir as a swimming pool, apparently a common occurrence in the warmer months.

Numerous Trinity Site memoranda refer to a "ranch building number 3" or a "southwestern ranch building" at Base Camp. It was assigned to laboratory use in March 1945 and must have been enclosed and habitable. 

Vital components for the ‘gadget' were brought into the McDonald Ranch house in a shock-proof case. According to some reports this is Sgt. Herbert Lehr actually delivering the plutonium core (or more likely half of it). Another reports tells us that the boxes were magnesium cases with rubber bumpers made out of test tube stoppers. Magnesium was used because it was light, dissipated heat, and did not reflect neutrons. The cases could hold the entire core and had a slot for the initiator. Th is another photo out there, showing that the case was in fact painted a “mustard yellow rust-preventing zinc-chromate primer” (at least when the cases arrived on Tinian). 

Interestingly, the U.S. later used yellow on all of it’s government issued civil defence instruments, and the international warning sign for radiation is black on yellow. On the other hand the Air force used yellow on the ground of its high visibility. Maybe the yellow originally came from the yellow-cake colour. Saturated yellow with black writing was also used to indicate that a wounded soldier required frequent penicillin injections.  

The McDonald Ranch is also sometimes called the Schmidt-McDonald ranch house. The house was built in 1913 by Franz Schmidt, a German immigrant and homesteader. In the 1920’s Schmidt sold the ranch to George McDonald and moved to Florida. The old ranch house remained empty until Manhattan Project personnel arrived in 1945. Then a spacious room in the northeast corner of the house was selected for the assembly of the plutonium core of the Trinity device. Workmen installed work benches, tables, and other equipment in this large room. Electricity for the ranch house was to be provided by a 5 kW generator. To keep the desert dust and sand out, the room's windows and cracks were covered with plastic and sealed with black tape. According to reports, while scientists assembled the initiator and the Pu-239 hemispheres, jeeps were positioned outside with their engines running for a quick getaway if needed. Detection devices were used to monitor radiation levels in the room, and when fully assembled the core was warm to the touch. The completed core was later transported the two miles to Ground Zero, inserted into the bomb assembly, and raised to the top of the tower.

On the morning of the July 15, 1945, a team of six to eight scientists, led by Robert Fox Bacher and Marshall Glecker Holloway and assisted by Louis Alexander Slotin, entered the room and began the final assembly of the plutonium core. The assembly process was completed about mid-afternoon with the insertion of the nuclear initiator at the core's centre. Others present during the assembly process included Robert Oppenheimer, Norris Bradbury, and Brigadier General Thomas Farrell. Once assembled, the core was taken by car directly to the base of the tower at Ground Zero, where it was encapsulated in the centre of the bomb.

The Trinity explosion on Monday morning, July 16, 1945, did not significantly damage the McDonald Ranch house. Even though most of the windows were blown out, and the chimney was blown over, the main structure survived intact. The nearby barn did not fare as well. The Trinity test blew part of its roof off. The ranch house stood empty and deteriorating until the Department of Energy and the Army provided funds for the National Park Service to completely restore the house to the way it appeared in July, 1945. When the work was completed, the house with many photo displays on Trinity was opened to the public for the first time in October 1984 during the semi-annual tour. The Schmidt-McDonald Ranch house is now part of the Trinity National Historic Landmark.

Back to planning the test

On the basis of a thorough Laboratory survey of proposed scientific measurements to be made at the test, justification for all construction and equipment requirements was sent in a detailed memo to Groves on Oct. 14, 1944. On Nov. 1, 1944, Groves wired Oppenheimer his approval of the necessary construction but asked that “the attention of key scientists not be diverted to this phase unnecessarily”. 

He needn’t have worried. By Aug. 1944 the outlook for the implosion program had turned bleak indeed. The test preparations lost their priority and the Laboratory turned nearly all its attention toward overcoming the serious difficulties that were developing. Urgency in securing manpower for research and development on the problem was so great that all of Bainbridge’s group, except for a few men in Louis Fussell’s section X-2C, were forced to abandon their work on the test and concentrate on development of a workable detonating system and other top priority jobs lest there be no test at all.

Louis Fussell was leader of the Group X-2C (Test Measurements) in the Explosives Division (Aug. 1944 to Aug. 1945) until it was dissolved. He then became, in March 1945, leader of Group X-5 (Detonating Circuit). 

From May, 1945 a temporary organisation was formed consisting of 7 groups designated TR-1 through TR-7 (TR for Trinity). Personnel from existing divisions in Los Alamos and from the SED (Special Engineering Detachment) were reassigned. The project was lead by Bainbridge, and Frank Oppenheimer (brother of J. Robert Oppenheimer) was his aide.

What to do if the test failed?

With doubt and uncertainty hanging over the project throughout 1944 it is not surprising that one of the first and most heavily emphasised efforts in the test preparations was planning for the recovery of the active material in case the nuclear explosion failed to take place. In 1944 there was barely enough plutonium available to conduct the essential experiments and the outlook for increased production was dim. It seemed absolutely essential that the active material not be wasted in an unsuccessful test.

Scientists toyed with the idea of using a water recovery method in which the bomb, surrounded by air space, would be suspended in a tank of water and fragments would be stopped by a 50 to 1 ratio of water to high explosive mass. 

They also investigated the possibility of detonating the bomb over a huge sand pile and putting the sand through placer operations to mine whatever plutonium might be imbedded there (see placer mining often used for precious metal deposits). Neither of these methods appeared particularly promising and the decision was made to contain the blast in a huge steel vessel.

Although the container, promptly dubbed Jumbo, became a high priority project at the outset, and all test plans, until the last minute, were based on the assumption that it would be used, there is little evidence that the idea met with much enthusiasm in Los Alamos.

As early as March 10, 1944, Oppenheimer wrote to General Groves outlining the plans and possibilities for “a sphere for proof firing”, pointing out that “the probability that the reaction would not shatter the container is extremely small.” He promised, however, that the Laboratory would go ahead with plans and fabrication of the vessel. But this was easier said than done, and by the following summer Jumbo had become the most agonising of the project’s endless procurement headaches. In late March 1944, Hans Bethe, head of the Theoretical Division, wrote in a memo to Oppenheimer that because of the numerous engineering problems, which he described in discouraging detail, “the problem of a confining sphere is at present darker than ever”.

Hans Albrecht Bethe (1906-2005) was initially a member of the Theoretical Group in the Chicago Metallurgical Laboratory (1942), but from March 1944 he became the leader of the Theoretical Division in Los Alamos. He later won the 1967 Nobel Prize in Physics for his work on the theory of stellar nucleosynthesis.

But the problem was tackled, nonetheless, by section X2-A of Bainbridge’s group with Robert W. Henderson and Roy W. Carlson responsible for engineering, design and procurement of the vessel. In May 1944 scale model “Jumbinos” were delivered to Los Alamos where numerous tests were conducted to prove the feasibility of the design. Feasible though the design appeared to be, there was scarcely a steel man in the country who felt he could manufacture the container. Specifications required that Jumbo must, without rupture, contain the explosion of the implosion bomb’s full complement of high explosive and permit mechanical and chemical recovery of the active material. To do this required an elongated elastic vessel 25 feet long and 12 feet in diameter with 14 inch thick walls and weighing 214 tons.

Roy W. Carlson is known to have worked on the understanding of blast phenomena, and in particular the confinement of an explosion by a steel vessel. From March 1945 he was a consultant for 'Structures' in the Trinity Project.


Personal letters explaining the urgency of the project and the importance of the specifications went out from Oppenheimer to steel company heads, but by May 23, 1944, Oppenheimer was forced to report to his Jumbo committee that the steel companies approached had expressed strong doubts that Jumbo could be manufactured to specifications. Meanwhile, he told them, feasibility experiments would continue on the Jumbinos and the order for the final vessel would be delayed a little longer. Eventually, the Babcock and Wilcox Corporation of Barberton, Ohio, agreed to take a crack at the job and the order was placed in Aug. 1944. It took them 13 months to make it and it cost $12 million. The following spring (1945) the tremendous steel bottle began its roundabout trip from Ohio on a specially built flat car, switching from one route to another wherever adequate clearance was assured. In May 1945, the jug was delivered to a siding, built for the purpose by the Manhattan District, at Pope, New Mexico, an old Santa Fe railroad station that served as a link with the Southern Pacific and the Pacific Coast in the 1890’s. There it was transferred to a specially built 64-wheel trailer pulled by two tractors for the 25-mile overland trip to the test site (a different report mention that Army D7 bulldozers pulled the trailer 28 miles, and on a newly build road).

Jumbo was positioned on-site hanging from a 60 foot tall steel tower. It was then lowered into a deep concrete foundation completely encasing its lower hemispherical end.  

But it was too late. During the last months before the test, all of the elaborate recovery schemes were abandoned. By then there was greater confidence in the success of the bomb and, more importantly, there was an increasing realisation that Jumbo would spoil nearly all the sought-after measurements which were, after all, the prime reason for conducting the test in the first place.

It has also been mentioned that by the time Jumbo actually arrived on-site officials were confident that the Hanford reactors could provide a stream of plutonium. Thus Oppenheimer was confident that there would be enough plutonium for a second test.


The fate of Jumbo, however, was not absolutely settled until the very last minute. On June 11, 1945, just a month before the test, Bainbridge, in a memo to Norris Bradbury, the then Laboratory director and in charge of bomb assembly, wrote that “Jumbo is a silent partner in all our plans and is not yet dead. . . We must continue preparations for (its) use until Oppenheimer says to forget it for the first shot”. 

And a silent partner it remained, Ultimately the magnificent piece of engineering was erected on a tower 800 feet (other reports say 800 m) from Ground Zero to stand idly by through the historic test (just in case it might be needed for second test). In fact the tower was destroyed during the test, but Jumbo survived intact. After the war, the Army blew the ends off Jumbo in an unsuccessful attempt to destroy it, and today the remains can be seen at the Trinity Site. It is said that Groves was concerned that Congress would criticise him for spending the $12 million on what was a white elephant, so he ordered Jumbo destroyed. However, it survived being hit by eight 500-pound shells.

Indirectly Jumbo was included in the report of Groves to the U.S. Secretary of War. The Jumbo was suspended from a 60-foot steel tower firmly anchored to concrete foundations, and a distance of 800 feet from the Trinity explosion. Groves compares this to a 15-20 story skyscraper. Forty tons of steel was used, equivalent to that used in a six story building. The blast “tore it from its foundations, twisted it, ripped it apart and left it flat on the ground”. Groves considered the Pentagon no longer a safe shelter.  

Commander Norris Edwin Bradbury (1909-1997) was initially the leader of Group X-1 (Implosion Research), and in March 1945 he also became leader of Group X-6 (Assembly and Assembly Tests), both in the Explosives Division. Also from March 1945 he was, with George Kistiakowsky, responsible for ‘Assembly’ in Project Trinity. Bradbury later became director of Los Alamos, and oversaw the development of thermonuclear weapons.   

Back at the Base Camp 

Between August 1944 and February 1945, however, Fussell’s section did manage to work on such preparations as acquiring and calibrating equipment, studying expected blast patterns, locating blast and earth shock instruments, and installing cables to determine electrical and weather characteristics, in addition to the design and construction of the test site Base Camp and the design and contract for Jumbo, i.e. about all the test program could demand with the plight of implosion so desperate and still uncertain. Contracts were let early in November 1944 for construction of Trinity camp, based on plans drawn up by Major Stevens in October. The Base Camp was completed Dec. 30, 1944 and a small detachment of about 12 military police took up residence to guard the buildings and shelters while additional construction continued.

The establishment of the Trinity Base Camp in the Jornada del Muerto valley east of the Rio Grande in New Mexico brought suddenly, albeit for a very brief time, a great influx of men and machines to a region hitherto home for only a few hardy farmers and ranchers.

Base Camp was not permanent, it was just temporary camp for scientists and technicians from the parent community of Los Alamos who had trekked from the Pajarito Plateau to the desolate Jornada to complete a unique military-scientific mission: the test of an atomic device. 

Beginning in the fall, Bainbridge and his Project Trinity group worked closely with Capt. Samuel P. Davalos, post engineer at Los Alamos in charge of the Operations Division's Technical Area Section, to develop plans for a base camp at the Trinity site. It had to include a bomb test area with technical facilities and a campsite that would serve the needs of at least 160 men (i.e. barracks, officers quarters, a mess hall, and other support facilities). At the end of Dec. 1944, when these basic facilities were completed, a small MP detachment under the command of Lt. Harold C. Bush arrived from Los Alamos to provide security for the satellite community. As 1945 unfolded, the activity of the more than two hundred camp residents intensified in a concerted effort to ready all technical facilities for the bomb test, tentatively scheduled for early summer. 

Project Trinity needed more than just a Base Camp, it needed warehouses; repair shops; bomb-proof structures; an explosives magazine; a stockroom to house equipment shipped from the Hill (the name given to Los Alamos); an unloading platform on the railroad siding at Pope, which was some 25 miles west of the site; a commissary; and (always) more barracks. They also constructed more than 20 miles of blacktopped roads for a fleet of one hundred motor vehicles, erected 200 miles of telephone wire, and installed electric water pumps and portable generators.

One report mentions 47 miles of roads built and upgraded, and a 20 mile road constructed between the site and a siding on the Santa Fe railroad in Pope, New Mexico (presumably used to bring Jumbo to the site). 

The same report mentioned that the main source of electric power on the test site was a 50 kW motor generator set that served all of Base Camp, and smaller generators in the bunkers at 10,000 North, West, and South. There were also two portable generators at the 1,000 North instrument bunker; and a 5 kW generator at the McDonald ranch house.

Conditions were difficult, with strict security requirements, extreme heat and exposure to a variety of poisonous reptiles and insects. Hence, project leaders at Trinity made a special effort to supply good food, reasonably comfortable quarters, and a variety of recreational sports and activities. Lieutenant Bush provided organised athletics, local hunting trips, a game room, and nightly movies.

Bainbridge and Davalos drew up plans for a Base Camp with accommodation and facilities for 160 personnel, along with the technical infrastructure to support the test. A construction firm from Lubbock, Texas built the barracks, officers' quarters, mess hall and other basic facilities. The requirements expanded and, by July 1945, 250 people worked at the Trinity test site. On the weekend of the test, there were 425 present.

Mention is made of Marvin Davis and his military police unit. They had planned to use horses to ride patrol, but ended up using jeeps and trucks. The horses were sometimes used for polo, and Capt. Bush, the Base Camp commander, managed to get them some real polo equipment. Those with experience were allowed to hunt deer and pronghorn. Some of the city boys had never seen a scorpion before. Other young soldiers arrived and were thrown in at the deep-end, Carl Rudder was responsible for maintaining all the generators, wells, and pumps, and Loren Bourg, being a trained fireman, became the site one-man fire department.

The planning continues

As the new year arrived, the implosion work began to show more promise and the Research Division under Robert Rathbun Wilson was asked to postpone even its highest priority experiments and turn its four groups, under Wilson, John Harry Williams, John Manley Manley, and Emilio Gino Segrè, to developing instruments for the test. By February 1945 the Laboratory was mobilising. Oppenheimer had long since been committed in Washington to a test in July 1945 and the deadline was fast approaching.

Emilio Gino Segrè (1905-1989) was leader of Group P-5 (Radioactivity) that later became Group R-4 (Radioactivity) in the Research Division of Robert R. Wilson. He discovered technetium and astaline, and then the antiproton for which he received the Nobel Prize in Physics in 1959.

To keep the implosion design as simple as possible, a near solid spherical core was chosen rather than a hollow one, although calculations showed that a hollow core would be more efficient in its use of plutonium. The hollow core design was initially pursued, but it was found difficult to produce the more stringent hollow pit implosion requirements that would be necessary. The core's sub-critical mass was instead manufactured into a  geometry that closely resembled a near perfect solid sphere, which could then be compressed to prompt super-criticality by a less technically demanding implosion, generated by the high explosive lens. This design became known as a "Christy Core" or "Christy pit" after physicist Robert Frederick Christy, who made the solid pit design a reality after it was initially proposed by Edward Teller. Along with the pit, the whole physics package was also informally nicknamed "Christy['s] Gadget".

Robert Frederick Christy (1916-2012) was an early recruit to the Theory Division in Los Alamos, he later was President of Caltech. He correctly estimated the amount of pure uranium-235 needed to small Los Alamos “Water Boiler” reactor go critical, i.e. 575 gm. He also suggested using a solid plutonium core, which became known as a “Christy pit”.  

In 1942 Edward Teller (1908-2003) was in the Theoretical Group in the Chicago Metallurgical Laboratory. From March 1944 to June 1944 he was leader of Group T-1 (Hydrodynamics of Implosion and Super) in the Theoretical Division in Los Alamos, and from June 1944 he had established an independent group of the ‘super’ fusion weapon. From Aug 1944 Teller was leader of Group F-1 (Super and General Theory) in F Division at Los Alamos. Later he was co-founder of Lawrence Livermore National Laboratory.   

In a conference at Los Alamos, attended by General Groves, it was decided then and there to freeze the implosion program and concentrate on one of several methods being investigated, i.e. lens implosion with a modulated nuclear initiator. The conference then outlined a detailed schedule for implosion work in the critical months ahead:

April 2, 1945: full scale lens mould delivered and ready for full scale casting.

April 15, 1945: full scale lens shot ready for testing and the timing of multi-point electrical detonation.

March 15, 1945 to April 15, 1945: detonators come into routine production.

April 15, 1945: large scale production of lenses for engineering tests begin. (Lenses direct explosive’s shock waves to a converging point)

April 15, 1945 to May 1, 1945: full scale test by magnetic method.

April 25, 1945: hemisphere shots ready.

May 15, 1945 to June 15, 1945: full scale plutonium spheres fabricated and tested for degree of criticality.

June 4, 1945: fabrication of highest quality lenses for test underway.

July 4, 1945: sphere fabrication and assembly begin.

I think this conference took place on Dec. 23, 1944, and it defined the measurement of the pressure of the blast wave and the time spread in the firing of the detonators as essential for the test. Desirable experiments were the photographic and spectrographic analyses of the fireball, and the measurement of the Earth’s motion during the explosion (in case of lawsuits).  

By the following month the schedule had already been shifted to establish July 4, 1945 as the actual test date and that was only the beginning of the date juggling. Overall direction of the implosion program was assigned early in March, 1945, to a committee composed of Samuel King Allison, Robert Fox Bacher, George Kistiakowsky, Charles Christian Lauritsen, Capt. William Sterling “Deak” Parsons and Hartley Rowe. For its job of “riding herd” on the program the committee was aptly named the Cowpuncher Committee and it was the Cowpunchers who had the responsibility for the intricate job of integrating all the efforts of Project Y, the arrival of critical material from Hanford and the activities at the Trinity site in order to meet the test deadline. 

From June 1940 to Jan. 1942 Samuel King Allison (1900-1965) was a member of the Committee/Section on Uranium. In 1942 he was leader of the Chemical Group in the Chicago Metallurgical Laboratory, and he became the laboratory Director from June 1943 to Nov. 1944. He also appears on the staff list of Los Alamos from 1944-1946. From 1946 to 1957 he was director of the Enrico Fermi Institute of Nuclear Studies

From March 1943 Robert Fox Bacher (1905-2004) was leader of the Experimental Physics Division in Los Alamos, and from Aug. 1944 he was leader of Weapon Physics Division. He was later vice president and provost of Caltech. 


Charles Christian Lauritsen (1892-1968) invented the Lauritsen electroscope, a type of quartz fibre radiation dosimeter. In the Manhattan Project he developed the proximity fuse, as well a running a Caltech program of rocket weapons. 

Capt. William Sterling “Deak” Parsons (United States Navy), from June 1943, was the leader of the Ordnance and Engineering Division in Los Alamos, and from Aug. 1944 he became one of the two Associate Laboratory Directors (with Enrico Fermi), and under the Laboratory Director Robert Oppenheimer. From March 1945 he was Officer-in-Charge of Project Alberta, to prepare atomic weapons for combat use. He later became Rear Admiral.  

Hartley Rowe was an experienced industrial designer who worked on the transition from research to production. He was a former technical advisor to General Eisenhower. Oddly enough, he had originally worked on the Panama Canal.

Trinity Base Camp was built by the Army in the winter of 1944 and was occupied by a detachment of military police from Dec. 1944, on. By summer 1945 it was a bustling hive of activity with more than 200 scientists, soldiers and technicians.

One report put the site population at about 325 people, 250 from Los Alamos, 30 from SED (Special Engineering Detachment), and 45 military police. 

Project Trinity takes form

Project Trinity, with Bainbridge as test director and William Penney and Victor Weisskopf as consultants, became an official organization and the top priority project of the Laboratory in March 1945. 

At the same time Project Alberta, for combat delivery of the weapons, was organized under Capt. Parsons with Norman Foster Ramsey Jr. and Norris Bradbury as technical deputies.

Norman Foster Ramsey Jr. was, from late 1943, leader of Group E-7 (Delivery) the Ordnance and Engineering Division in Los Alamos. From Aug. 1944 he was leader of Group O-2 (Delivery) in the Ordnance Division in Los Alamos. Later he was the first science advisor to NATO, and he also developed the atomic hydrogen maser. He received  the 1989 Nobel Prize in Physics, for the invention of the separated oscillatory field method, which had important applications in the construction of atomic clocks.

Bainbridge was a Harvard physics professor with a background in electrical engineering and a 3-year stint at the MIT Radiation Laboratory who had come to Los Alamos as a group leader in charge of high explosive development. As General Groves pointed out in his book, “Now It Can Be Told”, Bainbridge was “quiet and competent and had the respect and liking of the more than 200 enlisted men later on duty at Alamogordo”.

Bainbridge’s first task was to rush his organisation into preparations for a trial test, the detonation of 100 tons of conventional high explosives, which had been proposed in the winter of 1944 and scheduled for early May 1945. Since very little was known, in 1945, about blast effects above a few tons of TNT, such a test would provide data for the calibration of instruments for blast and shock measurements and would serve as a dress rehearsal to test the operation of the organisation for the final shot.

Meanwhile, a vast and complex laboratory was growing in several square miles of empty desert. There was a maze of roads to be built, hundreds of miles of wires to be strung over, on and under the ground, a complete communication system installed, buildings to be erected, supplies, equipment and personnel to be transported between Los Alamos and Trinity, all under the cloak of supreme secrecy.

One report mentions that the radio communications among site personnel were provided on two specially assigned FM channels by a large 50-watt station at Base Camp and six 25-watt stations elsewhere on the test site. Twenty 25-watt Motorola transmitters and receivers were deployed in vehicles. The test area was also equipped with two SCR-299 450-watt ground-to-plane transmitters and two SCR-511 walkie-talkie transceivers for use during the test.

The man who shouldered the monumental task of coordinating all that was on-going was John Harry Williams, leader of the Laboratory’s Electrostatic Generator group, who became responsible for Trinity services as head of TR-1. As Bainbridge wrote later, “The correlation of the construction program and the proper and successful designation of construction aid was exacting work requiring ‘superior judgment’, as the Army says, and long hours of hard work. This was done supremely well by Williams, to whom the Trinity project owes much for the successful completion of the operation”. Bainbridge has also pointed out the invaluable assistance provided by Sgt. J. A. Jopp, who was in charge of all the wire installation and construction at the site.


Procurement of an incredible assortment of equipment ranging from Kleenex to elaborate scientific instruments was a seemingly insurmountable job handled by Robert Van Gemert, also head of the Laboratory’s Supply and Property Department, aided and abetted by Frank Oppenheimer who served as Bainbridge’s trouble shooter. By April the number of urgent purchase requests had increased so rapidly that it became necessary to inflate the urgency rating’s that had been in use by the Procurement Office. Until things got out of hand that spring, four ratings X, A, B and C had been used in order of decreasing priority. By early May 1945, when everything seemed to warrant an X priority, it was announced that this super urgent rating would be subdivided into three others: XX, X1, and X2. XX would be used only if failure to obtain the material would produce a setback of major importance in the overall program of the Laboratory. It authorised the Procurement Office, through the Washington Liaison Office, to have recourse to the highest authority of the War Production Board and all government agencies and to use a special dispatch or cargo plane from anywhere in the United States to get delivery.

Robert J. Van Gemert, after the war, became head of the Supply and Property Department in Los Alamos. Later it was said that he used balsa wood business cards with English on one side and Japanese on the other. 

But the manufacturers were not impressed with X or XX priorities. Representatives from every armed service and government war project were pounding on their desks with equally high priorities and waiting six to 15 weeks for delivery while Trinity people were demanding three weeks delivery for the same item. The problem was further complicated by the fact that there was no direct communication between the Project and the purchasing offices, nor could Los Alamos buyers talk directly to the scientists at the site to discuss possible substitutions or compromises on specifications. Some items were just well-nigh impossible to get, like the seismographs that were needed to check earth shock at outlying areas around the state. The only instruments available were finally located at a firm which had already sold them to the Nazi sympathising Argentine government. It took an overriding directive, direct from General Grove’s office, to get the instruments shipped to Trinity instead.

Hundred of miles of wires had to be strung between base camp, the control point, the instrument bunkers and Ground Zero — one of the countless jobs that kept men at Trinity working at a feverish pace throughout the spring and summer of 1945.

There were four different types of bunkers, for protecting recording instruments, test personnel, motor generators, and cameras.  

Another crisis came when 10,000 feet of garden hose was lost during a shipping strike. A second order was placed but by June 29, 1945 the hose was still on the list of critical items not yet on hand. The hose was used to encase cables to sensitive instruments to protect them from the weather. Delayed delivery on a number of urgent requests led Oppenheimer to call a meeting in May 1945 to review the procurement situation. One of the principal reasons for the delays, it turned out, was the shortage of personnel in the Los Angeles, New York and Chicago purchasing’ offices. Although the number of requisitions had greatly increased there had been no increase in the number of buyers since January 1944, a situation blamed on salary restrictions. As a result of the meeting salary adjustments were agreed upon and more personnel secured for all three offices. Direct communications were established between the Project and the New York and Chicago offices and Project members were asked to submit improved drawings and specifications.

But slow or not, the materials did arrive and in June 1945 the amount of goods handled by the main warehouse at Los Alamos reached its peak. During May 1945 the warehouse handled an average of 35 tons a day, 89% was incoming; during June 1945 the daily average rose to 54 tons of which 87% was incoming, and during the first half of July 1945 it was 40 tons a day, 80% incoming. A new shipping group was organised that spring to handle the outgoing goods, most of’ them bound for Trinity or Tinian Island in the Pacific.

North Field was an airfield on Tinian in the Mariana Islands. It was the base for bombing operation against Japan, including the incendiary bombing of Tokyo and Operation Starvation (naval mines in harbours and sea lanes). It also became the base for the 509th Composite Group that flew the atomic bombing raids on Hiroshima and Nagasaki

Plenty of local procurement problems remained. First there was communication. Only five people on the project were allowed to telephone between Trinity and Los Alamos and these calls were routed to Denver, on to Albuquerque and finally to San Antonio, New Mexico. Teletype service was so bad, Van Gemert recalls, that you never knew if the test site was asking for a tube or a “lube” job. It soon became evident that the best way to communicate was to send notes back and forth by the truck drivers. At least two and often as many as ten trucks left Los Alamos every evening after dark to avoid both the blistering desert heat and unnecessary notice, and arrived at the test site early the next morning. Almost always there was a stop to be made at the U.S. Engineers yard in Albuquerque to pick up items addressed to Prof. W. E. Burke of the University of New Mexico’s physics department, who served as a blind to avoid a connection between the items and Los Alamos.

“We’d get things to Trinity any way we could”, Van Gemert says. Some of the ways were devious. A carload of telephone poles was desperately needed at the test site and no freight train was traveling fast enough to get it there in time. After considerable urging the Santa Fe railroad consented to attach the car to the rear of the Super Chief (the flagship passenger train of the Atchinson, Topeka and Santa Fe Railway) and sped the cargo to Albuquerque. Another time, for lack of freight space, 24 rolls of recording paper were luxuriously ensconced in a Super Chief drawing room for the trip from Chicago. 

To supplement the special items, the procurement people established a complete technical stockroom at the test site early in the game and trucked the entire stock from Los Alamos. The stockroom, known officially as FUBAR (fouled up beyond all recognition), was manned by enlisted men who used their spare time to manufacture the face shields needed to protect observers from the test blast. The shields were made of aluminum sheets, mounted on a stick handle, with welders’ goggles for a window.

There never seemed to be enough people to take care of all the work to be done on the test preparations and those who were available, from mess attendants to group leaders, worked at a fever pitch. A ten hour day was considered normal and it often stretched to at least 18 hours.

In the spring of 1945 a big part of the Laboratory was reorganised to take care of the test and many people found themselves involved in activities far removed from their normal duties. John Harry Williams, the high energy physicist, took the responsibility for construction and servicing of the base camp. John Henry Manley was wrapped up in neutron measurements as a Research Division group leader when he suddenly found himself in charge of blast measurements for the test. “I didn’t know anything about blast measurements”, he recalled 20 years later. “We’d never done anything like that before”. 

Throughout the spring and summer (of 1945) there was a constant stream of personnel traveling between Site Y and Trinity in a motley assortment of busses and cars, some of them barely able to make the long, monotonous trip.

Site Y was just another name for Los Alamos. 

Security precautions

Security precautions were rigid. In March 1945, Dana P. Mitchell, assistant director of the Laboratory, issued terse, precise travel instructions: “The following directions are strictly confidential and this copy is to be read by no one but yourself. You are to turn this copy in to me personally on your return to the site, ” the memo read, and continued with specific directions and mileages for reaching the site. “Under no condition,” it went on, “when you are south of Albuquerque are you to disclose that your are in any way connected with Santa Fe. If you are stopped for any reason and you have to give out information, state that you are employed by the Engineers in Albuquerque. Under no circumstances are telephone calls or stops for gasoline to be made between Albuquerque and your destination”. 

Travellers were then instructed to “stop for meals at Roy’s Cafe in Belen, which is on the left-hand side of the main road going south. If you leave the site at 7 a.m. you should make this stop around lunch time”. Even so, by mid-afternoon when the travellers reached the little junction town of San Antonio, most of them were hot, tired and thirsty and Jose Micra’s bar and service station became a popular, if illegal, stop. Micra still remembers the unusually heavy traffic in those days. One of his customers, John Manley, remembers Micra’s wall of bottles. “He had the whole south wall of his place lined with bottles”, Manley reports. "We used to worry an awful lot about that. If our big blast traveled that far, that’s the wall it would hit”. Luckily it didn’t.

Additional regulations required that all departing groups and individuals stop at the office of the intelligence officer for an explanation of “the security objectives of Trinity”. All personnel were required to sleep and eat at the camp rather than in nearby towns, and recreation trips for movies and dinners to nearby towns were prohibited to officers, enlisted men and civilians alike.

Practical details

In addition, all Trinity-bound personnel were required to report their impending departure to Oppenheimer’s office, to Intelligence Officer R. Taylor, and to Lt. Howard Bush who was trying to keep Trinity base camp running smoothly despite the constantly fluctuating population.

A small detachment of Military Police (some sources talk of 12 men), under the command of Lt. Howard Bush, took up residence on Trinity site in Oct. 1944. It was during this time that the construction company J.D. Leftwich of El Paso, contracted by the army, completed the first facilities (end Dec. 1944). 

By early 1945 there were more than 200 residents on the Trinity Base Camp, which included a blacksmiths shop, water storage tanks, a hay barn, officers’ quarters, a supply room, mess hall, barracks, latrine, PX (Post Exchange) and day room, coal storage, infirmary, laboratory, technical warehouse, office, garage, gasoline storage tanks, fire station, engineering office, plumbing shop, electrical shop, carpentry shop, and drinking water tanks.

As Bainbridge explained in a somewhat desperate sounding memo “to all concerned” in April 1945: “If your schedule is planned some days ahead it will operate to the comfort of all concerned if you tell Lt. Taylor who is going down and when they are going down. Lt. Taylor will notify Lt. Bush, who can then make proper arrangements for sufficient food for the mess. Lt. Bush is issued rations three days a week (Monday, Wednesday and Friday) and he is required on a Monday trip to leave a list of his requirements to be picked up on the following Wednesday trip. This means a minimum of four days notification is necessary if there is to be sufficient food on hand so that he can avoid the present difficulties which late-comers having to eat delicatessen store meat instead of the particular roast scheduled for that day. Please cooperate . . .“

There were other problems than supply and demand. Sanitary conditions in the mess hall were difficult to maintain because of the hard water. When water softening equipment was installed later it turned out that a miscalculation in water analysis resulted in a unit too small to handle the huge amounts of gypsum and lime encountered.

Some were almost comical, such as when Kenneth Ingvard Greisen was stopped for speeding in Albuquerque while he was driving detonators to Trinity four days before the test. He could have been delayed by several days had the officer checked the contents of his trunk.

In the barracks, desert creatures such as scorpions had to be carefully shaken out of clothes each morning before anyone dared dress. But despite the difficulties the camp ran well. The heat of the desert summer was relieved by swims in the cattle watering reservoirs at the old McDonald ranch. A herd of antelope disappeared from the desert range, a fact which has been attributed by the press to the ravages of the first atomic bomb. Former Trinity residents, however, admit that hunting with submachine guns was a favorite pastime and antelope steak was an almost daily part of the camp menu. So was range beef, lassoed near camp by amateur cowboys. A beer fund maintained by Laboratory people helped make up for the rules against leaving camp and there were nightly outdoor movies supplied from the Army’s endless assortment of Hollywood films.

Camp morale

“The choice of Lt. H. C. Bush as commanding officer of the base camp”, Bainbridge wrote in 1946, “was a particularly fortunate one. The wise and efficient running of the camp by Lt. Bush contributed greatly to the success of the test. It was a ‘happy camp’. The excellent camp morale and military-civilian cooperation did much to ameliorate the difficulties of operation under primitive conditions”. But there were times when the excellent camp morale was put to severe test. Back in Dec. 1944 Bainbridge had discussed with an unidentified colleague the dangers of a possible overshoot by bombers using the Alamogordo Bombing Range for their practice runs. “If they should go north of Area No. 3 by mistake in 1945”, he wrote, “they would have to go more than 15 miles beyond the boundary in order to interfere with us. The probability that they will overshoot is likely to be very small. Let them have their fun and settle with Ickes for the White Sands National Monument”. But within a few months they were trying to settle with Bainbridge. The chow line forms at the Base Camp mess hall. Perhaps the menu offers antelope steak. On May 10 1945, shortly after 1 a.m., three practice 100-pound bombs carrying five-pound black powder flash units were dropped near the Base Camp stables, setting them afire, straddling the main barracks and bringing a poker game to a sudden halt. Three days later another bomb dropped on the carpentry shop. There was no serious damage and no one was hurt.

An investigation revealed that a squadron of bombers from a base some 2,500 miles away was on its final long-range practice mission before going overseas. The lead planes had hit and completely obliterated the clearly marked bombing range targets and in the confusion the following planes assumed the well-lit camp site must be the place. Bainbridge’s suggestion that anti-aircraft guns loaded with smoke shells be used to defend the camp was rejected but no further bombing attacks were made.

On another occasion, however, a group of electricians working at a distant outpost stomped into camp headquarters, tossed a handful of spent machine gun shells on the CO’s desk and resigned. It was soon discovered that gunnery crews in Alamogordo bombers were encouraged to sharpen their trigger eyes on antelope herds roaming the bombing range. For the electricians it had been too close for comfort.

A trial explosion with 100 tons of TNT

The original date for a trial shot (rehearsal) of 100 tons of TNT was May 5, 1945 but was soon shifted to May 7, 1945 to allow for installation of additional testing equipment. Many additional requests had to be refused since any further delay would have put an intolerable burden on the whole group in its attempt to meet the July test deadline. Hundreds of crates of high explosive were brought to the site from Fort Wingate, New Mexico, and carefully stacked on the 25-foot by 25-foot wooden platform of a 20-foot wooden tower made up of 16 12-inch by 12-inch timber legs. The tower placed 800 yards from what would be actual test site for main test. Tubes containing 1,000 curies of fission products from a Hanford slug were interspersed in the pile to simulate, at a low level, the radioactive products expected from the nuclear explosion. The whole test was designed in scale for the atomic shot. The centre of gravity of the high explosive was in scale with the 100 foot height for the 4,000 to 5,000 tons expected in the final test, and measurements of blast effects, earth shock, and damage to apparatus and apparatus shelters were made at scaled-in distances. Only measurements to determine “cross talk” between circuits and photographic observations were, in general, carried out at the full distance proposed for the final shot. Then, as the last day of the European war dawned, the TNT was detonated and it was spectacular. A huge, brilliant orange ball rose into the desert sky lighting the pre-dawn darkness as far away as the Alamogordo base 60 miles southeast.

The actual explosive was 108 ‘long’ tons (774 boxes) of what was called Composition “B”, a mixture of TNT and RDX. The stack was 14-foot high and had to have wooden planking with cleats inserted in order to provide additional stability. 

A 'long' ton is 2,240 lbs., whereas a ‘short’ ton was 2,000 lbs., a (metric) tonne is 1,000 kg. or 2,205 lbs. It should be noted that tonnage is a maritime measure of volume, originally 100 cubic feet.   

Take 1,000 curies of cobalt-60 (a powerful gamma-ray source), this would normally be a disk source having a dimension of about 2.5 cm in diameter, and about 1.5 cm thick. You would need a lead shield of about 50 cm for protection. It is not possible to directly convert 1,000 curies in terms of radiation dose, because it depends upon the energy of the radiation emitted by the specific isotopes. But for a 1,000 curie cobalt-60 source, a short exposure would cause immediate illness, followed by death within a few weeks (>10 sieverts).

What was actually used was a dissolved irradiated uranium slug from the Hanford reactor (which naturally contained some plutonium). The solution had a beta activity of 1,000 curies (37 TBq) and a gamma activity of about 400 curies (15 TBq), and it was poured into a flexible tube threaded through the high explosives. As far as I can tell the dissolution was actually performed right next to the explosives, using a dissolver encased in 3 feet of concrete and topped with a 7 inch thick lead plug. Later the dissolving unit was covered in dirt and fenced off.  

Ralph Nobles, a physicist who helped set up the 100-ton TNT test, shared an amusing anecdote about the test preparations.

"They had a 'gear pump' set up on a table and were pumping the tracer solution through a plastic hose into uniform grid of pipes which had been built in the TNT cube. For some reason the pressure built-up in the plastic hose attached to the pump, and the hose popped off the pump and started wildly flailing around and spraying radioactive tracer solution in all directions. 

The radioactive tracer, though not highly dangerous, it was not something, in which one wanted to get soaked! So the nearby radio chemists started running away from the pump, to escape the radioactive liquid from the hose. The MP up here, on top TNT cube, saw the chemists running away, and must have assumed they were running because the TNT was about to explode! Where upon, he dropped his rifle and started moving so fast that his helmet flew off, he literally ran 'out from under his helmet' and I have never seen 40 feet of stairs descended faster. He couldn't have reached the ground much faster by jumping. 

However, he didn't stop at the ground and as he ran past my jeep, I wondered why he hadn't jumped in and yelled, 'that thing is about to blow, let's get the hell out of here,' but he didn't slow down or even seem to notice that we were there! When I last saw him, he appeared to be trying to set a running speed record!" 

The blast itself compressed and blew the surrounding earth into a saucer-shaped crater, expelling about 40% of the dirt. A scaling up of the RaLa shots suggested that 10% of the activity would remain in the soil within a 300-foot radius. However, only 2% of the activity of the dissolved radioactive material was deposited in the crater, with most being carried in the updraft of the explosion.

RaLa is Radioactive Lanthanum (140La) which was separated from its parent, radioactive barium. It was used as a very strong gamma source mounted at the center of an implosion high-explosive assembly. Using special gamma detectors, the density history of the materials in the assembly as a function of time (microseconds) was measured during the implosion. The 140La mixed with the products of the high-explosive implosion, and was scattered at the crater, into the cloud, and also was found in fallout down wind.

The rehearsal proved to be tremendously valuable and the high percentage of successful measurements in the Trinity test may be attributed in large measure to the experience gained from the test shot. Blast and earth shock data were valuable not only for calibrating instruments but for providing standards for the safe design of shock proof instrument shelters. Measurement of the effects from the radioactive material inserted in the stack of explosive was especially valuable in giving information on the probable amount and distribution of material which would be deposited on the ground. This information was essential for planning the recovery of equipment, the measurement of bomb efficiency, and protection of personnel for the final shot. The test also gave the men, accustomed to well equipped laboratories, a familiarity with the tribulations of field work, and perhaps most importantly, showed up some defects in the test operations while there was still time to correct them.

It has been reported that an electrical signal of unknown origin caused the explosion to go off 0.25 seconds early, ruining experiments that required split-second timing. The piezoelectric gauges developed by Anderson's team correctly indicated an explosion of 108 tons of TNT (450 GJ), but Luis Alvarez and Waldman's airborne condenser gauges were far less accurate.

The detonation itself produced a highly luminous sphere, with a hot column that mushroomed out at 15,000 feet. The illumination and sound was detected at the Alamogordo Air Base 60 miles away. 

Immediately after the test Bainbridge asked for lists of complaints about the operations from the various group leaders involved and on May 12, 1945, while the experience was still fresh in everyone’s minds, held a gripe session to discuss suggestions for improvements. Far and away the biggest complaint was transportation. Nearly everyone felt there were not enough roads between Ground Zero and the various shelters and the roads that did exist were in intolerable condition. The dust and ruts were hard on both personnel and instruments and the two-wheel drive GI sedans were constantly getting bogged down in a foot or so of soft, loose sand. They also asked for more vehicles and more repair men who could service the cars at night to avoid delays and keep up with the demand.

To overcome poor communications throughout the test site, new phone lines, public address systems to shelters and short wave radios in automobiles were requested as well as a building in which to hold meetings. Everybody complained of lack of help to get things done on time and asked in particular for more help on procurement, shipping and stock management and a direct teletype to the Los Alamos Procurement Office.

The group felt the operation was severely handicapped by the interminable delays caused by rigid restrictions on the movement of personnel in and out of the various areas just before the test. They asked for and got free access to all parts of the test area during the last few hours before the shot.

Only one man complained about camp food.

As a result of the meeting, 20 miles of black top road had to be laid, new structures built and a new communication system installed. After the test, too, a major effort had to be devoted to the final timing devices. Each experiment required different time schedules, some having to start ahead of Zero, others requiring a warning pulse only 1000th of a second ahead of the detonation. The circuits were the responsibility of Joseph McKibben and the electronic timing device was developed by Ernest William Titterton of Australia. 

Joseph Laws McKibben was also the inventor an “artificial muscle” which he designed to help his daughter’s polio-paralysed hands. 

Ernest William Titterton (1916-1990) was one of the British Mission, but he went on to also particulate in Operation Crossroads. He was also involved in the British bomb tests Operation Hurricane and Operation Totem. He later held the Chair of Nuclear Physics in Canberra.    

In addition to these chores there were the weak spots pointed out in the trial test to be overcome. And there was precious little time to do it. 

Back to planning for Trinity

As early as April 1945 hopes of meeting the original Independence Day deadline had begun to fade. Delays in the delivery of full scale lens molds and the consequent delay in the development and production of full scale lenses, as well as the tight schedule in production of active material made it necessary to reconsider the date, and on June 9, 1945, the Cowpuncher Committee agree that July 13, 1945, was the earliest possible date, and July 23, 1945 was more probable.

In a memo to all his group leaders on June 19, 1945, Oppenheimer explained that although July 4, 1945, had been accepted as a target date in March, “none of us felt that date could be met”. He then announced the Cowpuncher decision and explained, “In reaching this conclusion we are influenced by the fact that we are under great pressure, both internally and externally, to carry out the test and that it undoubtedly will be carried out before all the experiments, tests and improvements that should reasonably be made, can be made”. And so the pressure mounted, security tightened and preparation went on with increasing speed and intensity.

The air hung heavy that summer, rains failed to come and precipitation was half the normal amount. Temperatures rose to average four degrees above normal. Water became scarce and fires threatened, adding to the irritations and frustrations.

Dorothy McKibbin, who ran the Santa Fe liaison office for the Los Alamos project, could discern the increased tension, but because of rigid security, she had only her intuition to tell her what was happening. As many as 70 people checked into her office every day and one day she counted 100 phone calls. “The voices on the telephone showed strain and tautness, and I sensed we were about to reach some kind of climax in the project”, she recalls.

In the Laboratory, one or two hour meetings, attended by consultants, group and section leaders involved in the Trinity Project, were being held every Monday for consideration of new experiments, correlation of the work, detailed scheduling and progress reports. One of the most important corrective measures resulting from the 100-ton TNT test had been the setting of a date after which further apparatus, particularly electrical equipment, could not be introduced into the experimental area. The deadline would allow plenty of time for dry runs and would reduce the risk of last minute damage to electrical connections. In view of this, proposed experiments were described in writing in great detail and submitted to a special examining committee. If approved they were then submitted to the Monday meetings where they were considered with respect to the test programming as a whole before being accepted. “Any new experiment had to be awfully good to be included after the deadline”, Bainbridge reports.

For about a month before the test, John Williams held nightly meetings at Trinity to hear reports on field construction progress and to plan the assignment of men for the following day. Construction help was assigned on the basis of needs and priority of experiments which had been accepted for the test.

Meanwhile, J. M. Hubbard, who had joined the Trinity Project early in April 1945, as meteorology supervisor, had undertaken the job of determining the best test date from a weather point of view. Weather was a vital factor. Clear weather was best suited to observation planes in the air and visual and photographic measurements on the ground. Rain before or during the test could damage electrical circuits both for firing the “gadget” and operating the instruments.

The nuclear device detonated at Trinity was nicknamed ‘Gadget’, and looked like a large steel globe. The term Gadget' was a laboratory euphemism for a bomb, from which the laboratory's weapon physics division, 'G Division', took its name in August 1944. At that time it did not refer specifically to the Trinity Test device as it had yet to be developed, but once it was, it became the laboratory code name. The Trinity ‘Gadget' was officially a Y-1561 device, as was the "Fat Man” used a few weeks later in the bombing of Nagasaki. The two were very similar, with only minor differences, the most obvious being the absence of fusing and the external ballistic casing. 


Only six months before the test, according to General Groves, Joseph Oakland Hirschfelder, a Los Alamos physicist, had first brought up the possibility that fallout might be a real problem. For this reason it was considered essential that wind direction be such that the radioactive cloud would not pass over inhabited areas that might have to be evacuated, and there should be no rain immediately after the shot which would bring concentrated amounts of fallout down on a small area.

From fall 1943 Joseph Oakland Hirschfelder was leader of Group E-8 (Interior Ballistics) of the Ordnance and Engineering Division in Los Alamos, then from Sept. 1944 he was leader of Group T-7 (Damage) in the Theoretical Division. Later he was leader of Group O-5 (Calculations) in the Ordnance Division, and by March 1945 he was a consultant on ‘Damage' for the Trinity Project.

Using reports from each group on the particular weather conditions or surveys they would find most useful and coordinating them with complete worldwide weather information, Hubbard ultimately pinpointed July 18-19 or 20-21 (1945) as the ideal date with July 12-14, 1945, as second choice. July 16, 1945, was mentioned only as a possibility. However, on June 30, 1945, a review of all schedules was made at a Cowpuncher meeting for which all division leaders had submitted the earliest possible date their work could be ready. On the basis of these estimates, July 16, 1954, was established as the final date.

From the beginning, estimates of the success of the 'gadget' had been conservative. Although safety provisions were made for yields up to 20,000 tons (i.e. 20 kilotons TNT equivalent), test plans were based on yields of 100 to 10,000 tons.  By as late as July 10, 1945, the most probable yield was set at only 4,000 tons. 

Scientists not directly involved in the test established a betting pool on the yield and the trend was definitely toward the lower numbers, except for Edward Teller’s choice of around 45,000 tons. Oppenheimer himself reputedly picked 200 tons (or 300 tons, depending upon the report) and then bet $10 against Kistiakowsky’s salary that the ‘gadget' wouldn’t work at all. 

Ramsey chose zero (a complete dud), Kistiakowsky 1.4 kilotons, and Bethe chose 8 kilotons. Bethe's choice of 8 kilotons was exactly the value calculated by Segrè, with Bethe stating that he was swayed by Segrè's authority over that of a more junior member of Segrè's group who had calculated 20 kilotons. Enrico Fermi offered to take wagers among the top physicists and military present on whether the atmosphere would ignite, and if so whether it would destroy just the state, or incinerate the entire planet. This last result had been previously calculated by Bethe to be almost impossible, although for a while it had caused some of the scientists some anxiety. Bainbridge was furious with Fermi for scaring the guards who, unlike the physicists, did not have the advantage of their knowledge about the scientific possibilities. His own biggest fear was that nothing would happen, in which case he would have to head back to the tower to investigate.

Isidor Isaac Rabi, project consultant, won the betting pool with a guess of 18 kilotons, a number he picked only because all the low numbers had been taken by the time he entered the contest.

Isidor Isaac Rabi (1898-1988) received the 1944 Nobel Prize for Physics, for his discovery of nuclear magnetic resonance, which is used in magnetic resonance imaging.

It was not just the yield that was in doubt. Even as the scientists went about the last few weeks of preparations, the nagging uncertainty persisted about whether the bomb would work at all. This air of doubt is depicted in a gloomy parody said to have circulated around the Laboratory in 1945: 

“From this crude lab that spawned a dud

Their necks to Truman’s axe uncurled

Lo, the embattled savants stood

And fired the flop heard round the world”

Then, as if things weren’t looking dismal enough, a meeting of Trinity people held just before the test heard Hans Bethe describe in depressing detail all that was known about the bomb, and all that wasn’t. Physicist Frederick Reines remembers the utter dejection he felt after hearing the report. “It seemed as though we didn’t know anything”, he said.

Frederick Reines (1918-1998) was awarded the 1995 Nobel Prize for Physics, for his co-detection of the neutrino. Whilst at Los alamos he was director of Operation Greenhouse in 1951. 

It was only natural, Bethe wrote later, that the scientists would feel some doubts about whether the bomb would really work. They were plagued by so many questions: Had everything been done right? Was even the principle right? Was there any slip in a minor point which had been overlooked? They would never be sure until July 16, 1945.

The last days of preparation (July 1945)

By the first week in July 1945, plans were essentially complete and the hectic two weeks that remained were devoted to receiving and installing equipment, completing construction, conducting the necessary tests and dry runs and, finally, assembling the device.

The plans, as described in the official AEC history, “The New World”, were these: 

Working in shelters at three stations 10,000 yards south, west and north of the firing point, teams of scientists would undertake to observe and measure the sequence of events. The first task was to determine the character of the implosion. Kenneth Ingvard Greisen and Ernest Titterton would determine the interval between the firing of the first and last detonators. This would reveal the degree of simultaneity achieved. Darol Kenneth Froman and Robert R. Wilson would calculate the time interval between the action of the detonators and the reception of the first gamma rays coming from the nuclear reaction. From this value they hoped to draw conclusions as to the behavior of the implosion. With Bruno Benedetto Rossi’s assistance, Wilson would also gauge the rate at which fissions occurred.

Implosion studies were only a start. The second objective was to determine how well the bomb accomplished its main objective, i.e. the release of nuclear energy. Emilio Segrè would check the intensity of the gamma rays emitted by the fission products, while Hugh T. Richards would investigate the delayed neutrons. Herbert Lawrence Anderson would undertake a radiochemical analysis of soil in the neighbourhood of the explosion to determine the ratio of fission products to unconverted plutonium. No one of these methods was certain to provide accurate results, but the interpretation of the combined data might be very important.

The third great job at Trinity was damage measurements. John Henry Manley would supervise a series of ingenious arrangements to record blast pressure. Others would register earth shock while William G. Penney would observe the effect of radiant heating in igniting structural materials. In addition to these specific research targets, it was important to study the more general phenomena. This was the responsibility of Julian E. Mack. His group would use photography and spectrographic observations to record the behaviour of the ball of fire and its aftereffects. Some cameras would take colour motion pictures, some would take black and white at ordinary speeds and others would be used at exceedingly high speeds, up to 8000 frames per second, in order to catch the very beginning of the blast wave in the air. There also would be several spectrographs to observe the colour and spectrum of the light emitted by the ball of fire in the centre of the blast.

Kenneth Ingvard Greisen (1918-2007) was leader, from Aug. 1944 to May 1945, of Group X-1A (Photography with Flash X-Rays), in the Explosives Division of Los Alamos. He then became leader of Group X-7 (Detonator Developments) in same Division.

Darol Kenneth Froman (1906-1997), from March 1943, was leader of Group P-4 (Electronics) in the Experimental Physics Division of Los Alamos, and from Aug. 1944 he was leader of Group G-8 (The Electric Method) in the Weapon Physics Division.  

Bruno Benedetto Rossi (1905-1993), from Sept. 1943, was leader of Group P-6 (Detectors) in the Experimental Physics Division of Los Alamos. Originally he had a group working on improving electronic techniques, and in Sept, 1943 they were consolidated with a group working on improving counters (originally under Hans Staub), creating Group P-6. From Aug. 1944 Rossi was leader of Group G-6 (The RaLa Method) in the Weapon Physics Division. 

Herbert Lawrence Anderson (1914-1988) was Laboratory Research Coordinator in the Physics Group (under Enrico Fermi) in the Chicago Metallurgical Laboratory, in 1942. But by Aug. 1944 Anderson was leader of Group F-4 (Fission Studies) in F Division in Los Alamos.

Observation planes, one of them carrying Capt. Parsons, head of the overseas delivery project, would fly out of Albuquerque making passes over the test site to simulate the dropping of a bomb. They would also drop parachute-suspended pressure gauges near Ground Zero. One of the main reasons for the planes would be to enable Parsons to report later on the relative visual intensity of the explosion of the test bomb and that of the bomb to be dropped on Japan.

Safety precautions

Plans also were made to cover the legal and safety aspects of the test. To protect men and instruments, the observation shelters would be located 10,000 yards from Ground Zero and built of wood with walls reinforced with concrete and buried under huge layers of earth. Each shelter was to be under the supervision of a scientist until the shot was fired at which time a medical doctor would assume leadership. The medics were familiar with radiation and radiation instruments and would be responsible for efficient evacuation of the shelters on designated escape routes in case of emergency. Vehicles would be standing by ready to leave on a moments notice, manned by drivers familiar with the desert roads at night. Commanding the shelters would be R. R. Wilson and Dr. Henry Barnett at N 10,000, John Manley and Dr. Jim Nolan at W 10,000 and Frank Oppenheimer and Dr. Louis Hemplemann at S 10,000. 

N 10,000, W 10,000, and S 10,000 were the closest shelters, 10,000 yards north, west, and south of the tower (S 10,000 was the control bunker). Some reports incorrectly mention 10,000 m, and even convert that into 6.2 miles (as opposed to xx m). The three bunkers were called Able, Baker (South) and Pittsburgh, and they were heavily built wooden bunkers reinforced with concrete and covered with earth. Each had its own shelter chief: Robert Wilson at N-10,000, John Manley at W-10,000 and Frank Oppenheimer at S-10,000.

The fourth observation point was the test’s Base Camp (the abandoned Dave McDonald ranch), located about 10 miles southwest. 

The project’s leadership observed the explosion from a shelter on Campaña Hill, about 20 miles from the tower (today near Stallion Range Gate).

From March 1945 Capt. James (Jim) Nolan was a special consultant for the Project Alberta. 

A contingent of 160 enlisted men under the command of Major T. O. Palmer were stationed north of the test area with enough vehicles to evacuate ranches and towns if it became necessary and at least 20 men with Military Intelligence were located in neighboring towns and cities up to 100 miles away serving a dual purpose by carrying recording barographs in order to get permanent records of the blast and earth shock at remote points for legal purposes.

It has been written that in the final two weeks before the test, some 250 personnel from Los Alamos were at work at the Trinity site, and Lieutenant Bush's command had ballooned to 125 men guarding and maintaining the Base Camp. Another 160 men under Major T.O. Palmer were stationed outside the area with vehicles to evacuate the civilian population in the surrounding region should that prove necessary. They had enough vehicles to move 450 people to safety, and had food and supplies to last them for two days. Arrangements were made for Alamogordo Army Air Field to provide accommodation.

It is said that Groves, who was already concerned for the safety of Amarillo, Texas, a city of 70,000 only 300 miles away, placed a call to New Mexico Governor John J. Dempsey explaining that martial law might need to be implemented in the event of an emergency at the site.

The army located, listed and mapped every person living with a 40-mile radius in case evacuation became necessary. An evacuation detachment consisting of approximately 300 personnel was established in case protective measures or evacuation of civilians living offsite became necessary (including a detachment of 160 enlisted men).

A different report mentions 25 members of the Counter-Intelligence Corps stationed in towns and cities up to 100 miles from the site. 

The last week

On July 5, just six days after enough plutonium had been received, Oppenheimer wired Project consultants Arthur Holly Compton in Chicago and Ernest Orlando Lawrence in Berkeley: “Anytime after the 15th would be a good time for our fishing trip. Because we are not certain of the weather we may be delayed several days. As we do not have enough sleeping bags to go around, we ask you please not to bring any one with you”. 

Arthur Holly Compton (1892-1962) had been the chairman of the National Academy of Sciences review committee (April-Nov. 1941). From Jan. to June 1942 Compton was program chief of the OSRD Section S-1. In Jan. 1942: Compton decided to consolidate research on slow-neutron chain reaction into a new ‘Metallurgical Laboratory’ to be located on the University of Chicago campus, and he remained the Director until 1946 (Fermi was leader of the Physics Group). 

Ernest Orlando Lawrence (1901-1958) had been a member of the National Academy of Sciences review committee (April-Nov. 1941). From Jan. to June 1942 Lawrence was program chief of the OSRD Section S-1. He won the 1939 Nobel Prize in Physics for the invention of the cyclotron.  

A trial assembly of the 'Gadget' without the active components or explosive lenses was carried out by the bomb assembly team headed by Norris Bradbury at Los Alamos on July 3, 1945. It was then driven to Trinity and back. 

In one of the reports it was mentioned that two complete sets of high explosive castings “lenses” were available, the first on July 7, 1945, followed by a second set on July 10, 1945. Each was examined by Bradbury and Kistiakowsky, and the best ones were selected for use. The remainder were handed over to Edward Creutz, who conducted a test detonation at Pajarito Canyon near Los Alamos without nuclear material. 

The Trinity charges were assembled in Los Alamos on July 12, 1945, and sent to Trinity (on Friday 13th). When the Trinity charge was complete an inspection of the charge was made by removing each polar cap. Because of the specially prepared case, further inspection of the charge was possible. This case had been drilled with 1-inch holes located at the corner intersection of each casting. The charge was found to be satisfactory, the case was then closed, all booster holes were sealed, and the unit was wrapped in a Butvar (waterproof wrapping material) bag, sealed and lashed firmly to the truck in preparation for the haul to Trinity. The castings which were to be used in place of the dummy trap door plug, and were boxed together with one spare casting of each type. At midnight of July 12, 1945, the preassembled bomb started on its way to Trinity. G-2 escort cars preceded and followed the bomb. Before noon on July 13, 1945, the charge arrived at the base of the tower, and assembly operations began at 13:00.

There wasn’t much sleeping being done anywhere at Trinity those last frantic days. There were about 250 men from Los Alamos at the test site doing last minute technical work and many more were in Los Alamos contributing to the theoretical and experimental studies and in the construction of equipment. And all of them were working against time.

On July 1, 1945, the final schedule was broadcast at Trinity and circulated around the camp two days later. Rehearsals would be held July 11, 12, 13 and 14. Originally scheduled to be held in the afternoon, the times were changed after the first dry run when daily afternoon thunderstorms began to interfere with the flight of the observation planes and to produce electrical interference and pick up on the lines.

Meanwhile, Norris Bradbury, group leader for bomb assembly, had issued his countdown. Beginning on July 7, 1945, in Los Alamos the high explosive components were put through a number of tests to study methods of loading and the effects of transportation and a dry run on the assembly (the so-called “Schaffer Shake Tests”). On July 10, 1945, the crew began the tedious round-the-clock preparations of the components for delivery to Trinity, using night shifts to get the job done. Thursday, July 12, 1945, assembly began at V-Site and by late that night they were ready to “seal up all holes in the case; wrap with scotch tape (time not available for strippable plastic), and start loading on truck”. 

The set of tests starting on July 7, 1945, was a “hot run”. As far as I can see, the “Schaffer Shaker Test” was an 8-hour road trip of the bomb (without the nuclear material). The charge was inspected after 3 hours, and completely disassembled and inspected after the 8 hour trip. The charge was then reassembled, and then removed again.

At 01:00 on Friday, July 13, 1945, the pre-assembled high explosive components started for Trinity in a truck convoyed by Army Intelligence cars in front and behind with George Kistiakowsky accompanying the precious cargo in the forward car.

Measuring the energy (and yield) of the blast wave

Much emphasis was placed on measuring the energy in the blast wave. This was achieved by using a pair of beryllium-copper diaphragm microphones to record the peak pressure following the explosion. The idea was that the change in pressure generated by the blast wave could be measured accurately from 20 miles away. Another, more sophisticated, another method consisted of making a precise measurement of the velocity of sound at the site and comparing it with the velocity of the blast wave. Spring loaded piston gauges, water-filled pistons, diaphragm box gauges, and ball and cylinder gauges were calibrated to record a range of peak pressures from the blast. The mechanical gauges were insensitive to electrical disturbances and acted as backup to the electrical methods.  

According to the original test plans blast measurement instruments included piezo-electric gauges, paper diaphragm gauges, and condenser blast gauges (also used on the observer plane), but something called a Barnes’ box was not used. For ground shock measurements the plan was to use geophones and seismographs. For neutron measurements gold foils were used (fast ion chambers were not used), and for gamma-ray measurements they used so-called gamma ray sentinels. The photographic package used so-called “Fastaxes”, spectrographs, photometrics, and something called “ball of fire” studies. Finally a SCR-584 radar was also used. Some reports mention that so much measurement equipment was purchased and developed, that they did not have enough manpower to calibrate and test it all. 

Plans were made to estimate the energy of the bomb. One way was to determine the number of fissions by measuring the number and intensity of the gamma rays emitted. Prompt and delayed gamma rays could be measured separately. Ionisation chambers were used to measure the prompt gamma rays. The ionisation of the delayed gamma rays was measured by “suitable devices” placed within 10 or 20 miles of the ‘gadget'. The number and energy of the gamma rays could be used to derive the number of fissions and calculate the efficiency and yield of the bomb.

The energies and distribution of neutrons from the blast provided another method for calculating yield, but they were difficult to measure since they were more likely to be degraded and absorbed. Measurements of time-integrated neutron flux could be made using gold foils in protective tubes placed between 300 and 1000 meters from ground zero. The foils would be activated by slow neutrons from the blast. Arrangement were also made to perform direct examination of soil taken from the blast area. The presence of plutonium and fission products would support estimations of the efficiency of the explosion (two lead-lined tanks were available for taking samples).   

Another essential measurement was the time interval between the detonation of the high explosives and the beginning of the chain reaction. This would indicate if the nuclear reaction was started by the initiator or began prematurely. At the time it was not known the degree if simultaneity needed to detonate and efficient implosion. 

The plutonium arrives

The two hemispheres of plutonium made the trip to Trinity from Los Alamos on July 11, 1945, accompanied by a Lt. Richardson and several soldiers in a convoyed sedan and delivered to Bainbridge at the tower. A receipt for the plutonium was requested. “I was very busy and we were fighting against time”, Bainbridge recalled recently. “I thought "What kind of foolishness is this", and directed the men to the assembly site at McDonald ranch”.  Bainbridge remembers that Richardson and his crew seemed awfully eager to get rid of their strange cargo even though they weren’t supposed to know the real significance of it.

Of the several allotropes of plutonium, the metallurgists preferred the malleable δ phase. This was stabilised at room temperature by alloying it with gallium. Two equal hemispheres of plutonium-gallium alloy were plated with silver, and designated by serial numbers HS-1 and HS-2. The 6.19-kilogram (13.6 lb) radioactive core generated 15 W of heat, which warmed it up to about 100 to 110 °F (38°C to 43 °C), and the silver plating developed blisters that had to be filed down and covered with gold foil; later cores were plated with nickel instead. The Trinity core consisted of just these two hemispheres. Later cores also included a ring with a triangular cross-section to prevent jets forming in the gap between them.

Stories about the delivery of the plutonium diverge somewhat. A different source says that the plutonium core came from Los Alamos on July 12, 1945. The two scientists who came with it were Phillip Morrison and Marshall Glecker Holloway, who were both responsible for placing the core into the “gadget”. Morrison remembers that their driver was a young woman who drove high-speed all the way. 

Marshall Glecker Holloway (1912-1991), from Sept. 1952 he was charged with designing, building and testing a thermonuclear weapon, popularly known as a hydrogen bomb. This culminated in the Ivy Mike test in Nov. 1952 of that year (this was a test of principle and not of a weapon).

Another store goes that Brig. Gen. Thomas Francis Farrell, deputy to Maj. Gen. Leslie Groves, was asked to sign a receipt for the plutonium. Farrell later said, "I recall that I asked them if I was going to sign for it shouldn't I take it and handle it. So I took this heavy ball in my hand and I felt it growing warm, I got a certain sense of its hidden power. It wasn't a cold piece of metal, but it was really a piece of metal that seemed to be working inside. Then maybe for the first time I began to believe some of the fantastic tales the scientists had told about this nuclear power".

Eventually the receipt was signed at the ranch by Brig. Gen. T. F. Farrell, Groves’ deputy, and handed to Louis Alexander Slotin who was working on nuclear assembly. The acceptance of the receipt signalled the formal transfer of the precious plutonium-239 from the Los Alamos scientists to the Army for use in a test explosion.

Nuclear tests and the assembly of the active components were completed at the ranch and shortly after noon on Friday, July 13, 1945, and final assembly of the bomb began in a canvas tent at the base of the tower. Present were N. E. Bradbury, G. B. Kistiakowsky, R. S. Warner, Henry Linschitz, Lt. W. F. Schaffer, Leo. M. Jercinovic, Al Van Vessem, and Arthur B. Machen. By about 14:00 the sphere was in its cradle with the polar cap and dummy plug removed, ready for the tamper. This was followed by the insertion of the high-explosive, and quite a lot of manipulation to place the sphere in the right place to be lifted to the tower top. The sphere was in place on the tower at 09:00 on July 14, 1945. Cabling and detonators were then added.  

On July 14, 1945, just two days before the scheduled date for the Trinity test, Edward Creutz led a dress rehearsal practice test without any nuclear materials. Despite everything having worked perfectly with the 100-ton TNT blast in May, 1945, Creutz’s test was unsuccessful, and the device failed to detonate. Fortunately for the scientists concerned by this result, Hans Bethe was able to demonstrate the next day that the test failed because of warn out practice equipment. Since the equipment for the Trinity test was comparatively unused, many scientists were comforted by Bethe’s findings.

Scientists were also frustrated by a test run on the same day by Donald Frederick Hornig (he had just turned 25 years old). After spending several months working on the X-5 firing unit that would trigger the bomb’s detonation, Hornig had conducted several tests of his device with no problems. However, his final test failed, provoking many of the same fears as Creutz’s failed experiment. The same issue plagued both tests: the practice materials simply had become worn down after several months of experimentation.

Bradbury’s detailed step-by-step instructions for the assembly process, which was interrupted at frequent intervals for “inspection by generally interested personnel”, showed the careful, gingerly fashion in which the crew approached its history-making job.

“Pick up GENTLY with hook”

“Plug hole is covered with a CLEAN cloth”

“Place hypodermic needle IN RIGHT PLACE”

“Check this carefully”

“Insert HE - to be done as slowly as the G (Gadget) engineers wish. . . . Be sure shoe horn is on hand”

“Sphere will be left overnight, cap up, in a small dish pan”

In fact the assembly was made using a special tool kit. Several of these kits existed and some were used on Tinian to prepare the bombs used on Japan.

According to one report Marshall Holloway and Philip Morrison were responsible for the explosive assembly, and Robert Bacher was an advisor. Louis Slotin, Boyce Dawkins McDaniel and Cyril Stanley Smith were responsible for the mechanical assembly at the ranch house, and Holloway was again responsible for the mechanical assembly at the tower. 

By 17:00 on July 14, 1945, the active material and the high explosive came together for the first time. Neither Bradbury nor Raemer Edgar Schreiber, a member of the pit assembly crew, remembers any particular feeling of tension or apprehension during the operation although, Bradbury said, “There is always a certain amount of concern when you are working with high explosives”.

“We were given plenty of time for the assembly of active material,” Schreiber remembers. “By then it was pretty much a routine operation. It was simply a matter of working very slowly and carefully, checking and re-checking everything as we went along”.

The assembly departed from the routine only once, when the crew made the startling discovery that the two principal parts of the ‘gadget', carefully designed and precision machined, no longer fitted together. Marshall Holloway, in charge of pit assembly, came to the rescue and in only a couple of minutes had the problem solved. The plutonium component, which had generated a considerable amount of its own heat during the trip from Los Alamos, had expanded. The other section of the assembly had remained cold. The heat exchange resulting when the hot material was left in contact with the cold for only a minute or two soon had the two pieces slipping perfectly together.

An alternative to the above description provides more (and different) details. At the tower a temporary eyebolt was screwed into the 105-pound (48 kg) capsule, and a chain hoist was used to lower the capsule into the gadget. As the capsule entered the hole in the uranium tamper, it stuck. Robert Bacher realized that the heat from the plutonium core had caused the capsule to expand, while the explosives assembly with the tamper had cooled during the night in the desert. By leaving the capsule in contact with the tamper, the temperatures equalised and in a few minutes the capsule had slipped completely into the tamper. The eyebolt was then removed from the capsule and replaced with a threaded uranium plug, a boron disk was placed on top of the capsule, an aluminum plug was screwed into the hole in the pusher, and the two remaining high explosive lenses were installed. Finally, the upper Dural polar cap was bolted into place. Assembly was completed at about 16:45 on July 13, 1945. 

So the assembly of the nuclear capsule began on July 13 at the McDonald Ranch House, where the master bedroom had been turned into a clean room. The polonium-beryllium "Urchin" initiator was assembled, and Louis Slotin placed it inside the two hemispheres of the plutonium core. Cyril Smith then placed the core in the uranium tamper plug, or ‘slug’. Air gaps were filled with 0.5-mil (0.013 mm) gold foil, and the two halves of the plug were held together with uranium washers and screws which fit smoothly into the domed ends of the plug. The completed capsule was then driven to the base of the tower.

Early the next morning the tent was removed and the assembled ‘gadget' was raised to the top of the 100-foot tower where it rested in a specially constructed sheet steel house. But it was still without detonators. “Detonators were very fragile things in those days”, Bradbury explained. "We didn’t want to haul that “gadget” around with the detonators already in it. We might have dropped it”.

One account tells us that the 100-foot steel tower was a surplus Forest Service fire-watch tower. But another report tells us that the tower was the bottom half of a 200-foot Blaw-Knox tower (commonly used by the Army Signal Corps for mounting SCR-271 radar antennas, the fixed version of the SCR-270). The tower stood on four legs that went 20 feet (6.1 m) into the ground, with concrete footings. Atop it was an oak platform, and a shack made of corrugated iron that was open on the western side. Once built, it was tested using a 10,000 pound block of concrete, and later a bomb mockup.

We have mentioned several times the towers, in fact they were used to obtain a better indication of how the weapon would behave when dropped from a bomber, as detonation in the air would maximize the amount of energy applied directly to the target (as the explosion expanded in a spherical shape) and would generate less nuclear fallout

It is said that as the ‘gadget’ was being raised to the top, it came partially unhinged and began to sway. Many observers were stricken with panic at the possibility of the bomb accidentally falling from the tower and detonating, but it was eventually righted and made its way to the top of the tower without further incident.

Another story tells us that the workers piled up mattresses beneath the “gadget” to cushion a possible fall. 

So it was up to the detonator crew, headed by Kenneth Ingvard Greisen, to climb the tower and make the final installations and inspections and to return every six hours to withdraw the manganese wire whose induced radioactivity was a measure of neutron background (I think that these wires were called 'informers'). The necessary cables were connected to a dummy unit which would permit tests to continue while the bomb was armed. Late that night the job was essentially complete.

The seven man arming party, consisting of Bainbridge, Kistiakowsky, Joseph McKibben and four soldiers including Lieutenant H. C. Bush, drove out to the tower to perform the final arming shortly after 22:00 on July 15. Sgt. W. Stewart rigged the sound measurement instruments, and Sgt. J. C. Alderson obtained weather data for Ground Zero. Bush and one guard were there to “prevent  sabotage”. 

The 'gadget' was left in the care of an armed guard and the scientists and technicians were left with only the final routine preparations and last-minute adjustments on their equipment.

All planes at the Alamogordo base were grounded until further notice and arrangements had been made with the Civil Aeronautics Authority, the Air Corps and Navy to insure that the entire area would be barred to all aircraft during the last important hours. According to General Groves, it was quite upsetting to the base, for it was there that B-29 crews received their final training before leaving for the Pacific and every unit commander wanted his crew to have as many hours in the air as possible. All they knew was that their training schedules were being upset for some unexplained reason. Many men, Groves continued, were already on the landing field when the explosion occurred and not long after several thousand men were preparing for take-offs.

Reading through the above text one can see that descriptions diverge about the actual sequence and timing of the actual construction process, but this may in part be due to the difference between a planning schedule and the reality. In addition there is some uncertainty about who was present, when, what they did, and why. 

The “big-wigs” arrive

Meanwhile, the high-ranking observers began to assemble. On Sunday afternoon General Groves, who had been touring Manhattan District installations on the West Coast in order to be nearby in case the test hour was advanced, arrived at Trinity with Vannevar Bush and James Bryant Conant, members of the MED’s policy committee. 

It has been reported that there were a total of 425 people in or around the Trinity test site on the weekend of the explosion. 

A busload of consultants from Project Y left Los Alamos for the desert and automobiles were dispatched to Santa Fe to pick up Charles Allen Thomas, MED’s coordinator for chemical research, and to Albuquerque for Ernest O. Lawrence, Sir James Chadwick and William Leonard Laurence of the New York Times, the one newsman assigned by the Manhattan District to document the development of the bomb.

William Leonard Laurence (1888-1977) worked for the New York Times, and was the official historian of the Manhattan Project. He was the only journalist to witness the Trinity test and the atomic bombing of Nagasaki.

At the test site, after months of hectic activity, things became more relaxed as the final items on Bradbury’s hot run countdown indicate: 

Sunday, 15 July, 1945, all day: look for rabbit’s feet and four-leafed clovers. Should we have the chaplain down there? Period for inspection available from 09:00-10:00.

Monday, 16 July, 1945, 04:00: BANG!

The night before (Sunday 15 July, 1945)

But it wasn’t quite as simple as that. By Sunday evening the skies had darkened, thunder rolled in the surrounding mountains and lightning cracked through the overcast sky. It began to rain. Now that the test was ready, at long last, could it actually go?

Shortly before 11 p.m. Sunday night the arming party, consisting of Bainbridge, Kistiakowsky, Joe McKibben, two Pinny weathermen, Lt. Bush and a guard, assembled at the base camp for the final trip to the tower.

McKibben, who had the very important and punishing job of supplying the timing and remote operating signals, was dead tired. “He had had a more trying time for two weeks than most of us”, Bainbridge recalled. “Any one of 50 people with special test equipment who, needed timing and activating signals over their control wires had been asking McKibben and his group for rehearsals at all hours of the day and night for two weeks with very large amounts of business the week prior to July 16”.

But tired or not, McKibben had with him a two page check list of 47 jobs to be done before Zero hour. His preliminary jobs were finished by 23:00 and he was urged to get some sleep. “I remember he looked absolutely white with fatigue”, Bainbridge said, “and we wanted him alert and ready at test time”. 

Donald Hornig came out, went to the top of the tower to switch the detonating circuit from the dummy practice circuit to the real “gadget” and then returned to S 10,000 where he would be responsible for the “stop” switch. If anything went wrong while the automatic devices were operating seconds before the detonation, he would pull the switch and prevent the explosion.

In some reports only McKibben, Bainbridge, and Kistiakowsky were present at the tower.  

Kistiakowsky climbed about 30 feet up the tower to adjust a light at the radioed request of a cameraman and then returned to the car to sleep. Periodically Lt. Bush or the guard turned their flashlights on the tower to make certain there was no one trying to interfere with the cables. Hubbard and his assistants continued with their weather measurements while Bainbridge kept in touch with John Williams on the land telephone at S 10,000.

A separate report noted that the weather forecast for the July 16, 1945, was a storm, and as predicted, wind and rain began to better he Trinity tower on the night of July 15, 1945.  

“It was raining so hard”, McKibben remembers, “I dreamed Kisti (for Kistiakowsky) was turning a hose on me”. There was lightning, too, but not dangerously close to the tower. The rain continued. Back at the control dug-out Oppenheimer and General Groves consulted through the night.

The day (14 July, 1945)

On July 14, 1945, the tent was removed and the device, completely assembled except for the detonators, was raised to the top of the 100-foot tower.

“Every five or ten minutes Oppenheimer and I would go outside and discuss the weather,” Groves writes. “I was shielding him from the excitement swirling about us so that he could consider the situation as calmly as possible”. Fortunately, Groves continues, “although there was an air of excitement at the dugout, there was a minimum of conflicting advice and opinions because everyone there had something to do, checking and re-checking the equipment under their control”.

At 1 a.m. Groves urged the director to get some sleep. Groves himself joined Bush and Conant in a nearby tent for a quick nap without much luck. “The tent was badly set up”, Groves recalls, “and the canvas slapped constantly in the high wind”. 

By 2 a.m. the weather began to look better and it was decided that the shot probably could be fired that morning, but instead of the planned hour of 4 a.m. it was postponed until 5:30. The waiting and checking continued.

The rain stopped at 4:00 a.m. At 4:45 a.m. the crucial weather report came: “Winds aloft very light, variable to 40,000 surface calm. Inversion about 17,000 ft. Humidity 12,000 to 18,000 above 807.. Conditions holding for next two hours. Sky now broken becoming scattered”. The wind directions and velocities at all levels to 30,000 feet looked good from a safety standpoint. Bainbridge and Hubbard consulted with Oppenheimer and General Farrell through Williams on the telephone. One dissenting vote could have called off the test. The decision was made. The shot would go at 05:30.

Groves famously told Hubbard that “I will hang you” if his weather predictions were incorrect.

The arming party went into action. Bainbridge, McKibben and Kistiakowsky drove with Lt. Bush to the West 900 yard point where, according to McKibben’s check list, he “opened all customer circuits”. 

Back at the tower connections were checked, switches were thrown and arming, power, firing and informer leads were connected. Bainbridge kept in touch with Williams by phone, reporting each step before it was taken. “In case anything went sour,” Bainbridge explained, “the S 10,000 group would know what had messed it up and the same mistake could be avoided in the future”.

The lights were switched on at the tower to direct the B-29’s and the arming party headed for the control point at S 10,000, driving, they all insist, at the reasonable rate of about 25 miles an hour.

Two circling B-29’s observed the test, and I think one was shielded with lead. They carried members of Project Alberta, who would carry out airborne measurements during the atomic missions. These included Captain “Deak” Parsons, the Associate Director of the Los Alamos Laboratory and the head of Project Alberta; Luis Walter Alvarez, Harold Melvin Agnew, Bernard Waldman, Wolfgang Kurt Hermann Panofsky and William George Penney. In the end the overcast weather obscured their view of the test site.

Luis Walter Alvarez (1911-1988) won the 1968 Nobel Prize in Physics for his work on elementary particle physics. 

Harold Melvin Agnew (1921-2013) was director of the Los Alamos National Laboratory from 1970 to 1979, Democratic New Mexico State Senator from 1955 to 1961, and from 1979 he was President and Chief Executive Officer of General Atomics.

Arriving at S 10,000 about 05:00 Bainbridge broadcast the weather conditions so that leaders at the observation points would have the latest information and know what to worry about in the way of fallout.

Then from Kirtland Air Force Base came word from Captain Parsons. Weather was bad at Albuquerque and the base commander did not want the planes to take off. But the decision was already made.

Later the planes did take off but because of overcast only fleeting glimpses of the ground could be seen and Parsons was barely able to keep the plane oriented. Unable to drop their gauges with any degree of accuracy the airborne group became merely observers.

Just after 05:00 Bainbridge used his special key to unlock the lock that protected the switches from tampering while the arming party was at the tower.

At 05:10 Sam Allison began the countdown. Sam seems to think that he was the first person to count backwards over a 20-minute period. 

All through the night the spectators had been gathering to await the most spectacular dawn the world had ever seen.

They waited on high ground outside the control bunker. They waited at the observation posts at West and North 10,000. They waited in arroyos and in surrounding hills. A group of guards waited in slit trenches in Mockingbird Gap between Oscuro and Little Burro Peaks.

All had been instructed to lie face down on the ground with their feet toward the blast, to close their eyes and cover them as the countdown approached zero. As soon as they became aware of the flash they could turn over and watch through the darkened glass that had been supplied.

On Campaña Hill, 20 miles northwest of Ground Zero, a large contingent of scientists waited along with Laurence of the Times. They shivered in the cold and listened to instructions read by flashlight by David Dow, in charge of that observation post. They ate a picnic breakfast. Edward Teller warned about sunburn and he passed around some sunburn lotion in the pitch darkness. Fred Reines, a former Los Alamos physicist, waited with Greisen and I. I. Rabi, a project consultant, and heard the “Voice of America” burst forth on the short wave radio with “Star Spangled Banner” as if anticipating a momentous  event. Alvin Cushman Graves and Elizabeth Riddle Graves, a husband and wife scientific team, waited in a dingy Carrizozo motel with their recording instruments. Others, mostly military men, waited at spots as far away as 200 miles, their instruments ready to record the phenomena.

Four off-site monitoring posts were established in the towns of Nogal, Roswell, Socorro, and Fort Sumner

In San Antonio, restaurant owner Jose Micra was awakened by the soldiers stationed at his place with seismographs. “If’ you come out in front of your store now, you’ll see something the world has never seen before”, they told him. Just south of San Antonio, a group of hardy Los Alamos souls, who had climbed into the saddle of Chupadero Peak the day before, waited drowsily in their sleeping bags.

In Los Alamos, most people slept but some knew and went out to watch from the porches of their Sundt apartments. Others drove into the mountains for a better view. Mr. and Mrs. Darol Kenneth Froman and a group of friends waited in their car, gave up and were heading back down the mountain when 05:30 came. A group of wives, whose husbands had been off in the desert for endless weeks, waited in the chill air of Sawyer’s Hill. Months later one of them described the agonising hours. “Four o’clock. Nothing was happening. Perhaps something was amiss down there in the desert where one’s husband stood with other men to midwife the birth of the monster. Four fifteen and nothing yet. Maybe it had failed. At least, then, the husbands were safe. . . . Four thirty. The grey dawn rising in the East, and still no sign that the labor and struggle of the past three years had meant anything at all. . . . It hardly seemed worthwhile to stand there, scanning the sky, cold and so afraid”.

Elsewhere the world slept or fought its war and President Truman waited at Potsdam. Back at Trinity, over the intercoms, the FM radios, the public address system, Sam King Allison’s voice went on, counting first at five-minute intervals then in interminable seconds. “Aren’t you nervous?” Rabi asked Greisen as they lay face down on the ground. “Nope,” replied Greisen. “As we approached the final minute”, Groves wrote, “the quiet grew more intense. I was on the ground (at Base Camp) between Bush and Conant. As I lay there in the final seconds, I thought only of what I would do if the countdown got to zero and nothing happened”. Conant said he never knew seconds could be so long. At the control point, General Farrell wrote later, “The scene inside the shelter was dramatic beyond words. . . . It can be safely said that most everyone present was praying. Oppenheimer grew tenser as the seconds ticked off. He scarcely breathed. He held on to a post to steady himself”. 

The countdown went on. At minus 45 seconds Joe McKibben threw the switch that started the precise automatic timer. Now it was out of man’s control, except for Hornig who watched at his post at the stop switch.

It has often been written that Joe McKibben “pushed the button”, but what he actually did was start an automatic timer. He said "That kind of annoys me, I consider it a minor part of my work". 

Minus 30 seconds, and Williams and Bainbridge joined the others outside the control dugout.

Minus 10 seconds. Cool-headed Greisen changed his mind, “Now I’m scared,” he suddenly blurted to Rabi.

There are reports that an 18-year-old soldier named Val Logsdon Fitch was attending British scientist Ernest Titterton at a set of vacuum tubes that would deliver the detonating voltage across 6 miles of cable. Fitch would later go on to win the 1980 Nobel Prize in Physics for the decay of neutral K-mesons (Kaon).

Then, as the world teetered on the brink of a new age, Sam Allison’s voice cried, “Now!”

The explosion

The Trinity fireball, 0.053 seconds after detonation, as it shook the desert near the town of San Antonio, New Mexico, on July 16, 1945 

At that instant - 05:29:45 Mountain War Time on July 16, 1945 - came an incredible burst of light, bathing the surrounding mountains in an unearthly brilliance. Then came the shock wave that knocked over two men at S 10,000, then the thunderous roar.

The detonation time was actually set by Mountain Daylight Time, also known as Mountain War Time. All the reports love to mention the exact time of the explosion, however the time is not known with certainty, because scientists experienced difficulty in picking up station WWV for the time check (this is the time broadcasts from NIST). 

The explosion annihilated nearly all of the 100-foot metal tower from which the bomb was held and created a crater of a radioactive green (jade-like) glassy substance known as trinitite, which is today prized as a collector's item (it was originally named atomsite, but trinitite sounded better). The crater was almost 2,400 feet across and 10 feet deep in places, and only the metal and concrete stumps of the tower remained. 

Wrote Enrico Fermi shortly after the test: “My first impression of the explosion was the very intense flash of light, and a sensation of heat on the parts of my body that were exposed. Although I did not look directly towards the object, I had the impression that suddenly the countryside became brighter than in full daylight. I subsequently looked in the direction of the explosion through the dark glass and could see something that looked like a conglomeration of flames that promptly started rising. After a few seconds the rising flames lost their brightness and appeared as a huge pillar of smoke with an expanded head like a gigantic mushroom that rose rapidly beyond the clouds, probably to a height of the order of 30,000 feet. After reaching full height, the smoke stayed stationary for a while before the wind started dispersing it”.

Fermi then went on to explain the simple experiment he took time to conduct that helped considerably in making the first early estimates of the bomb’s success. “About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during and after the passage of the blast wave. Since, at the time, there was no wind, I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about two and a half meters, which at the time, I estimated to correspond to the blast that would be produced by 10,000 tons of TNT”.

The early estimates were between 15 and 20 kilotons, slightly more than the “Little Boy” bomb dropped on Hiroshima. A later report put it at 18.6 kilotons, and a fact sheet published even later stated that the bomb released energy was equivalent to 21 kilotons of TNT. Based upon the release of the calibrated photo sequence of the bomb explosion, the estimate is now about 25 kilotons. The official estimate for the total yield of the Trinity ‘gadget', which includes the energy of the blast component together with the contributions from the explosion's light output and both forms of ionising radiation, is 21 kilotons of TNT (88 TJ), of which about 15 kilotons of TNT (63 TJ) was contributed by fission of the plutonium core, and about 6 kilotons of TNT (25 TJ) was from fission of the natural uranium tamper. However, a re-analysis of data published in 2016 put the yield at 22.1 kilotons of TNT (92 TJ), with a margin of error estimated at 2.7 kilotons of TNT (11 TJ).

At the time the yield of Trinity was measured by observation of the velocity of expansion of the fireball as photographed by super-high-speed movie cameras (Fastax), by radiochemical analysis of the debris, and by observation of blast pressure versus time and distance. The fireball yield technique was confirmed by radiochemical data. The generation-time data were successfully recorded on the only calibrated oscilloscope fast enough to make the measurement. Observations of debris deposition patterns led to the first fallout model. Dozens of other experiments, such as blast pressures versus distance, neutron fluences (a measure of particle flux) in several energy ranges, gamma-ray emissions, and thermal radiation effects, also gave useful data.

Hans Bethe wrote that “it looked like a giant magnesium flare which kept on for what seemed a whole minute but was actually one or two seconds. The white ball grew and after a few seconds became clouded with dust whipped up by the explosion from the ground and rose and left behind a black trail of dust particles. The rise, though it seemed slow, took place at a velocity of 120 meters per second. After more than half a minute the flame died down and the ball, which had been a brilliant white became a dull purple. It continued to rise and spread at the same time, and finally broke through and rose above the clouds which were 15,000 feet above the ground. It could be distinguished from the clouds by its colour and could be followed to a height of 40,000 feet above the ground”.

Joe McKibben recalls that “we had a lot of flood lights on for taking movies of the control panel. When the bomb went off, the lights were drowned out by the big light coming in through the open door in the back”. “After I threw my last switch I ran out to take a look and realised the shock wave hadn’t arrived yet. I ducked behind an earth mound. Even then I had the impression that this thing had gone really big. It was just terrific”.

McKibben later said that in the silence, he stepped out the back door of S 10,000 and looked north over the bunker. "It was quite a pretty sight. Colored. Purplish. No doubt from the iron in the tower and a lot of soil off the ground that had been vaporised. I was surprised at the enormity of it and immediately felt it had gone big". Later he said "Then an amazing thing: It was followed by echoes from the mountains. There was one echo after another. A real symphony of echoes".

“The shot was truly awe-inspiring”, Bradbury has said. “Most experiences in life can be comprehended by prior experiences but the atom bomb did not fit into any preconception possessed by anybody. The most startling feature was the intense light”.

Bainbridge has said that the light was the one place where theoretical calculations had been off by a big factor. “Much more light was produced than had been anticipated”.

A military man is reported to have exclaimed, “The long-hairs have let it get away from them!” While scientists were able to describe the technical aspects of the explosion, for others it was more difficult. “Words are inadequate tools for acquainting those not present with the physical, mental and psychological effects. It had to be witnessed to be realised”, wrote General Farrell two days later. Nonetheless, many tried to describe the historic moment. Farrell himself wrote: “The effects could well be called unprecedented, magnificent, beautiful, stupendous, and terrifying. No man-made phenomenon of such tremendous power had ever occurred before. The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined. Seconds after the explosion came”, first, the air blast pressing hard against the people, to be followed almost immediately by the strong, sustained awesome roar which warned of doomsday and made us feel we puny things were blasphemous to dare tamper with the forces heretofore reserved for the Almighty”.

Robert Van Gemert comments were more down-to-earth, "I'm just amazed how those scientists whipped out so many bottles of gin or whatever they could find. And it was rapidly consumed, I can tell you that".

William L. Laurence, whose sole job was to write down the moment for history, wrote: “It was like the grand finale of a mighty symphony of the elements, fascinating and terrifying, uplifting and crushing, ominous, devastating, full of great promise and great foreboding”. Another time he said, “On that moment hung eternity. Time stood still. Space contracted to a pinpoint. It was as though the earth had opened and the skies split. One felt as though he had been privileged to witness the Birth of the World-to-be present at the moment of Creation when the Lord said: Let there be light”.

Oppenheimer, on the other hand, has said he was reminded of the ancient Hindu quotation:

“I am become Death, the destroyer of worlds” (or is it “shatterer of worlds”). 

The store is a bit more complex in that Oppenheimer later recalled that, while witnessing the explosion, he thought of a verse from the Hindu holy book, the Bhagavad Gita (XI,12): If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendour of the mighty one ... Years later he would explain that another verse had also entered his head at that time: “We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says, 'Now I am become Death, the destroyer of worlds.' I suppose we all thought that, one way or another”. 

At the time, however, probably few actually thought of the consequences of their work, beyond ending the war. Bradbury said recently, “For that first 15 seconds the sight was so incredible that the spectators could only gape at it in dumb amazement. I don’t believe at that moment anyone said to himself, ‘What have we done to civilisation? Feelings of conscience may have come later”.

Bainbridge reports that his reactions were mixed. “When the bomb first went off I had the same feelings that anyone else would have who had worked for months to prepare this test, a feeling of exhilaration that the thing had actually worked. This was followed by another quick reaction, a sort of feeling of relief that I would not have to go to the bomb and find out why the thing didn’t work”. But later he told Oppenheimer, “Now we’re all sons of bitches”.

Ernest O. Lawrence is quoted as saying that from his vantage point on Campaña Hill, “the grand, indeed almost cataclysmic proportion of the explosion produced a kind of solemnity in everyone’s behaviour immediately afterwards. There was a restrained applause, but more a hushed murmuring bordering on reverence as the event was commented upon”.

Robert Serber, was also on Campaña Hill, and he looked directly at the explosion with no eye protection. He saw a yellow glow, which grew almost instantly into an overwhelming white flash, so intense that he was completely blinded. There was a definite sensation of heat. The brilliant illumination seemed to last for about three to five seconds, changing to yellow and then to red; at this stage it appeared to have a radius of about twenty degrees. The first thing he succeeded in seeing after being blinded by the flash looked like a dark violet column several thousand feet high. This column must actually have been quite bright, or he would not have been able to distinguish it. By twenty or thirty seconds after the explosion he was regaining normal vision. At a height of perhaps twenty thousand feet, two or three thin horizontal layers of shimmering white cloud were formed, perhaps due to condensation in the negative phase of the shock wave. Some time later, the noise of the explosion reached him. It had the quality of distant thunder, but was louder. The sound, due to reflections from nearby hills, returned and repeated and reverberated for several seconds, very much like thunder. A column of white smoke appeared over the point of the explosion, rising very rapidly, and spreading slightly as it rose. In a few seconds it reached cloud level, and the clouds in the immediate neighbourhood seemed to evaporate and disappear. The column continued to rise and spread to a height of about twice the cloud level. There was no appearance of mushrooming at any height. A smoke cloud also was spreading near ground level.

Edwin M. McMillian, also on Campaña Hill, noted that descriptions were often quite different. After an exceedingly bright light which expanded very rapidly, there was sensation of heat on his face and hands, which lasted about a second. About two seconds later the sky and surrounding landscape were brightly illuminated, but not as strongly as in full sunlight. The "ball of fire" was still too bright for direct observation, but it rose and expanded, before slowly fading out. At some time during this stage, the layers of clouds visible above the explosion evaporated, forming a hole which rapidly got bigger.

At about thirty seconds, the general appearance was similar to a goblet; the ball was about a mile in diameter and about four miles above the ground, glowing with a dull red; a dark stem connected it with the ground, and spread out in a thin dust layer that extended to a radius of about six miles. When the red glow faded out a most remarkable effect made its appearance. The whole surface of the ball was covered with a purple luminescence, like that produced by the electrical excitation of air, and caused undoubtedly by the radioactivity of the material in the ball. This was visible for about five seconds; by this time the sunlight was becoming bright enough to obscure luminous effects. 

At about two minutes, the blast came. It was remarkably sharp, being more of a "crack" than a "boom". He did not feel any earth shock.

The later stages of motion of the cloud consisted of a slow drifting in the wind, showing the existence of several different wind directions at different altitudes. A current at a few hundred feet carried the lower part of the "stem" toward the North 10,000 station. The cloud was a different color than the ordinary clouds through which it passed, having a brownish tinge; this could have be caused by nitrogen dioxide formed from air by the intense ionisation.

At the control point, Farrell wrote, “The tension in the room let up and all started congratulating each other. All the pent-up emotions were released in those few minutes and all seemed to sense immediately that the explosion had far exceeded the most optimistic expectations and wildest hopes of the scientists”. 

Greisen also observed that “between the appearance of light and the arrival of the sound, there was loud cheering in the group around us. After the noise was over, we all went about congratulating each other and shaking hands. I believe we were all much more shaken up by the shot mentally than physically”.

Kistiakowsky, who had bet a month’s salary against $10 that the “gadget” would work, put his arms around the director’s shoulder and said, “Oppie, you owe me $10”.

One of the official reports lists all the measurements that worked and did not work during the 100-ton TNT test. There were no measurements of the detonator simultaneity, or the time interval between detonation and “nuclear action”. Measurements of delayed neutrons and gamma-rays worked. Nine of the eleven quartz blast gauges worked, the condenser gauges failed, but the 29 box gauges (holes covered with aluminum) all functioned. Only one of the five instruments measuring blast impulse using a piston action on a fluid worked. One of the three blast pressure measurements dropped from the observation plane worked. The geophones to measure Earth movement all worked.

All five Fastax cameras worked, as did the two Mitchell cameras. Two of the three Fairchild Aero cameras failed (because people did not push the right buttons). The photograph system for the primacord’s (to measure Mach wave and air velocity) suspended from balloons failed (failure to launch and poor quality balloon burst). The radar and radiosondes worked to measure temperature, humidity and wind velocities.

The same report listed the measurements that worked during the actual bomb test. Foil gauges estimated the explosion as 9.9 kiloton. Mechanical impulse gauges worked, as did the condenser gauge, and the microbarographs. Piezo-electric gauges provided no recordings, but piston gauges and foil gauges worked. Geophones and seismographs all worked.     

Estimate of the energy released in the first atomic bomb explosion

Geoffrey Iingram Taylor was approached in 1941 by the UK Ministry of Home Security and told that “it might be possible to produce a bomb in which a very large amount of energy would be released by nuclear fission”. His task was to report on the likely effect of such a weapon. Central to Taylor’s analysis used an approximation of the initial detonating device as a mathematical point, and the assumption of the spherical symmetry of the fireball. It must be said that both John von Neumann in the U.S. and Leonid Ivanovitch Sedov in the Soviet Union were also interested in the same problem (and they both developed exact solutions). The initial conclusion was that an atomic bomb would be less efficient that a high explosive releasing the same amount of energy. This is because more energy is dissipated as heat, whereas high explosives do not release the same amount of thermal energy. 

In 1947 Taylor added a second part to his approximation. The Atomic Energy Commission had declassified a sequence of 25 pictures of the Trinity test, with exact time (0.1 to 62 milli-seconds) and a length-scale. Measuring the expansion of the fireball, Taylor was able to independently estimate the yield as 17.5 kilotons. A figure that was still considered a military secret. 

Neutron, gamma-ray and thermal radiation

Some of the data below includes results obtained during the “Able” test at Bikini (1946).


For fast (prompt) neutrons the cross section for the sulphur (n,p) reaction is almost a step function, rising from 0 to 0.2 barns at 3 MeV. This reaction presents an excellent method for measuring the radial distribution of fast neutrons above 3 MeV in energy. Delayed neutrons all have energies about 0.6 MeV and thus do not interfere. One measurement was made at Trinity and many were made at Bikini. The results show that the average neutron must be scattered many times (six or more) before being degraded below 3 MeV. The absolute number of fast neutrons at Trinity was 6.5 x 1021 neutrons through the sphere at 200 m. It was impossible to make a theoretical estimate of the number of fast neutrons per fission which should have penetrate the bomb, so an estimate of the efficiency could not have been made from these data.

Slow neutrons came from two sources. Firstly, prompt neutrons are first slowed down in the high explosive to about 300 eV and then discharged into the air about 30-50 μs after the nuclear explosion. These have an average penetration of about 280 m in air before being absorbed as epithermals by nitrogen. Secondly, delayed neutrons are emitted from fission products. The half-lives of these neutron emitters is on the order of 1s or so. These neutrons have an energy of about 0.6 MeV and thus penetrate up to about 450 m in air. The total flux of slow neutrons as a function of distance was measured at Trinity using activation of cadmium-covered gold foils. Only 20% of the total number of slow neutrons are from the prompt neutrons, and their contribution is negligible after 0.5 s. There was a measured increase in intensity at 0.6 s is due to the arrival of the shock wave at 600 m. The deviation of the observed curve from the “expected” in the region 0.6 s to 3.5 s must be associated with air motion, but a detailed explanation was not possible. Due to the presence of the ground and the tenuous ball of fire, it is difficult to make absolute calculations of neutron intensities. 

The total gamma radiation at a number of positions was measured using x-ray film and paper, but the results were difficult to interpret. Pulling together data from “Able” indicated that the mean free path for gamma-rays at large distances was about 340 m, meaning that the gamma-rays had an energy near 5 MeV. It was also found that this indicated that about 40% of these gamma-rays were early fission gamma’s and the rest were from fission products. 

The theoretical expectation concerning the temperature of the radiating surface should be several hundred thousands of degrees for the first few microseconds, dropping to a minimum of about 4500°C at about 15 ms, then increasing for less than a second to 10,000°C and then cooling off more slowly. The minimum was corroborated, but the initial high temperatures were impossible to measure. 

Optical observations

About 100,000 photographic exposures were made, mostly motion frames. The expansion of the ball of fire before striking the ground was almost symmetric. 

At 3 ms there appeared at the bottom of the fire ball an irregular line of demarcation, below which the surface was appreciably brighter than above. This line rose like the top of a curtain until it disappeared at the top of the ball at about 11 ms. Shortly after the spikes struck the ground (about 2 ms) there appeared on the ground ahead of the shock wave a wide skirt of lumpy matter and within and above the skirt a smooth belt (interpreted as the Mach wave). Two successive visible fronts dropped behind the well-defined shock wave. The brighter, but less sharply limited ball of fire, fell behind it at about 16 ms. At about 32 ms there appeared immediately behind the shock wave a dark front of absorbing matter, which travelled slowly out until it became invisible at 0.85 s (or 375 m). The shock wave itself became invisible at about 0.10 s (2,400 m). 

The ball of fire grew even more slowly to a radius of about 300 m, until the dust cloud growing out of the skirt almost enveloped it. The top of the ball started to rise again at 2 s. At 3.5 s a minimum horizontal diameter, or neck, appeared one-third of the way up the skirt. The portion of the skirt above the neck formed a vortex ring. The neck narrowed, and the ring and fast-growing pile of matter above it rose as a new cloud of smoke, carrying a convection stem of dust behind it. A boundary within the cloud, between the ring and the upper part, persisted for at least 22 s. The stem appeared twisted like a left-handed screw. The cloud of smoke, surrounded by a faint purple haze, rose with its top traveling at 57 m/s, at least until the top reached 1.5 km. The later history of the cloud was not quantitatively recorded.

The velocity of the shock wave unexpectedly remained nearly constant at twice sound velocity during the expansion in radius from 250 to 400 m, decreasing by only 15% in this interval instead of dropping nearly to the ordinary velocity of sound. The predominant cause of the observed maintenance of velocity appeared to be radiant heating of the shock front by energy absorbed by the dark front as ultraviolet or visible radiation and transformed there to lower frequencies. 

Elsewhere the momentous event had not gone unnoticed 

The flash of light was seen in Albuquerque, Santa Fe, Silver City, Gallup and El Paso. Windows rattled in Silver City and Gallup. So intense was the light that a blind girl riding in an automobile near Albuquerque asked, “What was that?”

Other reports tell us that the flash of light was visible 160 miles away, that the shock wave was felt in Albuquerque, and that windows were shattered 120 miles away in Silver City.   

A rancher between Alamogordo and the test site was awakened suddenly. “I thought a plane had crashed in the yard. It was like somebody turned on a light bulb right in my face”. Another man, 30 miles away in Carrizozo, recalls, “It sure rocked the ground. You’d have thought it went off right in your back yard”. A sleepless patient in the Los Alamos hospital reported seeing a strange light. The wife, waiting on Sawyer’s Hill behind Los Alamos, saw it too, and wrote later: "Then it came. The blinding light like no other light one had ever seen. The trees, illuminated, leaping out. The mountains flashing into life. Later, the long slow rumble. Something had happened, all right, for good or ill”.

At North 10,000, Berlyn Brixner was in the open on top of the bunker at the controls of a fast movie camera with a blackened viewfinder. "I was one of the few people given permission to look directly at the bomb at zero time", says Brixner. His assignment as chief photographer was to shoot movies in 16-millimeter black-and-white, from every angle and distance and at every speed, of an unknown event beginning with the brightest flash ever produced on Earth". “The theoretical people had calculated a ... 10-sun brightness. So that was easy", Brixner said. "All I had to do was go out and point my camera at the sun and take some pictures. Ten times that was easy to calculate". 

The theoretical people also knew a little about radiation, which fogs film, and Brixner consequently shielded two of his near-tower cameras behind 12-inch-thick leaded glass. Some of his cameras were so fast they shot 100 feet of film in a second. Some were 20 miles away and ran for 10 minutes. All the cameras were turned on by signals from McKibben's control panel.

Brixner, at North 10,000, was stunned. "The whole filter seemed to light up as bright as the sun. I was temporarily blinded. I looked to the side. The Oscura Mountains were as bright as day. I saw this tremendous ball of fire, and it was rising. I was just spellbound! I followed it as it rose. Then it dawned on me. I'm the photographer! I've gotta get that ball of fire". He jerked the camera up.

One thing more, he says: "There was no sound! It all took place in absolute silence".

By the time the blast hit, 30 seconds after the flash, most of Brixner's 55 cameras in the desert were finished. Some had done their work in a second. There would be 100,000 frames to develop in black and white and a few in temperamental Kodachrome.

At North 10,000, Brixner and the others saw a kind of hazard the world had never known. "I was looking up, and I noticed there was a red haze up there, and it seemed to be coming down on us," he says. "Pretty soon the radiation monitors said, ‘The radiation is rising! We've got to evacuate!’ I said, “That's fine, but not until I get all the film from my cameras”. In the midst of the world's first fallout, somebody helped Brixner throw his last three cameras in an Army car, and they all got out of there fast. Film badges later showed they got low doses, at least by the standards of the day.

There was an interest in obtaining a good photographic record for spectrographic and yield analysis. Fastax cameras taking 10,000 frames a second were installed 800 yards from the blast, and were protected by steel and glass housings. All this was placed on a sled with a chain, so one of the lead-lined tanks could pull it away.   

Among the 250 lab workers and 125 soldiers on the Trinity site was a young civilian technician named Jack Aeby who was exempt from the draft because he'd suffered from tuberculosis. His job in the weeks leading to the test was to help the Italian physicist Emilio Segrè set radiation detectors near the tower. Some of the instruments were hung on barrage balloons tethered 800 yards from the tower. They were vaporized a millisecond after they transmitted their data. Aeby carried his personal 35 millimeter still camera, which Segré had got through security, and as the countdown started, he planning to take a new Anscochrome color transparency picture of the bomb. Aeby had carried a chair out into the darkness and was sitting there with his Perfex 44 camera on "bulb" and in the dark before "Zero" opened up the shutter, figuring that way he'd get a good image of the flash. He managed to get three pictures at different shutter speeds.

One of the reports list all the different tasks involved in completing the test, along with the actual names of the responsible staff from Los Alamos. This was from K. T. Bainbridge, through the different teams from pit assembly (G-1), detonator (X-5), asimultaneity (X-7), arming party, balloon flying, air blast and Earth shock measurements, delayed neutron and gamma-ray measurements, tank sampling, fission product chemistry, photography, medical group, etc. The total nominated list of Los Alamos staff totalled 125 men.    

In Los Alamos 230 miles to the north, a group of scientists' wives who had stayed up all night for the not so secret test, saw the light and heard the distant sound. One wife, Jane Wilson, described it this way, "Then it came. The blinding light [no] one had ever seen. The trees, illuminated, leaping out. The mountains flashing into life. Later, the long slow rumble. Something had happened, all right, for good or ill".

At the test site

At the test site, as the spectators watched the huge cloud billow into the sky, the medical officers took over leadership of the three observation points, watching their counters and maintaining contact with Paul Aebersold’s crew of monitors patrolling the roads within the test site. An Entry Permission Group, consisting of Bainbridge, Dr. Hemplemann and Aebersold kept track of the reports and made decisions on movement of personnel around the site.

The blast is said to have created a flash of light brighter than a dozen suns, and visible over the entire state of New Mexico, and parts of Arizona, Texas, and Mexico. The resultant mushroom cloud rose to over 38,000 feet within minutes, and the heat of the explosion was 10,000 times hotter than the surface of the sun! At ten miles away, this heat was described as like standing directly in front of a roaring fireplace. Every living thing within a mile of the tower was obliterated. The power of the bomb was estimated to be equal to 20,000 tons of TNT, or equivalent to the bomb load of 2,000 B-29 Superfortresses! 

At first there was no sign of danger. Then suddenly, the instruments at N 10,000 began clicking rapidly showing that radioactivity was on the rise. Dr. Henry Barnett, in charge of the shelter, gave the order to evacuate and soon the trucks and cars were roaring past W 10,000 and on to Base Camp. It later proved to be a false alarm. Film badges worn by the personnel at the observation point indicated that no radioactivity had reached the shelter (exposed to more than 0.1 roentgens). Before long those without further duties were permitted to return to Base Camp and those with instruments in the forward areas moved in to pick them up. 

As the sun came up, air currents were created which swept radioactivity trapped in the inversion layer into the valley. Geiger counters at S 10,000 began to go wild. The few men remaining there put on masks and watched anxiously as the radioactive air quickly moved away before the danger level was reached. Around 9:30 a.m. Bainbridge radioed the men in the slit trench at Mockingbird Gap to return to Base Camp.

This sudden rise in radiation levels at N 10,000 was because the instrumentation had not been accurately calibrated. 

More generally a 5 rem exposure limit for the 2-month period had been established. Protective clothing, monitoring instruments, film badges, and decontamination equipment were available. Dosimetry information on about 800 people for the period July 16, 1945 to Dec. 31, 1946, showed only 20 people received a dose in excess of the 5 rem limit (most of these were Army personnel at the test site on the day of the explosion). 

Shortly afterward a lead-lined tank, driven by Sgt. Bill Smith and carrying Herbert Anderson and Enrico Fermi, moved in to Ground Zero to recover equipment and to study debris in hope of getting information on long range detection of atomic explosions. The tank was equipped with a trap door through which earth samples could be safely picked up in the crater. Fermi later reported to his wife that he found “a depressed area 400 yards in radius glazed with green, glass-like substance where the sand had melted and solidified again”.  

The tanks (there were two of them) were white painted Sherman M-4’s equipped with their own air supply, and lined with 2-inches of lead, increasing their weight by 12 tons. The crater was far more radioactive than expected due to the formation of trinitite, and the crews of the two tanks were subjected to considerable exposure. Anderson's dosimeter and film badge recorded 7 to 10 Röntgens, and one of the tank drivers, who made three trips, recorded 13 to 15 Röntgens. One reports makes reference to samples being taken in the tank using a “rocket-fired ground scoops”. 

Meanwhile, General Groves, who had planned to wait at Base Camp until all danger of fallout was passed, hoped to make good use of the hours by discussing with Los Alamos people a number of problems connected with the next job on the agenda, the bombing of Japan. “These plans were utterly impracticable”, he wrote later, “for no one who had witnessed the test was in a frame of mind to discuss anything. The success was simply too great. It was not only that we had achieved success with the bomb; but that everyone - scientists, military officers and engineers - realised we had been personal participants and eyewitnesses to a major milestone in the world’s history”. But Groves had other problems to keep him busy anyway. 

The explosion had generated considerable excitement around the state and as far away as El Paso. At Associated Press in Albuquerque, the queries coming in were becoming more difficult to handle. Intelligence Officer Lt. Phillip Belcher, and the Laboratory’s Documents Division leader, was stationed at Albuquerque to keep any alarming dispatches about the explosion from going out. About 11 a.m. the AP man told Belcher he could no longer hold back the story. If nothing is put out now by the Army, he told him, AP’s own stories would have to go on the wire. The Army was prepared for this kind of determination. Weeks before a special press release had been prepared and sent to the Alamogordo Bombing Range with Lt. W. A. Parish from Groves’ office. With it, Parish also carried a letter to the commanding officer, Col. William Olmstead Eareckson, instructing him to follow Lt. Parish’s instructions, no questions asked. About 11 a.m. Parish was instructed to make his release: Alamogordo, July 16, 1945, - The Commanding Officer of the Alamogordo Army Air Base made the following statement today: “Several inquiries have been received concerning a heavy explosion which occurred on the Alamogordo Air Base reservation this morning. A remotely located ammunition magazine containing a considerable amount of high explosives and pyrotechnics exploded. There was no loss of life or injury to anyone, and the property damage outside of the explosives magazine itself was negligible. Weather conditions affecting the content of gas shells exploded by the blast may make it desirable for the Army to evacuate temporarily a few civilians from their homes”. The news ran in New Mexico papers and spread up and down the West Coast by radio.

As far as I can tell this press release was just one of many pre-prepared just in case. For example, the Army Public Relations Department had also prepared somber explanations in the event that disaster occurred and lives were lost.

It didn’t fool everyone. Some days later, Groves reports, he was dismayed when a scientist from the Hanford project said to him: “By the way, General, everybody at Du Pont sends their congratulations”. “What for?’ the general asked innocently. “This is the first time we’ve ever heard of the Army’s storing high explosives, pyrotechnics and chemicals in one magazine,” he replied.

At community radio station KRS in Los Alamos, Bob Porton, a GI, was about to rebroadcast the noon news, courtesy of KOB. "Suddenly, about 30 or 40 scientists all came in and stood around", he says. "We knew something was up". The lead story, Porton says, was this: "The commanding officer of Alamogordo Air Base announced this morning a huge ammunition dump had blown up, but there were no injuries". "All these scientists jumped up and down and slapped each other on the back" Porton says. "I was familiar with secrecy. I never asked any questions. But I knew it was something big". It was something big. What they'd heard was the coverup story for the first atomic bomb blast.

Colonel Eareckson has since been nominated by sympathetic historians as one of the unsung heroes of World War II. Not only was he forced to take the blame for this gross mishandling of explosives, but he had to take his orders that day from a mere lieutenant.

By late afternoon it was clear there would be no difficulty with fallout. Bainbridge finally left the control centre about 3 p.m. to return to the Base Camp for food and rest. General Groves, Conant and Bush left for Albuquerque to begin the trip back to Washington. Groves’ secretary, Mrs. Nora O’Leary, who had been standing by in Washington since early morning for word of the test, received a coded message from her boss and at 7:30 p.m. and sent the following message to Secretary of War Henry Lewis Stimson at Potsdam: “Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations. Local press release necessary as interest extends a great distance. Dr. Groves pleased. He returns tomorrow. I will keep you posted”.

A different report mentions that Groves sent the message “satisfactory plus and perhaps far greater than estimated”. Another report suggests that the Mrs. O’Leary may have passed the message to Stimson’s assistant George L. Harrison. The message arrived at the "Little White House" in the Potsdam suburb of Babelsberg and was at once taken to President Truman and Secretary of State James F. Byrnes.

Although it would be weeks before the measurement could be correlated and interpreted it was immediately apparent that the implosion weapon was a technical success. The fire bail, Fermi’s calculations with bits of paper and other data available immediately indicated the yield had been greater than the most optimistic predictions. It was therefore possible for Groves (through Harrison) to follow up his first message to Potsdam with another optimistic one the next day: “Doctor has just returned most enthusiastic and confident that the little boy is as husky as his big brother. The light in his eyes discernible from here to High Hold and I could hear his screams from here to my farm”. The message was clear. The power to crush Japan had taken on a new dimension. The device had worked beyond expectations, its flash seen for 250 miles, its thunder heard for 50 miles, and Groves was sure the plutonium bomb was as potent as the uranium gun.

Because Stimson's summer home at High Hold was on Long Island and Harrison's farm near Upperville, Virginia, this indicated that the explosion could be seen 200 miles (320 km) away and heard 50 miles (80 km) away.

The reality was that the blast measurement devices worked well, but the gamma ray measurement devices had been overloaded. Much of the film was slightly fogged or ruined, and the neutron detectors did not survive. Only about 1% of the fission products remained in the crater and its vicinity, the rest being dispersed as dust. In fact shortly before the field test, updated calculations indicated that fallout would be more substantial and widespread. 

Through the day of JuIy 16, 1945, cars of weary, excited men headed back toward Los Alamos. There was still a great deal of work to be done and for those who were going overseas, the test had simply been a rehearsal. A new Fat Man was scheduled to be delivered August 6, 1945. When they stopped for meals in Belen the men talked of inconsequential things and listened to mystified citizens discussing the strange sort of thunder they had heard that morning and the way “the sun came up and went right back down again”. Occupants of one car did not recognise the occupants of the other. Security was as tight as ever. It was not until they reached the guarded gates of Los Alamos that the flood of talk burst loose. Mrs. Fermi recalls the men returning late that evening. “They looked dried out, shrunken. They had baked in the roasting heat of the southern desert and they were dead tired. Enrico was so sleepy he went right to bed without a word. On the following morning all he had to say to the family was that for the first time in his life on coming back from Trinity he had felt it was not safe for him to drive. I heard no more about Trinity”. 

But during the day, rumours of the brilliant light so many had watched for and seen spread through the town. Although few people knew officially what had happened, most were able to sense or guess that the project to which each had contributed his part had been accomplished.

When he returned that night, Fred Reines found the town jumping. One of the janitors Reines knew spotted the returning scientist, grinned proudly and said, “We did it, didn’t we?”  "We sure did”, Reines told him.

There were at least two reports (one from 20 miles away) of a white substance like flour settling on everything, four or five days after the explosion. In fact many of the monitors reported that fallout had reached a number of areas beyond their jurisdiction. I am not sure if this refers to two houses that were rapidly evacuated, but in any case there are records showing that over the following 2 years, 7 visits were made to check on the visible conditions of the residents. Hempelmann is quoted as saying “a few people were probably overexposed, but they couldn’t prove it and we couldn’t prove it. So we just assumed we got away with it”.

There is a nice story of William and Helen Wrye, who had been in Amarillo and who returned to their ranch on the night the test. Their house was only 20 miles from Trinity, but they were tired and slept through the whole explosion. The next morning they were eating breakfast, when they saw soldiers with a little black box near their stock tank. William asked them what they were doing, and they said they were looking for radioactivity. William told them he did not have the radio on! The Wrye’s were one of the ranchers who saw deposits of a white substance like flour, which would glow in the night. He noted that cattle, and even his cat, sprouted white hair on the side that had been exposed to the blast. 

In the fall of 1945, the Kodak Company observed some spotting on their film and they traced it back to contamination in their cardboard. They concluded that it was 141Ce contamination from a nuclear explosion somewhere in the United States. It was fallout contamination of a river in Indiana which was used by a paper mill. This incident along with the next continental US tests in 1951 set a precedent. In subsequent atmospheric nuclear tests at the Nevada test site, United States Atomic Energy Commission officials gave the photographic industry maps and forecasts of potential contamination, as well as expected fallout distributions, which enabled them to purchase uncontaminated materials and take other protective measures.

Potsdam, and beyond

At Potsdam, where President Harry S. Truman and Prime Minister Winston Churchill were waiting to meet with Joseph Stalin to discuss a demand for unconditional surrender from the Japanese, the news of the successful test at “Trinity” that morning had a profound effect. Confidence in the test results and the reassurance that the first bomb could be ready for delivery on July 31, 1945, froze the previously tentative decision that the time had come to issue the surrender ultimatum. The atomic bomb had made invasion unnecessary and could provide the Japanese with an honourable excuse to surrender. The war could end quickly. There was no longer any need for help from Russia. Churchill and Truman approached the talks in extreme confidence.

It has been said that Truman had delayed his meeting with the Soviet and British leaders until he knew the results of the Trinity test. It is also said that Truman was informed of the costly, bloody assaults resulting from the naval attacks on the Japanese coast. 

There never had been much doubt that the gun-type uranium weapon would work. By July 14, 1945, two days before the implosion weapon was tested, the major portion of the uranium-235 component began its journey overseas from Los Alamos. A few hours before dawn on July 16, 1945, just as observers in the Jornada del Muerto were witnessing the incredible birth of the atomic age, the uranium bomb was hoisted aboard the cruiser Indianapolis at San Francisco (reports indicate that the Indianapolis carried two “Little Boy” uranium bombs to Tinian Island, of course, without the uranium-235 cores).

On July 26, 1945, the USS Indianapolis arrived at Tinian and two nights later transport planes arrived with the last necessary bit of uranium-235 and the uranium device was ready. A different report mentioned that 5 C-54 transport planes left Kirtland Air Force Base on July 26, 1945 (arriving at Tinian on July 29, 1945), with the “Little Boy” uranium components, and the “Fat Man” plutonium core and its initiator.

The Indianapolis was attacked and sunk by a Japanese submarine on July 30, 1945. The survivors were picked up four days later. 879 died and 317 survived. 

The actual attack order was issued on Aug. 1, 1945. The crews were selected at the same time. Seven B-29’s were used, with one as a stand-by plane located at Iwo Jima (Big Stink). Three planes (Straight Flush, Jabit III, and Full House) flew over the three possible targets to assess the weather and inform the atomic bomb carrier. Two B-29’s accompanied the bomb carrier. The bomb plane was Enola Gay, and the two observation planes (with cameras and instruments) were The Great Artist and Necessary Evil

The “Fat Man” bomb cases F-31 and F-32 arrived on Tinian, and the assembly started (unit F-33 was prepared for a practice bombing run). It has been said that the bombing crew were only informed that it was an immensely powerful bomb. Paul Tibbets, the pilot, only told them about the atomic bomb less than 2 hours before before arriving at the target.

The insertion of the charge, and arming was performed by Parsons and Morris R. Jeppson during the flight to Hiroshima.  

The world’s second man-made nuclear explosion (“Little Boy”) occurred over Hiroshima, Japan, on Aug. 6, 1945 (08:16:02 Hiroshima time), three weeks after the Trinity test. 

The aiming point was the T-shaped Aioi Bridge. The drop from 31,600 feet took 44,4 seconds, and in that time the Enola Gay had flown 11.5 miles from its target. It missed the target by 550 feet, and was detonated at 1,968 feet above the city. The only person in Enola Gay who actually saw the explosion was the tail gunner George Robert Caron. They were 358 miles from Hiroshima before Caron reported that he could no longer see the mushroom cloud. 

It has been said that the U.S. expected an immediate surrender on Aug. 7, 1945, and had decided on Aug. 7, 1945 to drop “Fat Man”. It has been reported that the Japanese approached the Soviet Ambassador to ask them to mediate surrender negotiations. But the meeting was cancelled, and the Soviet Union announced that it would be at war with Japan from Aug. 9, 1945.   

The “Fat Man” practice bomb F-33 was dropped on Aug. 8, 1945, and on the same day the bomb unit F-31 was completed with its plutonium core. It was then loaded onto the B-29 Bockscar. Because of the predicted bad weather in the following days it was decided to drop “Fat Man” visually. The two weather planes were Enola Gay and Laggin’ Dragon, and The Great Artiste (pilot Capt. Frederick C. Bock) and the Big Stink (pilot Major James I. Hopkins Jr.) were again the observer planes. 

On August 9, 1945, the third such explosion devastated the city of Nagasaki (the “Fat Man” was a plutonium bomb of the type tested at Trinity). 

Japan gave up the struggle five days later (Aug. 14, 1945), and surrender ceremonies were held Sept. 2, 1945. As a result of the data gathered on the size of the blast, the detonation height for the bombing of Hiroshima was set at 1,885 feet (575 m) to take advantage of the mach stem blast reinforcing effect. The final Nagasaki burst height was 1,650 feet (500 m) so the Mach stem started sooner. The knowledge that implosion worked led Oppenheimer to recommend to Groves that the uranium-235 used in a “Little Boy” gun-type weapon could be used more economically in a composite core with plutonium. It was too late to do this with the first “Little Boy”, but the composite cores would soon enter production.

It is worthwhile mentioning that Leo Szilárd, a Hungarian-American physicist, had long held strong moral objections to the use of the atomic bomb. It was the reason that, despite his early contributions to nuclear physics and the project, Groves sought to limit his role. Now, with the world on the precipice of its first nuclear detonation, Szilard was disturbed. He did not want to see the weapon used in combat, and so he drafted a petition for Truman demanding that the Japanese be warned before the atomic bomb was used on them. His petition was signed by dozens of employees at the Chicago Metallurgical Lab and the Oak Ridge project site. The petition never made it to Truman, and Szilárd and several of the petition’s signatories were later criticised by Groves and others.

On Sunday, September 9, 1945, Trinity Site was opened to the press for the first time. This was mainly to dispel rumours of lingering high radiation levels there, as well as in Hiroshima and Nagasaki. Led by General Groves and Oppenheimer, this widely publicized visit made Trinity front page news.

In one U.S. War Department press release mention is made of the total cost to arrive at the Trinity test, and presumably also the bombs dropped on Japan. The cost was $2 billion. But it should be noted that the production of the fissile isotopes of uranium-235 an plutonium-239 were enormous undertakings given the technology of the 1940’s, and accounted for 80% of the total costs of the project (e.g. Oak Ridge and Hanford).

What happened next?

A crashing let-down followed the long months of intense technical effort and the climax of victory; the Laboratory faltered and very nearly perished. Much of the credit for holding it together goes to Norris Edwin Bradbury, the Laboratory’s second director and the handful of men who shared his confidence in the facility’s future.

But as you might imagine there was a lot of “cleaning up” needed. Some cattle grazing on the Chupadera mesa suffered local beta burns and temporary loss of dorsal hair. Many of the animals were taken to Los Alamos or Oak Ridge for observation. The estimated dose for such effects was between 4,000 and 50,000 R (Röntgen). 

The day-to-day limit for workers on the site was 0.1 R/d in Los Alamos, but for Trinity it was set at 5 R/exposure, with a limit of 68 R over a two week period (“it would not even cause radiation sickness”). Later the limit was set to 75 Röntgens over 2 weeks (336 hours), or 15 R/hr peak. 

The surveying of off-site exposure in the region found two houses that had been missed in the earlier survey. The local radiation levels warranted “hasty evacuation” of the occupants.

Looking back two big issues were ignored. The first assumes a 21 kiloton bomb with a fissile content of about 6 kg of plutonium (239Pu). This would mean that about 4.8 kg of plutonium remained un-fissioned, and was sent in the atmosphere. But no surveys were performed until 3 years later. The second problem was that only external exposure was studied, not internally deposited radionuclides. Nose swabs were routine in Los Alamos, e.g. where the plutonium hemispheres were manufactured, but this was not followed up after the Trinity test. Plutonium has since been found in plants and soil in the region. 

One positive point was that Groves concluded that the Trinity site was “too small” and that a large site with a unpopulated area of at least 150 miles should be found.      

A official report in 1947 concluded that no one could receive any radiation injury through visiting the site of the explosion.  

A number of official reports were also published concerning the exposure to people living in north New Mexico. The estimate was an incremental risk of cancer mortality of 8 chances in 10,000 from a 50-year exposure to the new natural background, e.g. about 150 mrem per year, including contributions for cosmic radiation, natural terrestrial radioactivity, and natural radioactivity incorporated in the body. This was to be compared to an overall U.S. population lifetime risk of mortality from cancers induced by all causes which was about 2 chances in 10.

To preserve the secrecy of the atomic bomb mission and avoid claims against the Army, residents of New Mexico were not warned before the blast or informed of residual health hazards afterward, and no residents were evacuated. The device was detonated close to the ground, causing much soil to be drawn into the fireball. Some melted soil aggregated into larger droplets that became too heavy to remain suspended, fell to the shot crater, and formed puddles that cooled and became popular souvenirs known as trinitite or atomsite. Most material taken into the fireball eventually came to the surface as radioactive fallout. Exposure rates measured up to 15 or 20 R/h in public areas about 20 miles northeast of Ground Zero. Field teams used instruments that were crude, ill suited to field use, and incapable of measuring about 4.8 kg of un-fissioned plutonium that was dispersed. Vehicle shielding and contamination were not corrected for. Terrain and air flow patterns caused “hot spots” in and around Hoot Owl Canyon, which became known as “Hot Canyon,” and on Chupadera Mesa. Some residences were unknown to the Army and were not visited on test day. Ranchers reported that fallout “snowed down” for 4-5 days after the blast. Many residents collected rain water off their metal roofs into cisterns for drinking. It rained the night of test day, so fresh fallout was likely consumed. Most ranches had one or more dairy cows and a ranch near “Hot Canyon” maintained a herd of 200 goats. All evaluations of public exposures from Trinity published to date have been incomplete in that they have not reflected the internal doses that were received by residents from intakes of airborne radioactivity and contaminated water and foods. Too much remains undetermined about exposures from the Trinity test to put the event in perspective as a source of public radiation exposure or to defensibly address the extent to which people were harmed.

It is reported that the Army purchased 75 of the most injured cows, and the herd, and their descendants, resided at Oak Ridge for decades. 

Los Alamos becomes a “peacetime” laboratory

Before the summer of 1945 had ended, a mass exodus had begun. Many scientists, technicians and graduate students rushed to return to universities and industries from which they had been begged, borrowed or stolen for the wartime project. Many were lured away, and still others seriously tempted, by large salaries offered by universities attempting to rebuild their depleted technical staffs. In general, the Laboratory was staffed at the end of the war with people who were far from sure they wished to remain in Los Alamos.

For everyone there were some very large uncertainties. Neither the government nor the University of California had set down a plan for future operation of the Laboratory. The University had accepted the Los Alamos contract only as a patriotic gesture and there was no guarantee that the contract would continue. In the absence of national legislation on future use and control of atomic energy, there was little basis upon which to establish an appropriate policy for a laboratory whose initial mission was complete. Many thought the Laboratory would be abandoned. Others, suffering intense pangs of conscience, thought it ought to be, or that it should at least be turned over to basic and peaceful research.

All this was complicated by the question of whether or not a location on an isolated mesa top in New Mexico was adequate or satisfactory as a peacetime location for a laboratory of any kind. Furthermore, life there had never been easy and now, with the job finished, there was little incentive to endure it. The combination of an absentee contractor and Army administration of community and auxiliary services had aroused a state of antagonism and irritation that, for many people, could only be solved by leaving Los Alamos. By Oct. 1945, the Laboratory staff, which numbered 3,000 at its wartime peak, was nearing its all time low of only 1,000. Then, adding to the confusion, Oppenheimer announced plans to return to his peacetime duties, and appointed Bradbury to take over as temporary director.

An expert on conductivity of gases, properties of ions and atmospheric electricity, Bradbury had come to Los Alamos as an officer in the Naval Reserve after an outstanding academic career at Pomona, the University of California, Massachusetts Institute of Technology and Stanford, where he was a professor of physics. He was convinced that the nation would continue to need a laboratory for research into military applications of nuclear energy and that Los Alamos, now one of the world’s best-equipped research laboratories, was the logical place for it. He gambled that the government would eventually agree with him.

Meeting with key staff members in Oct. 1945, Bradbury laid his cards on the table. While awaiting legislation, he said, “we should set up the most nearly ideal project to study the use of nuclear energy”. However, he continued, “we have an obligation to the nation never to permit it to be in a position of saying it has something that it has not. The project cannot neglect the stockpiling and development of atomic weapons during this period”.

The re-construction of a peacetime laboratory had begun.

In the spring of 1946, the Laboratory took over technical direction of Operation Crossroads at Bikini in the Marshall Islands. The historic test series supplied highly significant technical data on the effects of atomic weapons on naval vessels and gave the staff additional experience in the conduct of weapons tests. It also gave the Laboratory a concrete objective when it was most needed and proved the Laboratories’ ability to conduct a major operation despite the loss of much of its experienced staff.

But still the situation in Los Alamos was uncertain. The final demoralising blow had come in Feb. 1, 1946 when the community water lines froze solid for weeks. Water was brought up from the Rio Grande in a procession of tank trucks and doled out in buckets and pans to grim-faced housewives. The disaster climaxed the bitter resentment of the system of Los Alamos community operation and hastened the exodus of still more unhappy people.

In May 1946, Bradbury played his ace: he announced that, effective in Sept. 1946, the Laboratory would cease to pay the way home for terminating employees. Those who had been unable to make up their minds, quickly decided, and the staff stabilised, leaving only those who shared Bradbury’s faith in Los Alamos.

The faith was confirmed often throughout the later part of 1946. In the spring, General Groves approved plans for the construction of permanent housing, and prefabricated units were added as quick relief for the critical housing shortage.

The biggest boost came in Aug. 1946, when Congress passed the McMahon Act, establishing the Atomic Energy Commission and putting atomic energy under civilian control. 

As 1947 began, the Commission took over and the University of California agreed to continue operating the Laboratory. With the Commission establishing as its first priority “the stabilisation and revitalisation of the Los Alamos Scientific Laboratory”, it became clear that Los Alamos would continue to play a key role in the nation’s atomic energy program.

The hydrogen bomb

Although the Laboratory continued development of advanced fission weapons, it shortly embarked upon its second major mission, i.e. development of the hydrogen bomb. Theoretical possibilities for a thermonuclear weapon, an idea born during a lunchtime discussion in early 1942, had been under study since the earliest days at Los Alamos by a special group headed by Edward Teller. Theoretically, the scientists knew that a fusion reaction was possible, but it required temperatures far higher than any previously created by man. With the success of the fission bomb, these high temperatures had been achieved. The thermonuclear bomb was now in the realm of practical possibility.

But there were still many major technical barriers. Once the cooperative efforts of Teller and Stanislaw Marcin Ulam made the necessary conceptual breakthrough, the Laboratory was able to launch an elaborate theoretical and experimental research program. The famous electronic brain, MANIAC, was built to handle the complex calculations of thermonuclear process, and the Laboratory went on a six-day week to get the job done. 

In Nov. 1952, two months before the Laboratory’s tenth anniversary, the world’s first full-scale thermonuclear explosion shook the Pacific atoll of Eniwetok with the detonation of the Los Alamos device, “Mike”.

Today radiation levels at the Trinity site remain about 10 times as high as natural background radiation. After being closed to the public for many years, the Trinity Site was declared a National Historic Landmark district in 1965 and was listed on the National Register of Historic Places in 1966. It is now open to visitors.

Visit Trinity

The visitor guide tells us radiation levels in the fenced off area are about 10 times the regions natural background. And that a 1 hour visit will result in a whole body exposure of between 1/2 and 1 milliRöntgen (mR). A U.S. adult receives an exposure of 90 mR annually, 35-50 mR from the Sun, and 20-35 mR from food, the rest from medical sources.

An official report (1967) reported that whole-body gamma-exposure rate at the Trinity Site varied from a high of 3 mR/h near Ground Zero to a low of 0.03 mR/h. Thus for a visitor the whole-body exposure would be less than 1 mR/h. Soil and air samples revealed that all the activity on the site is contained in pieces of trinitite. The most active piece of trinitite registered a surface exposure rate of 100 mR/h. The report concluded that the site was safe for visitors. 

A more extensive review was performed in 1985, with the objective to determine the dose (and associated risks) received after the Trinity test. Doses were estimated for inhalation or ingestion, and expressed in rem, as a unit that permits a direct comparison of doses from different types of radiation. The doses were compared with radiation protection standards.

The background external dose in the Los Alamos area is about 17 mrem/yr, plus 17 mrem/yr from cosmic rays, and 24 mrem/yr from natural radioactive potassium (40K), for a combined dose of about 158 mrem/yr. This is equivalent to a risk of cancer of 18 chances in 1 million per year, or 6 chances in 10,000 over 50 years of exposure to the natural background in that area. One can compare this to an overall U.S. population lifetime risk of mortality from cancer induced by all causes of 2 chances in 10.

Spending 1 week inside the fenced area of Ground Zero a visitor would receive about 8 mrem total dose, or about 1.6% of the radiation protection standard for a member of the public. 

It should be said that on July 16, 1945 (the day of the test) measurements at 30,000 feet to the north of Ground Zero recorded 10 R/h and at 30 feet from Ground Zero 6,000 R/h. By Aug. 14, 1945, this had fallen to 7 R/h. In 1947 the trinitite was collected in drums and buried inside the Ground Zero. In 1967 the radiation dose was between 3 mR/h and 0.03 mR/ inside the fenced Ground Zero.


For information, rad is the amount of radiation that deposits a specific amount of any type of energy (radiation) by ionisation in a gram of any type of material. The rem quantifies the biological response to radiation rather than the amount of energy deposited to the tissue. This is subject to the type of radiation, and reflects the fact that the “biological effectiveness” of neutrons is 5 to 10 more “effective” that gamma-rays or beta particles, and alpha particles are 10 to 20 times more “effective”. So rem is the amount of rads for a given radiation producing a specific biological damage, 1 rad produces 1 rem for gamma-rays, but 10 to 20 rem for alpha particles. The rem has the advantage of defining biological damage irrespective of the type of radiation. However the effect of 1 rem of radiation will be dependent upon what is irradiated, e.g. teeth or lungs, and that is why hole-body radiation is often mentioned as the reference. Dose or dose rates are expressed in rads or rems, depending on whether the reference is to energy deposited or to biological effect. Curie is a measure of the atom decaying each second, and thus is dependent upon the isotope in question.   


In 2010, the Los Alamos Document Retrieval and Assessment Project (LAHDRA) of the U.S. Centers for Disease Control and Prevention published their final report on radioactive exposure. They found that people were exposed to levels of up to 1,000 mSv in the first two weeks after the blast (10,000 times natural background radiation) and were also exposed to internal radiation through ingestion of contaminated fluids and food. There is, however, a lack of studies evaluating the internal doses of residents.

Moreover the U.S. government never undertook an epidemiological study to assess the link between nuclear fallout and cancer rates in the affected regions. Nevertheless, community organisations report a rise in the incidence of cancer and autoimmune diseases in families living in the affected area. While the U.S. government offered monetary compensation to people whose health had been affected by nuclear detonations at the Nevada test site, people affected by the tests near Alamogordo did not receive official recognition as “Downwinders” and were never given any compensation.

There is certainly some controversy about the so-called “Downwinders”. It is clear that 1945 the scientists were not absolutely sure what might happen in the Trinity test. People living in the larger region around the site were not evacuated, and the measurement instruments were unsophisticated. However, the expectation concerning the plume only moving across unpopulated areas was unrealistic. Later the different U.S. administrations never undertook an epidemiological study of the region affected by the plume and fallout.  © Bernard Smith 2017-18