Editing Manhattan Project (section)

== Plutonium ==
The second line of development pursued by the Manhattan Project used plutonium. Although small amounts of plutonium exist in nature, the best way to obtain large quantities is via a reactor. Natural uranium is bombarded by neutrons and [[nuclear transmutation|transmuted]] into [[uranium-239]], which rapidly decays, first into [[neptunium-239]] and then into [[plutonium-239]].<ref name="Smyth, pp. 130-132" /> As only a small amount will be transformed, the plutonium must be chemically separated from the remaining uranium, from any initial impurities, and from [[fission products]].<ref name="Smyth, pp. 130-132">{{harvnb|Smyth|1945|pp=130–132}}.</ref>

=== X-10 Graphite Reactor ===
{{Main|X-10 Graphite Reactor}}
[[File:X10 Reactor Face.jpg|thumb|Workers load uranium slugs into the X-10 Graphite Reactor.|alt=Two workmen on a movable platform similar to that used by window washers, stick a rod into one of many small holes in the wall in front of them.]]
In March 1943, DuPont began construction of a plutonium plant on a {{convert|112|acre|km2|1|adj=on}} site at Oak Ridge. Intended as a pilot plant for the larger production facilities at Hanford, it included the air-cooled [[X-10 Graphite Reactor]], a chemical separation plant, and support facilities. Because of the subsequent decision to construct water-cooled reactors at Hanford, only the chemical separation plant operated as a true pilot.<ref name="Jones, pp. 204-206">{{harvnb|Jones|1985|pp=204–206}}.</ref> The X-10 Graphite Reactor consisted of a huge block of graphite, {{convert|24|ft|m}} per side, weighing around {{convert|1500|ST}}, surrounded by {{convert|7|ft|m}} of high-density concrete as a radiation shield.<ref name="Jones, pp. 204-206" />

The greatest difficulty was encountered with the uranium slugs produced by Mallinckrodt and Metal Hydrides. These had to be coated in aluminum to avoid corrosion and the escape of fission products into the cooling system. The Grasselli Chemical Company attempted to develop a [[tinning#Hot-dipping|hot dipping process]] without success. [[Alcoa]] tried canning, developing a new process for flux-less welding; 97% of the cans passed a standard vacuum test, but high temperature tests indicated a failure rate of more than 50%. Nonetheless, production began in June 1943. The Metallurgical Laboratory eventually developed an improved welding technique with the help of [[General Electric]], which was incorporated into the production process in October 1943.<ref>{{harvnb|Hewlett|Anderson|1962|pp=208–210}}.</ref>

The X-10 Graphite Reactor went critical on 4 November 1943 with about {{convert|30|ST}} of uranium. A week later the load was increased to {{convert|36|ST}}, raising its power generation to 500&nbsp;kW, and by the end of the month the first 500&nbsp;mg of plutonium was created.<ref>{{harvnb|Hewlett|Anderson|1962|p=211}}.</ref> Gradual modifications raised the power to 4,000&nbsp;kW in July 1944. X-10 operated as a production plant until January 1945, when it was turned over to research.<ref name="Jones 1985 209">{{harvnb|Jones|1985|p=209}}.</ref>

=== Hanford reactors ===
{{Main|Hanford Engineer Works}}
Although an air-cooled design was chosen for the reactor at Oak Ridge to facilitate rapid construction, this was impractical for the much larger production reactors. Initial designs by the Metallurgical Laboratory and DuPont used helium for cooling, before they determined that a water-cooled reactor was simpler, cheaper and quicker to build.<ref>{{harvnb|Groves|1962|pp=78–82}}.</ref> The design did not become available until 4 October 1943; in the meantime, Matthias concentrated on improving the Hanford Site by erecting accommodations, improving the roads, building a railway switch line, and upgrading the electricity, water and telephone lines.<ref>{{harvnb|Jones|1985|p=210}}.</ref>

[[File:Hanford B-Reactor Area 1944.jpg|thumb|left|Aerial view of Hanford [[B-Reactor]] site, June 1944|alt=An aerial view of the Hanford B-Reactor site from June 1944. At center is the reactor building. Small trucks dot the landscape and give a sense of scale. Two large water towers loom above the plant.]]
As at Oak Ridge, the most difficulty was encountered while canning the uranium slugs, which commenced at Hanford in March 1944. They were [[pickling (metal)|pickled]] to remove dirt and impurities, dipped in molten bronze, tin, and [[silumin|aluminum-silicon alloy]], canned using hydraulic presses, and then capped using [[arc welding]] under an argon atmosphere. Finally, they were tested to detect holes or faulty welds. Disappointingly, most canned slugs initially failed the tests, resulting in an output of only a handful per day. But steady progress was made and by June 1944 production increased to the point where it appeared that enough canned slugs was available to start [[B-Reactor|Reactor B]] on schedule in August 1944.<ref>{{harvnb|Hewlett|Anderson|1962|pp=222–226}}.</ref>

Work began on Reactor B, the first of six planned 250&nbsp;MW reactors, on 10 October 1943.<ref>{{harvnb|Thayer|1996|p=139}}.</ref> The reactor complexes were given letter designations A through F, with B, D and F sites developed first, as this maximized the distance between the reactors. They were the only ones constructed during the Manhattan Project.<ref>{{harvnb|Hanford Cultural and Historic Resources Program|2002|p=1.16}}</ref> Some {{convert|390|ST}} of steel, {{convert|17400|cuyd}} of concrete, 50,000 concrete blocks and 71,000 concrete bricks were used to construct the {{convert|120|ft|m|adj=on}} high building.

Construction of the reactor itself commenced in February 1944.<ref>{{harvnb|Hewlett|Anderson|1962|pp=216–217}}.</ref> Watched by Compton, Matthias, DuPont's [[Crawford Greenewalt]], [[Leona Woods]] and Fermi, who inserted the first slug, the reactor was powered up beginning on 13 September 1944. Over the next few days, 838 tubes were loaded and the reactor went critical. Shortly after midnight on 27 September, the operators began to withdraw the [[control rod]]s to initiate production. At first all appeared well but around 03:00 the power level started to drop and by 06:30 the reactor had shut down completely. The cooling water was investigated to see if there was a leak or contamination. The next day the reactor started up again, only to shut down once more.<ref>{{harvnb|Hewlett|Anderson|1962|pp=304–307}}.</ref><ref name="Jones, pp. 220-223">{{harvnb|Jones|1985|pp=220–223}}.</ref>

Fermi contacted [[Chien-Shiung Wu]], who identified the cause of the problem as [[neutron poison]]ing from [[xenon-135]], which has a [[half-life]] of 9.2 hours.<ref>{{harvnb|Howes|Herzenberg|1999|p=45}}.</ref> Fermi, Woods, [[Donald J. Hughes]] and [[John Archibald Wheeler]] then calculated the [[nuclear cross section]] of xenon-135, which turned out to be 30,000 times that of uranium.<ref>{{harvnb|Libby|1979|pp=182–183}}.</ref> DuPont engineer George Graves had deviated from the Metallurgical Laboratory's original design in which the reactor had 1,500 tubes arranged in a circle, and had added an additional 504 tubes to fill in the corners. The scientists had originally considered this overengineering a waste of time and money, but Fermi realized that by loading all 2,004 tubes, the reactor could reach the required power level and efficiently produce plutonium.<ref>{{harvnb|Thayer|1996|p=10}}.</ref> Reactor D was started on 17 December 1944 and Reactor F on 25 February 1945.<ref name="Thayer 1996 141">{{harvnb|Thayer|1996|p=141}}.</ref>

=== Separation process ===
[[File:Hanford Engineer Works.png|thumb|upright=1.6|Map of the Hanford Site. Railroads flank the plants to the north and south. Reactors are the three northernmost red squares, along the Columbia River. The separation plants are the lower two red squares from the grouping south of the reactors. The bottom red square is the 300 area.|alt=A contour map showing the fork of the Columbia and Yakima rivers and the boundary of the land, with seven small red squares marked on it]]
Meanwhile, the chemists considered how plutonium could be separated from uranium when its chemical properties were not known. Working with the minute quantities of plutonium available at the Metallurgical Laboratory in 1942, a team under Charles M. Cooper developed a [[fluoride selective electrode|lanthanum fluoride process]] which was chosen for the pilot separation plant. A second separation process, the [[bismuth phosphate process]], was subsequently developed by Seaborg and Stanly G. Thomson.<ref>{{harvnb|Hewlett|Anderson|1962|pp=184–185}}.</ref> Greenewalt favored the bismuth phosphate process due to the corrosive nature of lanthanum fluoride, and it was selected for the Hanford separation plants.<ref>{{harvnb|Hewlett|Anderson|1962|pp=204–205}}.</ref> Once X-10 began producing plutonium, the pilot separation plant was put to the test. The first batch was processed at 40% efficiency but over the next few months this was raised to 90%.<ref name="Jones 1985 209" />

At Hanford, top priority was initially given to the installations in the 300 area: buildings for testing materials, preparing uranium, and assembling and calibrating instrumentation. One of the buildings housed the canning equipment for the uranium slugs, while another contained a small test reactor. Notwithstanding its priority, work on the 300 area fell behind schedule due to the unique and complex nature of the facilities, and wartime shortages of labor and materials.<ref>{{harvnb|Jones|1985|pp=214–216}}.</ref>

Early plans called for the construction of two separation plants in each of the areas known as 200-West and 200-East. This was subsequently reduced to two, the T and U plants, in 200-West and one, the B plant, at 200-East.<ref>{{harvnb|Jones|1985|p=212}}.</ref> Each separation plant consisted of four buildings: a process cell building or "canyon" (known as 221), a concentration building (224), a purification building (231) and a magazine store (213). The canyons were each {{convert|800|ft}} long and {{convert|65|ft}} wide. Each consisted of forty {{convert|17.7|by|13|by|20|ft|adj=on}} cells.<ref>{{harvnb|Thayer|1996|p=11}}.</ref>

Work began on 221-T and 221-U in January 1944, with the former completed in September and the latter in December. The 221-B building followed in March 1945. Because of the high levels of radioactivity involved, work in the separation plants had to be conducted by remote control using closed-circuit television, something unheard of in 1943. Maintenance was carried out with the aid of an overhead crane and specially designed tools. The 224 buildings were smaller because they had less material to process, and it was less radioactive. The 224-T and 224-U buildings were completed on 8 October 1944, and 224-B followed on 10 February 1945. The purification methods that were eventually used in 231-W were still unknown when construction commenced on 8 April 1944, but the plant was complete and the methods were selected by the end of the year.<ref>{{harvnb|Hewlett|Anderson|1962|pp=219–222}}.</ref> On 5 February 1945, Matthias hand-delivered the first shipment of 80&nbsp;g of 95%-pure plutonium nitrate to a Los Alamos courier in Los Angeles.<ref name="Thayer 1996 141" />

=== Weapon design ===
{{Main|Project Y}}
[[File:Thin Man plutonium gun bomb casings.jpg|thumb|A row of Thin Man casings. Fat Man casings are visible in the background.|alt=Long, tube-like casings. In the background are several ovoid casings and a tow truck.]]
In 1943, development efforts were directed to a [[gun-type fission weapon]] with plutonium called [[Thin Man (nuclear bomb)|Thin Man]]. Initial research on the properties of plutonium was done using cyclotron-generated plutonium-239, which was extremely pure but could only be created in very small amounts. Los Alamos received the first sample of plutonium from the Clinton X-10 reactor in April 1944 and within days Emilio Segrè discovered a problem: the reactor-bred plutonium had a higher concentration of plutonium-240, resulting in up to five times the spontaneous fission rate of cyclotron plutonium.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=226–229, 237}}</ref>

This rendered it unsuitable for use in a gun-type weapon, for the plutonium-240 would start the chain reaction too soon, causing a [[predetonation]] that would disperse the critical mass after a minimal amount of plutonium had fissioned (a [[fizzle (nuclear test)|fizzle]]). A higher-velocity gun was suggested but found to be impractical. The possibility of separating the isotopes was also considered and rejected, as plutonium-240 is even harder to separate from plutonium-239 than uranium-235 from uranium-238, and attempting it "would postpone the weapon indefinitely".<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=242–244}}</ref>

Work on an alternative method of bomb design, known as implosion, had begun earlier under the direction of the physicist [[Seth Neddermeyer]]. Implosion used explosives to crush a subcritical sphere of fissile material into a smaller and denser form. The critical mass is assembled in much less time than with the gun method. When the fissile atoms are packed closer together, the rate of neutron capture increases,<ref>{{harvnb|Hewlett|Anderson|1962|pp=312–313}}.</ref> so it also makes more efficient use of fissionable material.<ref>{{harvnb|Hewlett|Anderson|1962|p=246}}.</ref> Neddermeyer's 1943 and early 1944 investigations showed promise, but also made it clear that an implosion weapon was more complex than the gun-type design from both a theoretical and an engineering perspective.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=129–130}}</ref> In September 1943, [[John von Neumann]], who had experience with [[shaped charge]]s, proposed using a spherical configuration instead of the cylindrical one that Neddermeyer was working on.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=130–131}}</ref>

[[File:Fat Man design model.png|thumb|left|An implosion-type nuclear bomb|alt=Diagram showing fast explosive, slow explosive, uranium tamper, plutonium core and neutron initiator]]

An accelerated effort on the implosion design, codenamed [[Fat Man]], began in August 1944 when Oppenheimer implemented a sweeping reorganization of the Los Alamos laboratory to focus on implosion.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=245–248}}</ref> Two new groups were created at Los Alamos to develop the implosion weapon, X (for explosives) Division headed by explosives expert [[George Kistiakowsky]] and G (for gadget) Division under Robert Bacher.<ref>{{harvnb|Hewlett|Anderson|1962|p=311}}.</ref><ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|p=245}}</ref> The new design featured [[explosive lens]]es that focused the implosion into a spherical shape.<ref name="Hoddeson et al, pp. 294-296" /> The design of lenses turned out to be slow, difficult and frustrating.<ref name="Hoddeson et al, pp. 294-296">{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=294–296}}</ref> Various explosives were tested before settling on [[composition B]] and [[baratol]].<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|p=299}}</ref> The final design resembled a soccer ball, with 20 hexagonal and 12 pentagonal lenses, each weighing about {{convert|80|lb}}. Getting the detonation just right required fast, reliable and safe electrical [[detonator]]s, of which there were two for each lens for reliability.<ref name="Hansen. p. V-123" /> They used [[exploding-bridgewire detonator]]s, a new invention developed at Los Alamos by a group led by [[Luis Walter Alvarez|Luis Alvarez]].<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=301–307}}</ref>

To study the behavior of converging [[shock wave]]s, Robert Serber devised the [[RaLa Experiment]], which used the short-lived [[radioisotope]] [[lanthanum-140]], a potent source of [[gamma radiation]]. The gamma ray source was placed in the center of a metal sphere surrounded by the explosive lenses, which in turn were inside in an [[ionization chamber]]. This allowed the taking of an X-ray movie of the implosion. The lenses were designed primarily using this series of tests.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=148–154}}</ref> In his history of the Los Alamos project, [[David Hawkins (philosopher)|David Hawkins]] wrote: "RaLa became the most important single experiment affecting the final bomb design".<ref>{{harvnb|Hawkins|Truslow|Smith|1961|p=203}}.</ref>

Within the explosives was an aluminum pusher, which provided a smooth transition from the relatively low-density explosive to the next layer, the [[Tamper (nuclear weapons)|tamper]] of natural uranium. Its main job was to hold the critical mass together as long as possible, but it would also reflect neutrons into the core and some of its uranium would fission. To prevent predetonation by an external neutron, the tamper was coated in a thin layer of neutron-absorbing boron.<ref name="Hansen. p. V-123" /> A polonium-beryllium [[modulated neutron initiator]], known as an "urchin",<ref>{{harvnb|Hansen|1995a|p=I-298}}.</ref> was developed to start the chain reaction at precisely the right moment.<ref>{{harvnb|Hewlett|Anderson|1962|p=235}}.</ref> This work on the chemistry and metallurgy of radioactive polonium was directed by [[Charles Allen Thomas]] of the [[Monsanto Company]] and became known as the [[Dayton Project]].<ref>{{harvnb|Gilbert|1969|pp=3–4}}.</ref> Testing required up to 500 [[Curie (unit)|curies]] per month of polonium, which Monsanto was able to deliver.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=308–310}}</ref> The whole assembly was encased in a [[duralumin]] bomb casing to protect it from bullets and flak.<ref name="Hansen. p. V-123">{{harvnb|Hansen|1995b|p=V-123}}.</ref>

[[File:Remote handling of a kilocurie source of radiolanthanum.jpg|thumb|Remote handling of a kilocurie source of radiolanthanum for a [[RaLa Experiment]] at Los Alamos|alt=A shack surrounded by pine trees. There is snow on the ground. A man and a woman in white lab coats are pulling on a rope, which is attached to a small trolley on a wooden platform. On top of the trolley is a large cylindrical object.]]
The ultimate task of the metallurgists was to determine how to cast plutonium into a sphere. The difficulties became apparent when attempts to measure the density of plutonium gave inconsistent results. At first contamination was suspected, but it was soon determined that there were multiple [[allotropes of plutonium]].<ref>{{harvnb|Hewlett|Anderson|1962|pp=244–245}}.</ref> The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures. Attention then shifted to the even more malleable δ phase that normally exists in the 300&nbsp;°C to 450&nbsp;°C range. It was found that this was stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with [[alpha particles]], which would exacerbate the pre-ignition problem. The metallurgists then hit upon using a [[plutonium-gallium alloy]], which stabilized the δ phase and could be [[hot pressing|hot pressed]] into the desired spherical shape. As plutonium was found to corrode readily, the sphere was coated with nickel.<ref>{{harvnb|Baker|Hecker|Harbur|1983|pp=144–145}}</ref>

The work proved dangerous. By the end of the war, half the chemists and metallurgists had to be removed from work with plutonium when unacceptably high levels of the element was detected in their urine.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|p=288}}</ref> A minor fire at Los Alamos in January 1945 led to a fear that a fire in the plutonium laboratory might contaminate the whole town, and Groves authorized the construction of a new facility for plutonium chemistry and metallurgy, which became known as the DP-site.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|p=290}}</ref> The hemispheres for the first plutonium [[pit (nuclear weapon)|pit]] (or core) were produced and delivered on 2 July 1945. Three more hemispheres followed on 23 July and were delivered three days later.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=330–331}}</ref>

In contrast to the plutonium Fat Man, the uranium gun-type Little Boy weapon was straightforward if not trivial to design. Overall responsibility for it was assigned to Parsons's Ordnance (O) Division, with the design, development, and technical work at Los Alamos consolidated under [[Lieutenant Commander (United States)|Lieutenant Commander]] [[Francis Birch (geophysicist)|Francis Birch]]'s group. The gun-type design now had to work with enriched uranium only, and this allowed the design to be greatly simplified. A high-velocity gun was no longer required, and a simpler weapon was substituted.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|pp=245–249}}{{sfn|Rhodes|1986|p=541}}

Research into the Super was also pursued, although it was considered secondary to the development of a fission bomb. The effort was directed by Teller, who was its most enthusiastic proponent.{{sfn|Hawkins|Truslow|Smith|1961|pp=95–98}} The F-1 (Super) Group calculated that burning {{convert|1|m3|sp=us}} of liquid [[deuterium]] would release the energy of {{convert|10|MtTNT}}, enough to devastate {{convert|1000|sqmi}}.{{sfn|Hawkins|Truslow|Smith|1961|pp=214–216}} In a final report on the Super in June 1946, Teller remained upbeat about the prospect of it being successfully developed, although that opinion was not universal.<ref name="PBS">{{cite web |url=https://www.pbs.org/wgbh/amex/bomb/peopleevents/pandeAMEX71.html |title=American Experience . Race for the Superbomb . Super Conference |publisher=[[PBS]] |access-date=28 August 2016 |archive-date=28 August 2016 |archive-url=https://web.archive.org/web/20160828134429/http://www.pbs.org/wgbh/amex/bomb/peopleevents/pandeAMEX71.html |url-status=live }}</ref>

=== Trinity ===
{{Main|Trinity (nuclear test)}}
Because of the complexity of an implosion-style weapon, it was decided that, despite the waste of fissile material, a full-scale [[nuclear test]] was required. Oppenheimer codenamed it "Trinity".<ref>{{harvnb|Jones|1985|p=465}}.</ref> In March 1944, planning for the test was assigned to [[Kenneth Bainbridge]], who selected the [[Alamogordo Bombing Range]] for the test site.<ref>{{harvnb|Hewlett|Anderson|1962|pp=318–319}}.</ref> A base camp was constructed with barracks, warehouses, workshops, an explosive magazine and a commissary.<ref>{{harvnb|Jones|1985|pp=478–481}}.</ref> A pre-test explosion was conducted on 7 May 1945 to calibrate the instruments. A wooden test platform was erected {{convert|800|yd}} from future Trinity Ground Zero and piled with about {{convert|100|ST}} of high explosives{{efn|The charge consisted of {{convert|89.75|ST|t}} tons of [[TNT]] and {{convert|14.91|ST|t}} tons of [[Composition B]] (with the total explosive power of approximately 108 tons of TNT), actually a few tons more than the stated "100-tons".<ref>{{cite book |last=Walker |first=Raymond L. |date=1950 |title=100-ton Test: Piezo Gauge Measurements |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015086443556&seq=3 |publisher=U.S. Atomic Energy Commission, Technical Information Division |page=1}}</ref><ref>{{cite book |last=Loring |first=William S. |date=2019 |title=Birthplace of the Atomic Bomb: A Complete History of the Trinity Test Site |url=https://books.google.am/books?id=h0CIDwAAQBAJ&pg=PT141#v=onepage&q&f=false |location=Jefferson, North Carolina |publisher=McFarland & Company, Inc., Publishers |page=133 |isbn=978-1-4766-3381-7}}</ref>
}} spiked with [[nuclear fission product]]s.<ref name="Jones, p. 512" /><ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=360–362}}</ref>

[[File:Trinity device readied.jpg|thumb|left|The explosives of "the gadget" were raised to the top of the tower for the final assembly.|alt=Men stand around a large oil-rig type structure. A large round object is being hoisted up.]]
Groves did not relish the prospect of explaining to a Senate committee the loss of a billion dollars worth of plutonium, so a cylindrical containment vessel codenamed "Jumbo" was constructed to recover the active material in the event of a failure. It was fabricated at great expense from {{convert|214|ST}} of iron and steel.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=174–175}}</ref> By the time it arrived, however, confidence in the implosion method was high enough, and the availability of plutonium was sufficient, that Oppenheimer decided not to use it. Instead, it was placed atop a steel tower {{convert|800|yd}} from the weapon as a rough measure of the explosion's power. Jumbo survived, although its tower did not, adding credence to the belief that Jumbo would have successfully contained a [[Fizzle (nuclear explosion)|fizzled explosion]].<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=365–367}}</ref><ref name="Jones, p. 512">{{harvnb|Jones|1985|p=512}}.</ref>

[[File:Trinity Detonation T&B.jpg|thumb|right|The [[Trinity (nuclear test)|Trinity test]] of the Manhattan Project was the first detonation of a [[nuclear weapon]].]]
For the actual test, the weapon, nicknamed "the gadget", was hoisted to the top of a {{convert|100|ft|m|adj=on}} steel tower, as detonation at that height would give a better indication of how the weapon would behave when dropped from a bomber. Detonation in the air maximized the energy applied directly to the target and generated less [[nuclear fallout]]. The gadget was assembled under the supervision of [[Norris Bradbury]] at the nearby [[McDonald Ranch House]] on 13 July, and precariously winched up the tower the following day.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=367–370}}</ref>

At 05:30 on 16 July 1945 the gadget exploded with an [[TNT equivalent|energy equivalent]] of around 20 kilotons of TNT, leaving a crater of [[trinitite]] (radioactive glass) in the desert {{convert|250|ft}} wide. The shock wave was felt over {{convert|100|mi}} away, and the [[mushroom cloud]] reached {{convert|7.5|mi}} in height. It was heard as far away as [[El Paso, Texas]], so Groves issued a cover story about an ammunition magazine explosion at Alamogordo Field involving gas shells.<ref>{{harvnb|Hoddeson|Henriksen|Meade|Westfall|1993|pp=372–374}}</ref><ref>{{harvnb|Jones|1985|pp=514–517}}.</ref>

Oppenheimer later claimed that, while witnessing the explosion, he thought of a verse from the [[Hindu]] holy book, the ''[[Bhagavad Gita]]'' (XI,12):
{{verse translation|italicsoff=true|lang=sa
|कालोऽस्मि लोकक्षयकृत्प्रवृद्धो लोकान्समाहर्तुमिह प्रवृत्तः। ऋतेऽपि त्वां न भविष्यन्ति सर्वे येऽवस्थिताः प्रत्यनीकेषु योधाः॥११- ३२॥
|If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendor of the mighty one&nbsp;...{{sfn|Jungk|1958|p=201}}<ref>{{cite web |url=http://www.asitis.com/11/12.html |title=Bhagavad Gita As It Is, 11: The Universal Form, Text 12 |access-date=19 July 2013 |publisher=A.C. Bhaktivedanta Swami Prabhupada}}</ref>}}

together with verse (XI,32), which he translated as "Now I am become Death, destroyer of worlds".<ref name="Time Inc p. 133">{{cite magazine |title=J. Robert Oppenheimer |first=Lincoln |last=Barnett |author-link=Lincoln Barnett |magazine=[[Life (magazine)|Life]] |issn=0024-3019 | url=https://books.google.com/books?id=GVIEAAAAMBAJ&dq=oppenheimer+%22if+the+radiance+of+a+thousand+suns+were+to+%22&pg=PA133 | access-date=2023-08-29 | page=133}}</ref><ref name="TIME.com 1948 k845">{{cite magazine |title=The Eternal Apprentice |magazine=[[Time (magazine)|Time]] | date=1948-11-08 | url=https://content.time.com/time/subscriber/article/0,33009,853367-8,00.html | access-date=2023-08-29}}</ref>{{efn|The first instance in print of Oppenheimer's ''Gita'' story is apparently from 1948. Oppenheimer at times translated it to "shatterer of worlds" as well. The quote with "destroyer of worlds" comes from a taped interview of Oppenheimer did with NBC in 1965. Oppenheimer's translation is not considered a standard or literal one, and was likely influenced by the style of his [[Sanskrit]] teacher, [[Arthur Ryder]], who translated the line as: "Death am I, and my present task / Destruction." A more common translation has the identification not as "Death," but as "Time." In the passage, the Hindu god [[Krishna]] is revealing himself and his true form to Prince Arjuna, imploring Arjuna to fulfill his duty and take part in a war, and assuring him that the fate of those killed is really up to Krishna, not mortal men.<ref>{{cite journal|last=Hijiya|first=James A.|title=The 'Gita' of J. Robert Oppenheimer|journal=Proceedings of the American Philosophical Society|date=June 2000|volume=144|number=2|pages=123–167}}</ref>}}

The test was significantly more successful than had been anticipated; this was immediately cabled to Stimson, who was then at the [[Potsdam Conference]], and Groves hastily prepared a lengthier report sent via courier. President [[Harry S. Truman]] was powerfully and positively affected by the news. Stimson noted in his diary that when he shared it with Churchill, Churchill remarked: "Now I know what happened to Truman yesterday. I couldn't understand it. When he got to the meeting after having read this report, he was a changed man. He told the Russians just where they got on and off and generally bossed the whole meeting."<ref>{{harvnb|Groves|1962|pp=303–304}}.</ref>