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In 1943, development efforts were directed to a gun-type fission weapon with plutonium called Thin Man. The idea was to fire one subcritical mass of plutonium at another and the collision would create a nuclear explosion. However, Emilio Segrè discovered that the reactor-bred plutonium had a higher concentration of plutonium-240, resulting in up to five times the spontaneous fission rate of cyclotron plutonium. This made reactor plutonium unsuitable for use in a gun-type weapon, causing a predetonation that would release enough energy to disperse the critical mass with a minimal amount of plutonium reacted (a fizzle). Work on an alternative method of bomb design, known as implosion, had begun earlier under the direction of the physicist Seth Neddermeyer.

Implosion involved a subcritical sphere of fissile material was crushed into a smaller, denser form by implosion using explosives. The mass reaches a critical mass as the fissile atoms are packed closer together, increasing the rate of neutron capture. It takes far less time to assemble the critical mass using this method than it would with the gun method since the metal just needs to move a relatively little distance. Neddermeyer's implosion research between 1943 and the beginning of 1944 showed promise, but it was also obvious that the issue would be far more challenging than the gun design from a theoretical and engineering standpoint. In September of 1943, John von Neumann made the case that implosion would not only lessen the risk of detonation before time and fizzle but would also make better use of the fissionable material. Von Neumann had knowledge of the shaped charges used in armour-piercing shells. In place of the cylindrical structure that Neddermeyer was developing, he suggested utilizing a spherical one.

Oppenheimer implemented a reorganization of the Los Alamos laboratory to focus on implosion by August of 1944. Two new groups were created to develop the implosion weapon, codenamed Fat Man, which used explosive lenses to focus the explosion onto a spherical shape using a combination of both slow and fast high explosives. The design of lenses that detonated with the proper shape and velocity turned out to be slow, difficult and frustrating. To study the behavior of converging shock waves, Robert Serber devised the RaLa Experiment, which used the short-lived radioisotope lanthanum-140, a potent source of gamma radiation. This experiment allowed the taking of an X-ray movie of the implosion, which was the most important single experiment affecting the final bomb design.

The Dayton Project was a work with the chemistry as well as metallurgy (production and purification) of radioactive polonium, directed by Charles Allen Thomas of the Monsanto Company. Testing required up to 500 curies per month of polonium, which Monsanto was, luckily, able to deliver. The assembly was encased in a duralumin bomb casing to protect it from bullets and flak. The ultimate task of the metallurgists was to determine how to cast plutonium into a sphere. The difficulty became apparent when attempts to measure the density of plutonium gave inconsistent and incorrect results.


At first contamination was believed to be the cause, but it was soon determined that there were multiple allotropes of plutonium. The phase was found to be stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with alpha particles. The metallurgists then hit upon a plutonium-gallium alloy, which stabilized the phase and could be hot pressed into the desired spherical shape. By the end of the war, half the experienced chemists and metallurgists had to be removed from work with plutonium when unacceptably high levels of the element appeared in their urine. Groves authorized the construction of a new facility for plutonium chemistry and metallurgy, known as the DP-site.


The hemispheres for the first plutonium core were produced and delivered on the 2nd of July, 1945. The uranium gun-type Little Boy weapon was straightforward to design, with overall responsibility for it assigned to Parsons's Ordnance (O) Division and the design, development, and technical work at Los Alamos consolidated under Lieutenant Commander Francis Birch's group. Now that just enriched uranium could be used in the gun-type design, it was possible to substantially simplify it. It was no longer necessary to use a high-velocity gun and a less complex weapon was used instead.

It was decided that a test would be necessary at first despite the waste of fissile material due to the complexity of an implosion-style weapon. Groves granted approval for the test, provided that the active substance was recovered. Oppenheimer decided against a controlled fizzle and went forward with the Trinity nuclear test, which was conducted at full power.

Harvard physics professor Kenneth Bainbridge was given the task of preparing the test in March of 1944. Bainbridge decided to conduct the test at a bombing range close to Alamogordo Army Airfield. The Trinity Base Camp and its amenities, which included barracks, warehouses, workshops, an explosive magazine, and a commissary, were built by Bainbridge and Captain Samuel P. Davalos


Groves did not enjoy the idea of having to defend the loss of $1 billion worth of plutonium in front of a Senate committee, therefore a cylindrical containment vessel with the codename "Jumbo" was built to retrieve the active substance in the case of a failure. It was built by Babcock & Wilcox in Barberton, Ohio, out of 194,000 kg of iron and steel, measuring 7.6 m long and 3.7 m broad. It was delivered in a special train car to a siding near Pope, New Mexico, and then pulled by two tractors on a trailer for the final 40 km to the test location. By the time it finally 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 730 m from the weapon as a rough measure of how powerful the explosion would be. In the end, Jumbo survived, although its tower did not, adding credence to the belief that Jumbo would have successfully contained a fizzled explosion.

On the 7th of May, 1945, a pre-test explosion was carried out to calibrate the equipment. 730 meters from Ground Zero, a wooden test platform was built and loaded with 90,700 kg of TNT that had been laced with nuclear fission products in the form of an irradiated uranium slug from Hanford that was then dissolved and injected into tubing inside the explosion. Brigadier General Thomas Farrell, Oppenheimer and Groves' new deputy commander, saw this explosion. Data from the test proved crucial for the upcoming Trinity test.

The "gadget" weapon was raised to the top of a 30 m steel tower for the real test because detonation there would provide a better idea of how the weapon would behave when dropped from a bomber.













Detonation in the air reduced nuclear fallout and increased the amount of energy applied directly to the target. On the 13th of July, the device was put together at the neighbouring McDonald Ranch House under the direction of Norris Bradbury, and the next day it was perilously winched up the tower. Bush, Chadwick, Conant, Farrell, Fermi, Groves, Lawrence, Oppenheimer, and Tolman were among the observers. On the 16th of July, 1945, at 05:30, the device exploded with the force of around 20 kilotons of TNT, leaving a 76-meter-wide crater of trinitite (radioactive glass) in the desert. The mushroom cloud rose to a height of 12.1 km, and the shock wave could be felt more than 160 km away. It was heard as far away as El Paso, Texas, so Groves had to issue a cover story about an ammunition magazine explosion at Alamogordo Field.


Later, Oppenheimer said that while he watched the explosion, he remembered the following verse from the Bhagavad Gita (XI,12), a sacred text to Hindus:

"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 clarify that he had also been thinking of another verse at the 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."




An implosion-type nuclear bomb
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The explosives of "the gadget" were raised to the top of the tower for the final assembly.
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Trinity Test (July 16th, 1945)
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