Trinity and Gadget the first atomic bomb

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It is often assumed that “Little Boy” the atomic bomb dropped on Hiroshima was the first atomic device to be detonated, however truth is “the Gadget” was the 1st atomic bomb.

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Trinity was the code name of the first detonation of a nuclear weapon. It was conducted by the United States Army at 5:29 am on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, on what was then the USAAF Alamogordo Bombing and Gunnery Range (now part of White Sands Missile Range). The only structures originally in the vicinity were the McDonald Ranch House and its ancillary buildings, which scientists used as a laboratory for testing bomb components. A base camp was constructed, and there were 425 people present on the weekend of the test.

The code name “Trinity” was assigned by J. Robert Oppenheimer, the director of the Los Alamos Laboratory, inspired by the poetry of John Donne.

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The test was of an implosion-design plutonium device, informally nicknamed “The Gadget”, of the same design as the Fat Man bomb later detonated over Nagasaki, Japan, on August 9, 1945. The complexity of the design required a major effort from the Los Alamos Laboratory, and concerns about whether it would work led to a decision to conduct the first nuclear test. The test was planned and directed by Kenneth Bainbridge.

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The term “Gadget” was a laboratory euphemism for the 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 fuzing and the external ballistic casing. The bombs were still under development, and small changes continued to be made to the Fat Man design.

The Gadget was an implosion device, which means the plutonium core is surrounded by many small explosives, these compress the plutonium and bring it closer to the point of causing it to go super critical. All those wires are attached to different explosives which burn at different frequencies. The trick of the 20 explosions is that they push the pieces of uranium (or plutonium) together to a ball with an over-critical mass, which explodes. They have to time this extremely accurately, however. Microseconds differences will make the ball lopsided and less effective. Part of the solution is to make each and every cable the same length which is why the Gadget looks like a ball of wires.

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Assembly of the nuclear capsule began on 13 July 1945 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.

The Gadget, the first atomic bomb, 1945 (2)

For the test, the gadget was lifted to the top of a 100-foot (30 m) bomb tower. It was feared by some that the Trinity test might “ignite” the earth’s atmosphere, eliminating all life on the planet, although calculations had determined this was unlikely even for devices “which greatly exceed the bombs now under consideration”. Less wild estimates thought that New Mexico would be incinerated. Calculations showed that the yield of the device would be between zero (if it did not work) or 20 kilotons of TNT. In the aftermath of the test, it appeared to have been a blast equivalent to 18 kilotons of TNT..

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Ground zero after the test

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