The gadget
The "gadget" was the code-name given to the first nuclear explosive developed under the Manhattan Project during World War II, which was tested at the Trinity test site on July 16, 1945. It was so called because it was not a deployable weapon, and because revealing words like "bomb" were not used during the project for fear of espionage. It was an implosion-type plutonium device, similar in design to the "Fat Man" bomb used three weeks later in the atomic bombing of Nagasaki, Japan.
Design
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A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.
The initial design, based on work done on cyclotron-supplied plutonium would have envisioned a gun-type device, akin to Little Boy. However, mass-produced plutonium emitted enough neutrons to make it likely that it would pre-detonate with low yield, making this design, known as "Thin Man", unusable.
Subsequently, an implosive design was selected, resulting in the design known as Fat Man. Both Fat Man and the gadget are not strictly "Fat Man type", as the design was modified into a production design, and both were strictly one-off prototypes.
Test
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The "gadget" was tested at Trinity Site, New Mexico, near Alamogordo. It was feared by some that the Trinity test might "ignite" the Earth's atmosphere, eliminating all life on the planet, although a classified report produced several years earlier had demonstrated that this was not possible.[1] Less wild estimates thought that New Mexico would be incinerated. Calculations showed that the yield of the device would be between 0 (if it did not work) and 20 kilotons (metric, equivalence of TNT). In the aftermath of the test, it appeared to have been a blast equivalent to 18 kt of TNT. It was also a highly covert operation.