Courtesy of Los Alamos National Laboratory
April 1943
Los Alamos opens

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Gun Assembly
From the beginning, the development of an atomic bomb centered around the gun assembly method for both uranium and plutonium. This method involved firing a non-critical projectile at a non-critical target. When the two pieces came together, they became critical and detonated. To ensure the chain reaction began the instant the two pieces came together, a source of neutrons was needed to initiate the chain reaction. This was accomplished by having a piece of polonium on one fissionable section and a piece of beryllium on the other fissionable section. When these two came together they reacted as follows:

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Implosion
The second means for assembling uranium or plutonium was implosion. This method was only briefly discussed at the initial lectures at Los Alamos. Implosion involves detonating a surrounding explosive so as to cause the fissionable material to fly together in the middle. The first sketch of this method showed four wedges mounted on a ring. When the explosive was fired the wedges formed a critical mass. After Los Alamos opened, implosion experiments were carried out which attempted to compress hollow shells of material. Eventually, implosion was used to compress a solid, sub-critical mass of plutonium in the Trinity and Fat Man bombs.

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