To bring the subcritical masses together into a supercritical mass, two techniques are used:
Gun-triggered
Implosion
Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:
The foil is broken when the subcritical masses come together and polonium spontaneously emits alpha particles.
These alpha particles then collide with beryllium-9 to produce beryllium-8 and free neutrons.
The neutrons then initiate fission.
Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core’s expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.
Gun-triggered Fission Bomb
The simplest way to bring the subcritical masses together is to make a gun that fires one mass into the other. A sphere of U-235 is made around the neutron generator and a small bullet of U-235 is removed. The bullet is placed at the one end of a long tube with explosives behind it, while the sphere is placed at the other end. A barometric-pressure sensor determines the appropriate altitude for detonation and triggers the following sequence of events:
The explosives fire and propel the bullet down the barrel.
The bullet strikes the sphere and generator, initiating the fission reaction.
The fission reaction begins.
The bomb explodes.
Little Boy was this type of bomb and had a 14.5-kiloton yield (equal to 14,500 tons of TNT) with an efficiency of about 1.5 percent. That is, 1.5 percent of the material was fissioned before the explosion carried the material away.