Atomic Age: ATOMIC CHAIN REACTION

When a piece of paper is lighted with a match, the paper particles first heated set others on fire; these in turn ignite others, and so on. The same sort of chain reaction must be started for a successful large-scale atomic explosion. Above, the rare form of uranium, U-235, is shown breaking down into barium and krypton (only one of several possible disintegrations). The “match” is a neutron source at left. (Radium mixed with beryllium is a common source of neutrons.)

The uranium nucleus splits into barium and krypton atoms, which are highly excited, unstable and artificially radioactive. They throw off gamma and beta radiation, and finally, in an effort to lose mass, they spout neutrons. If these neutrons are slowed by such substances as graphite, paraffin, heavy water or ordinary water, they will touch off other uranium nuclei. In a tiny fraction of a second the reaction will run through a good-sized sample of uranium, containing trillions of atoms, and the result will be a cataclysmic blast.

This diagram merely illustrates the principle on which the atomic bomb works, not the specific processes occurring in the bombs dropped on Japan. Actually, there is no need for a reaction multiplying as fast as that shown above. An increase of a few percent of neutrons in each cycle is enough to do the trick.