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...

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