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Plutonium-238 is an alpha emitter.
Uranium 238 is bombarded by neutrons, and forms Neptunium 238. Neptunium decays to form Plutonium 238.
In this period plutonium-238 from Russia was cheaper.
Plutonium-238 has 144 neutrons, plutonium-240 has 146 neutrons.
Atomic mass and half life are also different.