americium atomic mass unit atomic weight beta decay californium chain reaction cross section curium decay product fertile material fissile fission gamma ray half-life implosion index to isotope pages isotopes of americium isotopes of neptunium isotopic light water reactor mox fuel manhattan project neptunium neutron neutron capture nuclear fuel nuclear fuel cycle nuclear proliferation nuclear reprocessing nuclear transmutation nuclear weapon nuclear weapon yield plutonium proton radioisotope thermoelectric generator spent nuclear fuel spontaneous fission standard deviation thermal reactor uranium