Plutonium-240 (240
or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.[3]

Plutonium-240, 240Pu
Namesplutonium-240, Pu-240
Protons (Z)94
Neutrons (N)146
Nuclide data
Natural abundanceTrace
Half-life (t1/2)6561(7) years[1]
Isotope mass240.0538135(20)[2] Da
Decay modes
Decay modeDecay energy (MeV)
Alpha decay5.25575(14)[2]
Isotopes of plutonium
Complete table of nuclides

240Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of 240Pu limits plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically disperses the core before full implosion is reached.[4][5] It decays by alpha emission to uranium-236.

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