The
radioactive decay of one
isotope to another by the emission of a
beta particle (a.k.a. an
electron). The
beta particle is emitted by a
neutron, which becomes a
proton (an
electron anti-neutrino is also emitted). This is the process by which
Plutonium-239 is formed during the fissioning of
Uranium. (U238 + n -> U239 ->
Np239 + e- ->
Pu239 + e-)