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-)