Natural Science Forum / Physics / Particle Physics / October 2006

Beta particles are electrons or positrons (electrons with positive
electric charge, or antielectrons). Beta decay occurs when, in a
nucleus with too many protons or too many neutrons, one of the protons
or neutrons is transformed into the other. In beta minus decay, a
neutron decays into a proton, an electron, and an antineutrino: n Æ p
+ e - +.

In beta plus decay, a proton decays into a neutron, a positron, and a
neutrino: p Æ n + e+ +n.

Both reactions occur because in different regions of the Chart of the
Nuclides, one or the other will move the product closer to the region
of stability. These particular reactions take place because
conservation laws are obeyed. Electric charge conservation requires
that if an electrically neutral neutron becomes a positively charged
proton, an electrically negative particle (in this case, an electron)
must also be produced. Similarly, conservation of lepton number
requires that if a neutron (lepton number = 0) decays into a proton
(lepton number = 0) and an electron (lepton number = 1), a particle
with a lepton number of -1 (in this case an antineutrino) must also be
produced. The leptons emitted in beta decay did not exist in the
nucleus before the decay-they are created at the instant of the
decay.

http://www.lbl.gov/abc/wallchart/chapters/03/2.html