Beta particles are negatively charged electrons that are emitted from the nucleus as part of a radioactive disintegration. The beta particles emitted have a continuous range of energies up to a maximum. This appeared at first to be a violation of the conservation of energy. To overcome this problem, in 1931 Wolfgang Pauli proposed that another particle was emitted which he called the neutrino (v). He suggested that this particle had a very small mass and zero charge. It could carry away the excess momentum to account for the difference between the maximum beta energy and the spectrum of energies that the emitted beta particles displayed. In fact, we now refer to the neutrino emitted in beta-minus decay as the antineutrino, indicated by the '-' over the symbol v. P- decay is an example of a weak interaction, and is different to most other fundamental decays as parity is not conserved.
The following shows an example of a beta decay scheme for 131I:
The half-life for 131I decay is 8.02 days. The most abundant P particle emitted from 131I has a maximum energy of 0.606 MeV and there are many associated y rays, the most abundant (branching ratio = 0.81) having an energy of 0.364 MeV.
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