Single Beta Decay
Beta decay is a nuclear transition, where the atomic number Z of the nucleus changes by one unit, while atomic mass A remains the same. This results in three possible decay modes:
β− − decay
β+ − decay
The basic underlying mechanism for (1) is given by
on the quark level respectively, see Figure 1. The other decay modes are understood in an analogous way.
The corresponding decay energies are given by the following relations, where denotes the mass of the neutral atom (not the nucleus) :
β− - decay:
The Q-value corresponds exactly to the mass difference between the mother and the daughter atom. It represents the available energy in a nuclear transition.
β+ - decay:
Because all masses are given for atoms, this decay requires the rest mass of two electrons. Therefore, the mass difference between both has to be larger than for β+ -decay to occur.
The Q-values of the last two reactions are related by
If Q is larger than , both electron capture and β+-decay are competitive processes, because they lead to the same daughter nucleus. For smaller Q-values only electron capture will occur. Obviously, for any of the modes to occur the corresponding Q-value has to be larger than zero .
 Kai Zuber, Neutrino Physics, Institute of Physics (2004)
Figure 1: Neutron Beta Decay