In contrast to alpha decay, where only a few discrete decay energies are possible for each nuclear species, the spectrum of emitted beta particles is continuous. All beta-particle energies are possible from zero to a fixed maximum, called the endpoint energy. The endpoint energy in beta decay corresponds to the mass difference between the parent atom and the residual product, as would be expected from conservation of energy. However, the average energy of the beta particles is less than one-half the endpoint energy. When this was first discovered, it was thought that there might be a conflict with the demands of energy conservation.
It was eventually realized, by the early 1930s, that the electron shared the available energy (i.e., the energy corresponding to the mass difference between the initial and final constituents), with an elusive partner, which was termed the neutrino (v). In present usage, the "neutrino" emitted in decay is more precisely termed an anti-neutrino (F). The neutrino itself is emitted in the analogous process of [+ decay. When the distinction between the neutrino and anti-neutrino is not important, they are both generically termed neutrinos.13 In standard physics terminology, the and the v are called "particles," and the [3+ and the v are termed "anti-particles." According to very general considerations, in each beta decay one particle and one anti-particle are emitted.14
13 Unless otherwise indicated, we will use "neutrino" as a generic term and indicate the specific species by v or V.
14 There are two issues here. One is the trivial matter of establishing the (arbitrary) convention for choosing which is called the particle and which the anti-particle. We do not here consider the second and much more significant issue: Why are there both particles and anti-particles and what are the relationships between the particles and anti-particles? Ordinary matter is made up of particles and is termed matter, as distinct from anti-matter. There is no evidence for the existence of bulk anti-matter outside of science fiction, although individual anti-particles are observed in cosmic rays and can be created and observed under special laboratory conditions.
For many years it was common to say that the mass of the neutrino was zero. However, experimental studies during the past decade have provided convincing evidence that the neutrino has a small, nonzero mass. The precise value has not been determined, but it is less than 0.001% of the mass of the electron. Although the distinction between so small a mass and a zero mass is of fundamental importance in some areas of physics and astrophysics, it has no relevance to the understanding of phenomena related to nuclear energy.
The neutrino has zero charge and can typically pass through very large amounts of material without stopping. For example, a flux of neutrinos is not appreciably depleted in passing through the Earth.15
Was this article helpful?