All the beta-radioactive nuclides important in nuclear reactors decay by emitting negative electrons. The daughter nuclide then has an atomic number one higher than the parent, as in the example of f|Sr given in Sec. 2.1. Beta emission differs from alpha emission in that beta particles from a particular nuclide undergoing decay have all energies between zero and a maximum energy characteristic of that nuclide. Figure 2.2 is an example of how beta-particle energies are distributed. The average energy is usually around one-third the maximum. This distribution of energy is explained by postulating that a second particle, the neutrino, is emitted along with the electron and that the sum of the energies carried by the electron and the neutrino equals the maximum observed beta energy. The average neutrino energy is thus about twice the average electron energy.

Neutrinos carry no charge, have little if any mass, and have practically no observable effects. Their range in matter is so great that their energy cannot be utilized. They have no present practical importance.

Beta-radioactive isotopes are known for every element. The half-lives and maximum energies of a few of the most important are listed in Table 2.2, together with their source.

Maximum energies range from 10,000 eV to about 4 MeV. Half-lives range from microseconds to billions of years, with large half-lives tending to correlate with lower energies.

The dependence of range of beta particles in aluminum on energy is shown in Fig. 2.3. Although beta particles have a range greater than alpha particles, they can be stopped by relatively thin layers of water, glass, or metal. The range of beta particles in tissue is great enough, however, to cause bums when the skin is exposed. Beta-active isotopes that may become fixed in the body are very toxic. 90Sr, which becomes fixed in bone, is an example. Those, like 8SKr or 14 C, that are turned over quickly by the body, are much less toxic.