2.1. Nuclear Decay

Nuclides (X) are the nuclei of atoms of a specific isotope. They are characterised by the number of positively charged protons ( Z), neutrons ( N) and the energy state of the nucleus. In terms of mass ( A) and atomic number ( Z) a nuclide is

(2.1)

where A = Z + N. To be regarded as distinct a nuclide must contain enough energy to have a measurable lifetime, usually more than 10 ^-10 s. An isotope refers to a type of atom whereas a nuclide refers to its nucleus. Isotopes have the same atomic number Z as the nuclide, but different numbers of neutrons and hence atomic mass A:

(2.2)

Nuclides may be stable or unstable. Radionuclides are nuclides that possess properties of spontaneous disintegration. About 1700 nuclides are known, of which about 300 are stable and the rest radioactive.

The spontaneous decay of unstable nuclides with the emission of particles or photons is a statistical process termed radioactive decay. The decay modes are alpha, beta, gamma, electron capture, proton emission, neutron emission, cluster radioactivity and spontaneous fission.

(2.3)

Depending on the mode of decay radionuclides change their parameters as shown in Table 2.1.