ABSTRACT
Radionuclides are elements that have unstable nuclei which disintegrate or change spontaneously with a loss of energy in the form of ionising radiation (that is any radiation which displaces orbital electrons from atoms, so producing ions). The instability of an atomic nucleus is caused by an imbalance of the number of protons (Z) and neutrons (N) in the nuclei. Stable nuclei have neutron and proton numbers which are closely related. This is illustrated in Figure 8.1, which shows that stable nuclei occur only within a narrow band of increasing neutron and proton numbers. Unstable nuclei break down, ultimately forming stable nuclei. The most unstable nuclei disintegrate rapidly and do not now exist in measurable quantities in the environment. Other unstable nuclides, however, have a slow decay rate and still exist in significant amounts.
