Adapted for the Web by Stephen M. Karesh, PhD & Marsha Lipps CNMT
MODES OF DECAY
• Nuclei are composed of combinations of nucleons (protons and neutrons); certain combinations of these nucleons (i.e., certain nuclides) possess a high degree of stability while others are relatively unstable.
• Thus, one radioactive nuclide may have a high n/p ratio while for another the ratio may be low. A radionuclide may have an odd number of protons and an even number of neutrons, while for another the reverse may be true.
Because radioactive nuclides do differ from one another in so many respects, it is not surprising, then, that various modes of decay are possible, depending upon the nature of the nuclide and the type of instability.
• Unstable nuclei are said to be radioactive because they emit radiation as they undergo spontaneous decay. This radiation is emitted either from the nucleus itself or as a result of alterations in the configuration of orbital electrons about it. The nature of this radiation is a function of the mode of decay of a particular nuclear species.
• There are approximately 3,000 known nuclides, approximately 8-10% of which are stable; the other ~90% is radioactive and only a very small percent of these isotopes occurs in nature. In the following diagram, the black dots represent stable isotopes; all other colors represent different half-lives; for example, red = seconds, pink = minutes, orange = hours, yellow = days, etc.
Types of radiation resulting from radioactive decay can be summarized in a simple chart. Only X-rays, Auger electrons and internal conversion electrons arise from the outer orbitals; all other emissions are from the nucleus.