TABLE OF CONTENTS Radioactivity is the spontaneous disintegration of an atom that is accompanied by emission of radiation. There are many radioactive elements that are isotopes (having the same atomic number but different atomic mass) of nonradioactive elements. An atom of a radioactive isotope has the same number of orbital electrons as an atom of its nonradioactive counterpart and, in general, will behave chemically and biologically like the nonradioactive species. Therefore experimental and diagnostic as well as analytical procedures can utilize atoms of radioactive isotopes as tracers. The difference between the radioactive and the nonradioactive atoms of identical elements is the number of neutrons in the nucleus, the number of protons and electrons being the same for all. (Some elements have more than two isotopes.)
All radioactive nuclides exhibit the phenomenon of radioactive decay with emission of one or more of several types of radiation. The modes of decay with associated radiations are summarized in Table 11.1. In many cases, the product nucleus is left in an excited state after a decay [ ?, ?, electron capture (EC)]. Stability is achieved by emission of gamma radiation from the nucleus. Sometimes this process takes place very quickly; otherwise gamma radiation may be delayed, perhaps by many days. The latter situation is known as an isomeric transition, and the nucleus is said to be in a metastable state, denoted by m after the mass number.
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