Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 24

Not all elements have stable isotopes; they do exist for most of the light and mid‐weight elements, those with atomic numbers (number of protons) up to and including bismuth (Z = 83). However, there are no stable isotopes of technetium (Z = 43), promethium (Z = 61), or for all elements with atomic numbers higher than 83. Prominent examples are radium (Z = 88) and uranium (Z = 92), which are found naturally as a mix of isotopic forms that are all radioactive.

For those nuclei with a stable state there is an optimal ratio of neutrons to protons. For the lighter elements this ratio is approximately 1:1; for increasing atomic weights, stability is more likely when the number of neutrons exceeds the number of protons. A plot depicting the number of neutrons as a function of the number of protons is called the line of stability (Figure 1.12).