Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Species of atoms characterized by the number of protons, neutrons, and the energycontent of the atomic nucleus are called nuclides. Isotopes, isobars, isotones, andisomers are families of nuclides that share specific properties (Table 2-2). An easyway to remember these relationships is to associate the p in isotopes with the samenumber of protons, the a in isobars with the same atomic mass number, the n inisotones with the same number of neutrons, and the e in isomer with the differentnuclear energy states.Only certain combinations of neutrons and protons in the nucleus are stable. On aplot of Z versus N these nuclides fall along a "line of stability" for which the N/Zratio is approximately 1 for low Z nuclides and approximately 1.5 for high Znuclides, as shown in Fig. 2-7. A higher neutron-to-proton ratio is required in heavyelements to offset the coulombic repulsive forces between protons by providingincreased separation of protons. Nuclei with an odd number of neutrons and anodd number of protons tend to be unstable, whereas nuclei with an even numberof neutrons and an even number protons more frequently are stable. The numberof stable nuclides identified for different combinations of neutrons and protons isshown in Table 2-3. Nuclides with an odd number of nucleons are capable of producinga nuclear magnetic resonance (NMR) signal, as described in Chapter 14.Combinations of neutrons and protons that are not stable do exist but over timethey will permute to nuclei that are stable. Two kinds of instability are neutronexcess and neutron deficiency (i.e., proton excess). Such nuclei have excess internalenergy tompared with a stable arrangement of neutrons and protons. They achievestability by the conversion of a neutron to a proton or vice versa, and these eventsare accompanied by the emission of energy. The energy emissions include particulateand electromagnetic radiations. Nuclides that decay (i.e., transform) to morestable nuclei are said to be radioactive, and the transformation process itself is calledradioactive decay or radioactive disintegration. There are several types of radioactivedecay and these are discussed in detail in Chapter 18. A nucleus may undergo sev-TABLE 2-2. NUCLEAR FAMILIES: ISOTOPES, ISOBARS, ISOTONES,AND ISOMERSIsotopesIsobarsIsotonesAtomic number (Z)Mass number (A)Number of neutrons (A-Z)Atomic and mass numbers butdifferent energy states1-131 and 1-125: Z = 53Mo-99 and Tc-99: A = 99531-131: 131 - 53 = 7854Xe-132: 132 - 54 = 78Tc-99m and Tc-99: Z = 43A= 99Energy of Tc-99m > Tc-99: ilE = 142 keY