Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology. Rachel A. Powsner

the atomic nucleus also has an inner structure (Figure 1.8). Experiments showed that the nucleus consists of two types of particles: protons, which carry a positive charge, and neutrons, which carry no charge. The general term for protons and neutrons is nucleons. The nucleons have a much greater mass than electrons. Table 1.1 reviews the properties of the various subatomic particles.

A simple but useful model of the nucleus is a tightly bound cluster of protons and neutrons. Protons naturally repel each other since they are positively charged; however, there is a powerful binding force called the nuclear force that holds the nucleons together very tightly (Figure 1.9). The work (energy) required to overcome the nuclear force, the work to remove a nucleon from the nucleus, is called the nuclear binding energy. Typical binding energies are in the range of 2 million to 9 million electron volts (MeV) (approximately one thousand to one million times the electron binding force). The magnitude of the binding energy is related to another fact of nature: the measured mass of a nucleus is always less than the mass expected from the sum of the masses of its neutrons and protons. The “missing” mass is called the mass defect, the energy equivalent of which is equal to the nuclear binding energy. This interchangeability of mass and energy was immortalized in Einstein’s equation E = mc².

Figure 1.8 The nucleus of an atom is composed of protons and neutrons.

Table 1.1 Properties of the subatomic particles

Name(s)	Symbol	Mass a	Charge
Neutron	N	1839	None
Proton	P	1836	Positive (+)
Electron	e–	1	Negative (–)
Beta particle (beta minus particle, electron)b	Β–	1	Negative (–)
Positron (beta plus particle, positive electron)	β+	1	Positive (+)
Gamma ray (photon)	γ	None	None
X‐ray	X‐ray	None	None
Neutrino	ν	Near zero	None
Antineutrino		Near zero	None

      ^a Relative to an electron.

      ^b There is no physical difference between a beta particle and an electron; the term beta particle is applied to an electron that is emitted from a radioactive nucleus. The symbol β without a minus or plus sign attached always refers to a beta minus particle or electron.

Figure 1.9 Nuclear binding force is strong enough to overcome the electrical repulsion between the positively charged protons.

       Isotopes, isotones, and isobars:

      Each atom of any sample of an element has the same number of protons (the same Z: atomic number) in its nucleus. Lead found anywhere in the world will always be composed of atoms with 82 protons. The same does not apply, however, to the number of neutrons in the nucleus.

Figure 1.10 Standard atomic notation.

Figure 1.11 Nuclides of the same atomic number but different atomic mass are called isotopes, those of an equal number of neutrons are called isotones, and those of the same atomic mass but different atomic number are called isobars. Stable nuclear configurations are shaded gray, radioactive configurations are white.

      (Adapted from Brucer, M. Trilinear Chart of the Nuclides, Mallinkrodt Inc, 1979.)

An isotope of an element is a particular variation of the nuclear composition of the atoms of that element. The number of protons (Z: atomic number) is unchanged, but the number of neutrons (N) varies. Since the number of neutrons changes, the total number of neutrons and protons (A: the atomic mass) changes. The chemical symbol for each element can be expanded to include these three numbers (Figure 1.10).

Two related entities are isotones and isobars. Isotones are atoms of different elements that contain identical numbers of neutrons but varying numbers of protons. Isobars are atoms of different elements with identical numbers of nucleons. Examples of these are illustrated in Figure 1.11. Nuclide is a general term for the composition of a nucleus and includes isotopes, isotones, isobars, and other nuclear configurations.

       The stable nucleus:

      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