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Basic Atomic and Nuclear Physics
Radioactivity is a process involving events in individual atoms and nuclei. Before discussing radioactivity, therefore, it is worthwhile to review some of the basic concepts of atomic and nuclear physics.
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Quantities and Units
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Types of Quantities and Units
Physical properties and processes are described in terms of quantities such as time and energy. These quantities are measured in units such as seconds and joules. Thus a quantity describes what is measured, whereas a unit describes how much .
Physical quantities are characterized as fundamental or derived. A base quantity is one that “stands alone”; that is, no reference is made to other quantities for its definition. Usually, base quantities and their units are defined with reference to standards kept at national or international laboratories. Time (s or sec), distance (m), and mass (kg) are examples of base quantities. Derived quantities are defined in terms of combinations of base quantities. Energy (kg · m 2 /sec 2 ) is an example of a derived quantity.
The international scientific community has agreed to adopt so-called System International (SI) units as the standard for scientific communication. This system is based on seven base quantities in metric units, with all other quantities and units derived by appropriate definitions from them. The four quantities of mass, length, time and electrical charge are most relevant to nuclear medicine. The use of specially defined quantities (e.g., “atmospheres” of barometric pressure) is specifically discouraged. It is hoped that this will improve scientific communication, as well as eliminate some of the more irrational units (e.g., feet and pounds). A useful discussion of the SI system, including definitions and values of various units, can be found in reference 1.
SI units or their metric subunits (e.g., centimeters and grams) are the standard for this text; however, in some instances traditional or other non-SI units are given as well (in parentheses). This is done because some traditional units still are used in the day-to-day practice of nuclear medicine (e.g., units of activity and absorbed dose). In other instances, SI units are unreasonably large (or small) for describing the processes of interest and specially defined units are more convenient and widely used. This is particularly true for units of mass and energy, as discussed in the following section.
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Mass and Energy Units
Events occurring at the atomic level, such as radioactive decay, involve amounts of mass and energy that are very small when described in SI or other conventional units. Therefore they often are described in terms of specially defined units that are more convenient for the atomic scale.
The basic unit of mass is the unified atomic mass unit, abbreviated u. One u is defined as being equal to exactly 
the mass of an unbound 12 C atom * at rest and in its ground state. The conversion from SI mass units to unified atomic mass units is
* Atomic notation is discussed in Section D.2.
The universal mass unit often is called a Dalton (Da) when expressing the masses of large biomolecules. The units are equivalent (i.e., 1 Da = 1 u). Either unit is convenient for expressing atomic or molecular masses, because a hydrogen atom has a mass of approximately 1 u or 1 Da.
The basic unit of energy is the electron volt (eV ). One eV is defined as the amount of energy acquired by an electron when it is accelerated through an electrical potential of 1 V. Basic multiples are the kiloelectron volt (keV ) (1 keV = 1000 eV ) and the megaelectron volt (MeV ) (1 MeV = 1000 keV = 1,000,000 eV ). The conversion from SI energy units to the electron volt is
Mass m and energy E are related to each other by Einstein's equation E = mc 2 , in which c is the velocity of light (approximately 3 × 10 8 m/sec in vacuum). According to this equation, 1 u of mass is equivalent to 931.5 MeV of energy.
Relationships between various units of mass and energy are summarized in Appendix A . Universal mass units and electron volts are very small, yet, as we shall see, they are quite appropriate to the atomic scale.
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Radiation
The term radiation refers to “energy in transit.” In nuclear medicine, we are interested principally in the following two specific forms of radiation:
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Particulate radiation, consisting of atomic or subatomic particles (electrons, protons, etc.) that carry energy in the form of kinetic energy of mass in motion.
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Electromagnetic radiation, in which energy is carried by oscillating electrical and magnetic fields traveling through space at the speed of light.
Radioactive decay processes, discussed in Chapter 3 , result in the emission of radiation in both of these forms.
The wavelength, λ, and frequency, ν, of the oscillating fields of electromagnetic radiation are related by:
where c is the velocity of light.
Most of the more familiar types of electromagnetic radiation (e.g., visible light and radio waves) exhibit “wavelike” behavior in their interactions with matter (e.g., diffraction patterns and transmission and detection of radio signals). In some cases, however, electromagnetic radiation behaves as discrete “packets” of energy, called photons (also called quanta ). This is particularly true for interactions involving individual atoms. Photons have no mass or electrical charge and also travel at the velocity of light. These characteristics distinguish them from the forms of particulate radiation mentioned earlier. The energy of the photon E, in kiloelectron volts, and the wavelength of its associated electromagnetic field λ (in nanometers) are related by
Figure 2-1 illustrates the photon energies for different regions of the electromagnetic spectrum. Note that x rays and γ rays occupy the highest-energy, shortest-wavelength end of the spectrum; x-ray and γ-ray photons have energies in the keV-MeV range, whereas visible light photons, for example, have energies of only a few electron volts. As a consequence of their high energies and short wavelengths, x rays and γ rays interact with matter quite differently from other, more familiar types of electromagnetic radiation. These interactions are discussed in detail in Chapter 6 .
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Atoms
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