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A. General Characteristics
From the discussions in the previous section, we know that the atoms of any element have two distinct parts: the nucleus, which contains the protons and neutrons, and the extranuclear space, which contains the electrons. The electrons in the atom, particularly those farthest from the nucleus, determine the chemical properties of the element. We will discuss electrons and the chemical properties of elements in detail in the next chapter.. In the remainder of this chapter, we will describe properties of the nucleus and, in particular, the characteristics of nuclear decay, which is also called radioactivity or radioactive decay of the nucleus.

In nuclear decay, the nuclei of radioactive atoms decay spontaneously to form other nuclei, a process that always results in a loss of energy and often involves the release of one or more small particles. Some atoms are naturally radioactive. Others that are normally stable can be made radioactive by bombarding them with subatomic particles. Often, one isotope of an element is radioactive and others of the same element are stable. A radioactive isotope is called a radioisotope.

Radioactivity is a common phenomenon. Of the 350 isotopes known to occur in nature, 67 are radioactive. Over a thousand radioactive isotopes have been produced in the laboratory. Every element, from atomic number 1 through number 109, has at least 1 natural or artificially produced radioactive isotope. Of the 3 known isotopes of hydrogen, one is radioactive - hydrogen-3, more commonly known as tritium. Oxygen, the Earth's most abundant element, has 8 known isotopes, 5 of which are radioactive (oxygen-13, -14, -15, -19, and -20). Iodine, an element widely used in nuclear medicine, has 24 known isotopes ranging in mass from 117 to 139 amu. Of these, only iodine-127 is stable; this isotope is the only naturally occurring one. Uranium has 14 known isotopes, all of which are radioactive.

B. Radioactive Emissions
Nuclei undergoing nuclear decay release various kinds of emissions. We will discuss three of these emissions: alpha particles, beta particles, and gamma rays. All three are forms of ionizing radiation, so called because their passage through matter leaves a trail of ions and molecular debris.

1. Alpha () particles

An alpha particle is identical to a helium atom that has been stripped of its two electrons; thus, an alpha particle contains two protons and two neutrons. Because an alpha particle has no electrons to balance the positive charge of the two protons, it has a charge of +2 and can be represented as He2+. If a particle has a charge, whether negative or positive, it can be shown as a superscript. Thus He2+ means a helium atom that has lost two electrons and has a +2 charge. The symbol O2- means an oxygen atom that has added two electrons and thus has a charge of -2. Atoms that have acquired a charge by losing or gaining electrons are called ions.

Besides He2+, other symbols for this particle are

When ejected from a decaying nucleus, alpha particles interact with all matter in their path, whether it be photographic film, lead shielding, or body tissue, stripping electrons from other atoms as they go. In their wake, they leave a trail of positive ions (atoms from which electrons have been removed) and free electrons. A single alpha particle, ejected at high speed from a nucleus, can create up to 100,000 ions along its path before it gains two electrons to become a neutral helium atom.

In air, an alpha particle travels about 4 cm before gaining the two electrons. Within body tissue, its average path is only a few thousandths of a centimeter. An alpha particle is unable to penetrate the outer layer of human skin. Because of this limited penetrating power, external exposure to alpha particles is not nearly as serious as internal exposure. If a source of alpha emissions is taken internally, the alpha radiation can do massive damage to the surrounding tissue; therefore alpha emitters are never used in nuclear medicine.

2. Beta ( ) particles
A beta particle is a high-speed electron ejected from a decaying nucleus; it carries a charge of -1. (The next section discusses how a nucleus can eject an electron even though it does not contain electrons.) A beta particle is represented as

Like alpha particles, beta particles cause the formation of ions by interacting with whatever matter is in their path. Beta particles are far less massive than alpha particles and carry a charge with only half the magnitude of that of the alpha particle. (This property depends only on the size of the charge, not its sign.) Thus beta particles produce less ionization and travel farther through matter before combining with a positive ion to become a neutral particle. The path of a beta particle in air can be 100 times that of an alpha particle. About 25 cm of wood, 1 cm of aluminum, or 0.5 cm of body tissue will stop a beta particle.

Because beta particles cause less ionization than alpha particles, beta particles are more suitable for use in radiation therapy, since the likelihood of damage to healthy tissue is greatly reduced. Beta emitters such as calcium-46, iron-59, cobalt-60, and iodine-131 are widely used in nuclear medicine.

3. Gamma () rays
The release of either alpha or beta particles from a decaying nucleus is generally accompanied by the release of nuclear energy in the form of gamma rays, represented as

Gammma rays have no charge or mass and are similar to X rays. Even though they bear no charge, gamma rays are able to produce ionization as they pass through matter. The degree of penetration of gamma rays through matter is much greater than that of either alpha or beta particles. The path length of a gamma ray can be as much as 400 m in air and 50 cm through tissue. Because of their penetrating power, gamma rays are especially easy to detect. Virtually all radioactive isotopes used in diagnostic nuclear medicine are gamma emitters. Each of the beta emitters listed in the previous paragraph is also a gamma emitter. Additional gamma emitters commonly used in nuclear medicine include chromium-51, arsenic-74, technetium-99, and gold-198.

The characteristics of alpha particles, beta particles, and gamma rays are summarized in Table 4.4.

TABLE 4.4 Characteristics of radioactive emissions
Name Symbol Charge Mass (amu) Penetration through matter
alpha particle +2 4 4.0 cm air
0.005 cm tissue
no penetration through lead
beta particle -1 5.5 X 10-4 6-300 cm air
0.006-0.5 cm tissue
.0005-0.03 cm lead
gamma ray 0 0 400 m air
500 cm tissue
3 cm lead

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