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Radioactive isotopes possess unstable nuclei whose nuclear configurations tend to be spontaneously transformed by radioactive decay. Radioactive decay occurs when the nucleus of an unstable parent isotope is transformed into that of a daughter isotope. Daughter isotopes have different atomic numbers and/or different atomic mass numbers from their parent isotopes. Three major radioactive decay processes (Figure 3.12) have been recognized: alpha (α) decay, beta (β) decay, and electron capture.

Alpha decay involves the ejection of an alpha (α) particle plus gamma (γ) rays and heat from the nucleus. An alpha particle consists of two protons and two neutrons, which is the composition of a helium (⁴He) nucleus. The ejection of an alpha particle from the nucleus of a radioactive element reduces the atomic number of the element by two (2p⁺) while reducing its atomic mass number by four (2p⁺ + 2n⁰). The spontaneous decay of uranium‐238 (²³⁸U) into thorium‐234 (²³⁴Th) is but one of many examples of alpha decay.

Beta decay involves the ejection of a beta (β) particle plus heat from the nucleus. A beta particle is a high‐speed electron (e⁻). The ejection of a beta particle from the nucleus of a radioactive element converts a neutron into a proton (n⁰ − e⁻ = p⁺) increasing the atomic number by one while leaving the atomic mass number essentially unchanged. The spontaneous decay of radioactive rubidium‐87 (Z = 37) into stable strontium‐87 (Z = 38) is one of many examples of beta decay.

Electron capture involves the addition of a high‐speed electron to the nucleus with the release of heat in the form of gamma rays. It can be visualized as the reverse of beta decay. The addition of an electron to the nucleus converts one of the protons into a neutron (p⁺ + e⁻ = n⁰). Electron capture decreases the atomic number by one while leaving the atomic mass number unchanged. The decay of radioactive potassium‐40 (Z = 19) into stable argon‐40 (Z = 18) is a useful example of electron capture. It occurs at a known rate, which allows the age of many potassium‐bearing minerals and rocks to be determined. Only about 9% of radioactive potassium decays into argon‐40; the remainder decays into calcium‐40 (⁴⁰Ca) by beta emission.

The heat released by radioactive decay is called radiogenic heat. Radiogenic heat is a major source of the heat generated within Earth. It is an important driver of global tectonics and many of the rock‐producing processes discussed in this text, including magma generation and metamorphism.

The time required for one half of the radioactive isotope to be converted into a new isotope is called its half‐life and may range from seconds to billions of years. Radioactive decay processes continue until a stable nuclear configuration is achieved and a stable isotope is formed. The radioactive decay of a parent isotope into a stable daughter isotope may involve a sequence of decay events. Table 3.2 illustrates the 14‐step process required to convert radioactive uranium‐238 (²³⁸U) into the stable daughter isotope lead‐206 (²⁰⁶Pb). All of the intervening isotopes are unstable, so that the radioactive decay process continues. The first step, the conversion of²³⁸U into thorium‐234 by alpha decay, is slow, with a half‐life of 4.47 billion years (4.47 Ga). Because many of the remaining steps are relatively rapid, the half‐life of the full decay, sequence is just over 4.47 Ga. As the rate at which²³⁸U atoms are ultimately converted into²⁰⁶Pb atoms is known, the ratio²³⁸U/²⁰⁶Pb can be used to determine the crystallization ages for minerals, especially for those formed early in Earth's history, as explained below.

Those isotopes that decay rapidly, beginning with protactinium‐234 and radon‐222, produce large amounts of decay products in short amounts of time. Radioactive decay products, especially alpha particles, can produce notable damage to crystal structures and significant tissue damage in human populations (Box 3.2). Radioactive isotopes also have significant applications in medicine, especially in cancer treatments. Radioactive decay provides a significant energy source through nuclear fission in reactors. Radioactive materials remain important global energy resources, even though the radioactive isotopes in spent fuel present long‐term hazards, expecially with respect to their disposal. As noted above, radioactive decay is also the primary heat engine within Earth and is partly responsible for driving plate tectonics and core–mantle convection. Without radioactive heat, Earth would be a very different kind of home.

Figure 3.12 Three types of radioactive decay: alpha decay, beta decay, and electron capture (gamma decay) and the changes in nuclear configuration that occur as the parent isotope decays into a daughter isotope.

Table 3.2 The 14‐step radioactive decay sequence that occurs in the conversion of the radioactive isotope²³⁸U into the stable isotope²⁰⁶Pb.

Parent isotope	Daughter isotope	Decay process	Half‐life
Uranium‐238	Thorium‐234	Alpha	4.5 × 109 years
Thorium‐234	Protactinium‐234	Beta	24.5 days
Protactinium‐234	Uranium‐234	Beta	1.1 minutes
Uranium‐234	Thorium‐230	Alpha	2.3 × 105 years
Thorium‐230	Radium‐226	Alpha	8.3 × 104 years
Radium‐226	Radon‐222	Alpha	1.6 × 103 years
Radon‐222	Polonium‐218	Alpha	3.8 days
Polonium‐218	Lead‐214	Alpha	3.1 minutes
Lead‐214	Bismuth‐214	Beta	26.8 minutes
Bismuth‐214	Polonium‐214	Beta	19.7 minutes
Polonium‐214	Lead‐210	Alpha	1.5 × 10−4 seconds
Lead‐210	Bismuth‐210	Beta	22.0 years
Bismuth‐210	Polonium‐210	Beta	5.0 days
Polonium‐210	Lead‐206	Alpha	140 days