Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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(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.

Schematic illustration of 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.
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

      Inhalation of radon gas is the second largest cause of lung cancer worldwide, second only to cigarette smoking. In the 1960s, underground uranium miners began to show unusually high incidences of lung cancer. The cause was shown