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Nuclear Decay

Unstable nuclei, called radioactive isotopes, will undergo nuclear decay to make it more stable. There are only certain types of nuclear decay which means that most isotopes can't jump directly from being unstable to being stable. It often takes several decays to eventually become a stable nucleus. When unstable nuclei decay, the reactions generally involve the emission of a particle and or energy. Half-lives are characteristic properties of the various unstable atomic nuclei and the particular way in which they decay. Alpha and beta decay are generally slower processes than gamma decay. Half-lives for beta decay range upward from 10-2 sec and, for alpha decay, upward from about 10-6 sec. Bismuth-209 has the longest half-life of 2x1019 years. Half-lives for gamma decay may be too short to measure (~ 10-14 second), though a wide range of half-lives for gamma emission has been reported.

Table 14-01 below summarizes the various types of nuclear decay with a few examples.

Type Emission Penetrating Power Example
Alpha Decay Helium nuclei 1, stopped by skin, very damaging due to ionization 92U238 90Th234 + 2He4
Applicable to nuclei with Z>83, see Figure 14-02
Beta Decay Electron, high speed 100, penetrates human tissue to ~ 1 cm 53I131 54Xe131 + -1e0
Applicable to nuclei with high neutron-proton ratio
Gamma Decay Photons, high energy 10000, highly penetrating but not very ionizing 92U238 90Th234 + 2He4 + 2 photon
Energy lost from settling within the nucleus after transmutation
Positron Emission Positron 100 6C11 5B11 + 1e0
Applicable to nuclei with a low neutron-proton ratio
Electron Capture Electron, inner shell ~ Infinite for Neutrino 37Rb81 + -1e0 36Kr81 + neutrino
Applicable to nuclei with a low neutron-proton ratio

Table 14-01 Types of Nuclear Decay

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