Alpha-Decay Systematics for Elements with 50 < Z < 83
From observing the slope of the mass-defect curve, one notes that most isotopes whose mass numbers are ≳ 140 are unstable toward a-emission. However, with the exception of naturally occurring 147Sm, α-decay was not observed for elements below bismuth until 30 years ago. This was due to the fact that the rate for a-decay is a very sensitive exponential function of the decay energy. The energy available for decay increases rapidly with mass, so that in the region above lead α-decay becomes a dominant decay mode. It was also known that, if one could produce nuclides sufficiently far to the neutron-deficient side of the β-stability line, then with nuclei Z < 83 would undergo α-particle emission. This was shown to be true experimentally in 1949 when Thompson, Ghiorso, Rasmussen and Seaborg1 reported the discovery of α-radioactivity in proton-rich isotopes of gold, mercury and the rare earths. Since that time, with the availability of new accelerators, the number of known α-active nuclides below bismuth has steadily increased.
KeywordsLead Isotope Mass Formula Neutron Number Mercury Isotope Dominant Decay Mode
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