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Double beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process. ==History== The idea of double beta decay was first proposed by Maria Goeppert-Mayer in 1935. In 1937 Ettore Majorana theoretically demonstrated that all results of beta decay theory remain unchanged if the neutrino is its own anti-particle, i.e. a Majorana particle. In 1939 Wendell H. Furry proposed if neutrinos are a Majorana particle, double beta decay can proceed without emission of any neutrino, via the process now called the neutrinoless beta decay. In 1930–40s parity violation in weak interactions was not known and consequently calculations showed that neutrinoless double beta decay should be much more likely to occur than ordinary double beta decay (if neutrinos are Majorana particles). The predicted half-lives were on the order of 1015–16 years. Efforts to observe the process in laboratory date back to at least 1948 when Edward L. Fireman made the first attempt to measure the half-life of the isotope. Radiometric experiments through about 1960 produced negative results or false positives, not confirmed by later experiments. In 1950 for the first time the half-life of the was measured by geochemical methods, to be 1.4 years, reasonably close to the modern value.〔 In 1956 after the V-A nature of weak interactions was established it became clear the half-life of neutrinoless double beta decay would significantly exceed that of ordinary double beta decay. Despite significant progress in experimental techniques in 1960–70s, double beta decay was not observed in a laboratory until the 1980s. Experiments had only been able to establish the lower bound for the half-life—about 1021 years. On the other hand, geochemical experiments detected double beta decay of and .〔 Double beta decay was first observed in a laboratory in 1987 by the group of Michael Moe at UC Irvine on . Since then many experiments have observed ordinary double beta decay in other isotopes. None of those experiments has produced positive results for the neutrinoless process, raising the half-life lower bound to 1025 years. Geochemical experiments continued through the 1990s, producing positive results for a few more isotopes.〔 Double beta decay is the rarest known kind of radioactive decay; as of 2012 it has been observed for only 12 isotopes (including double electron capture in observed in 2001), and all have a mean lifetime over 1018 yr (table below).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double beta decay」の詳細全文を読む スポンサード リンク
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