Double beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process.
In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted. In the process of beta minus decay, unstable nuclei decay by converting a neutron in the nucleus to a proton and emitting an electron and an electron antineutrino. In order for beta decay to be possible, the final nucleus must have a larger binding energy than the original nucleus. For some nuclei, such as germanium-76, the nucleus with atomic number one higher has a smaller binding energy, preventing beta decay from occurring. However, the nucleus with atomic number two higher, selenium-76, has a larger binding energy, so the double-beta decay process is allowed. This process is allowed because the Q-value of the first beta reaction is less than the Q-value of the second beta reaction.
Although efforts to observe the process date back to 1948, it was first observed in a laboratory setting in 1987 by a group led by Michael Moe at the UC Irvine on isotope 82Se. Geochemical observation of the decay products (by extraction of krypton and xenon from very old selenium and tellurium minerals) are known since 1950. Double-beta decay is the rarest known kind of radioactive decay; it has been observed for only 12 isotopes, and all of them have a mean lifetime of more than 1019 yr.
For some nuclei, the process occurs as conversion of two protons to neutrons, with emission of two electron neutrinos and absorption of two orbital electrons (double electron capture). If the mass difference between the parent and daughter atoms is more than 1.022 MeV/c2 (two electron masses), another decay branch is accessible, with capture of one orbital electron and emission of one positron. When the mass difference is more than 2.044 MeV/c2 (four electron masses), the emission of two positrons is possible. These theoretically possible double-beta decay branches have not yet been observed.
Read more about Double Beta Decay: List of Known Double-beta Decay Isotopes, Neutrinoless Double-beta Decay
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