| In the process of beta decay unstable nuclei decay by converting a neutron in the nucleus to a proton and emitting an electron and anti-neutrino. 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 nuclei with atomic number one higher has a smaller binding energy, preventing beta decay from occurring. In the case
of germanium-76 the nuclei with atomic number two higher, selenium-76 has a larger binding energy, so the "double beta decay" process is allowed.
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| In double beta decay two neutrons in the nuclei are converted to protons, and two electrons and two anti-neutrinos are emitted. This process was first observed in 1986. It is the rarest known kind
of radioactive decay; it was observed for only 10 isotopes, and all of them have the mean life time more than 1019 yr.
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| For some nuclei, the process occurs as conversion of two protons to neutrons, with emission of two neutrinos and absorption of two orbital electrons (double electron capture). If mass difference
between the parent and daughter atoms is more than 1022 keV (two electron masses), another branch of the process becomes possible,
with capture of one orbital electron and emission of one positron. And, at last, when the mass difference is more than 2044
keV (four electron masses), the third branch of the decay arises, with emission of two positrons. All these kinds of double
beta decay are predicted but not observed yet.
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