Double beta decay

In the process of beta 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.

In double beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron anti-neutrinos are emitted. This process was first observed in laboratory in 1986 (the first effort of experimental observation was dated by 1948). Geochemical observation of the decay products (by extraction of Kr and Xe from very old Se and Te minerals) are known since 1950. Double beta decay is the rarest known kind of radioactive decay; it was observed for only 10 isotopes, and all of them have a mean life time 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 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 2.044MeV/c² (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.

Neutrinoless double beta decay

The processes described above are also known as two neutrino double beta decay, as two neutrinos (or anti-neutrinos) are emitted. If the neutrino is a Majorana particle, meaning that the anti-neutrino and the neutrino are actually the same particle then it is possible for neutrinoless double beta decay to occur. In neutrinoless double beta decay the emitted neutrino is immediately absorbed (as its anti-particle) by another nucleon of the nucleus, so the total kinetic energy of the two electrons would be exactly the difference in binding energy between the initial and final state nuclei. Numerous experiments have been carried out and proposed to search for neutrinoless double beta decay, as its discovery would indicate that neutrinos are indeed Majorana particles and allow a calculation of neutrino mass. For further information, see the NEMO3 page.

List of known double-beta decay isotopes

More than 60 naturally occurring isotopes are capable of undergoing double-beta decay. Only ten of them were observed to decay[1] (via the two-neutrino mode, allowed by the Standard Model): ⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹¹⁶Cd, ¹²⁸Te, ¹³⁰Te, ¹⁵⁰Nd, and ²³⁸U.

Many isotopes are, in theory, capable both of double-beta decay and other decays. In most cases, the double-beta decay is so rare as to be nearly impossible to observe against the background of other radiation. However, the double beta decay rate of 238U (also an alpha emitter) has been measured radiochemically; ²³⁸Pu is produced by this type of radioactivity. Two of the nuclides (⁴⁸Ca and ⁹⁶Zr) from the list above can decay also via single beta decay but this decay is extremely suppressed and was never observed.