Double beta decay

As in single beta decay, this process allows the atom to move closer to the optimal ratio of protons and neutrons.

As a result of this transformation, the nucleus emits two detectable beta particles, which are electrons or positrons.

In neutrinoless double beta decay, a hypothesized process that has never been observed, only electrons would be emitted.

The idea of double beta decay was first proposed by Maria Goeppert Mayer in 1935.

[3] In 1939, Wendell H. Furry proposed that if neutrinos are Majorana particles, then double beta decay can proceed without the emission of any neutrinos, via the process now called neutrinoless double beta decay.

[4] It is not yet known whether the neutrino is a Majorana particle, and, relatedly, whether neutrinoless double beta decay exists in nature.

[5] As parity violation in weak interactions would not be discovered until 1956, earlier calculations showed that neutrinoless double beta decay should be much more likely to occur than ordinary double beta decay, if neutrinos were Majorana particles.

[5] Efforts to observe the process in laboratory date back to at least 1948 when E.L. Fireman made the first attempt to directly measure the half-life of the 124Sn isotope with a Geiger counter.

In 1950, for the first time the double beta decay half-life of 130Te was measured by geochemical methods to be 1.4×1021 years,[7] reasonably close to the modern value.

Despite significant progress in experimental techniques in 1960–1970s, double beta decay was not observed in a laboratory until the 1980s.

[5] Double beta decay was first observed in a laboratory in 1987 by the group of Michael Moe at UC Irvine in 82Se.

None of those experiments have produced positive results for the neutrinoless process, raising the half-life lower bound to approximately 1025 years.

For some nuclei, such as germanium-76, the isobar one atomic number higher (arsenic-76) has a smaller binding energy, preventing single beta decay.

However, the isobar with atomic number two higher, selenium-76, has a larger binding energy, so double beta decay is allowed.

Similar suppression of energetically barely possible single beta decay occurs for 148Gd and 222Rn,[10] but both these nuclides are rather short-lived alpha emitters.

[11] The table below contains nuclides with the latest experimentally measured half-lives, as of December 2016, except for 124Xe (for which double electron capture was first observed in 2019).

Searches for double beta decay in isotopes that present significantly greater experimental challenges are ongoing.

[17] The following known beta-stable (or almost beta-stable in the cases 48Ca, 96Zr, and 222Rn[10])[18] nuclides with A ≤ 260 are theoretically capable of double beta decay, where red are isotopes that have a double-beta rate measured experimentally and black have yet to be measured experimentally: 46Ca, 48Ca, 70Zn, 76Ge, 80Se, 82Se, 86Kr, 94Zr, 96Zr, 98Mo, 100Mo, 104Ru, 110Pd, 114Cd, 116Cd, 122Sn, 124Sn, 128Te, 130Te, 134Xe, 136Xe, 142Ce, 146Nd, 148Nd, 150Nd, 154Sm, 160Gd, 170Er, 176Yb, 186W, 192Os, 198Pt, 204Hg, 216Po, 220Rn, 222Rn, 226Ra, 232Th, 238U, 244Pu, 248Cm, 254Cf, 256Cf, and 260Fm.

[9] The following known beta-stable (or almost beta-stable in the case 148Gd) nuclides with A ≤ 260 are theoretically capable of double electron capture, where red are isotopes that have a double-electron capture rate measured and black have yet to be measured experimentally: 36Ar, 40Ca, 50Cr, 54Fe, 58Ni, 64Zn, 74Se, 78Kr, 84Sr, 92Mo, 96Ru, 102Pd, 106Cd, 108Cd, 112Sn, 120Te, 124Xe, 126Xe, 130Ba, 132Ba, 136Ce, 138Ce, 144Sm, 148Gd, 150Gd, 152Gd, 154Dy, 156Dy, 158Dy, 162Er, 164Er, 168Yb, 174Hf, 180W, 184Os, 190Pt, 196Hg, 212Rn, 214Rn, 218Ra, 224Th, 230U, 236Pu, 242Cm, 252Fm, and 258No.

Neutrinoless double beta decay is a lepton number violating process.

With only two electrons in the final state, the electrons' total kinetic energy would be approximately the binding energy difference of the initial and final nuclei, with the nuclear recoil accounting for the rest.

where G is the two-body phase-space factor, M is the nuclear matrix element, and mββ is the effective Majorana mass of the electron neutrino.

Therefore, observing neutrinoless double beta decay, in addition to confirming the Majorana neutrino nature, can give information on the absolute neutrino mass scale and Majorana phases in the PMNS matrix, subject to interpretation through theoretical models of the nucleus, which determine the nuclear matrix elements, and models of the decay.

In order to remove backgrounds from cosmic rays, most experiments are located in underground laboratories around the world.

Some members of the Heidelberg-Moscow collaboration claimed a detection of neutrinoless beta decay in 76Ge in 2001.

[36] As of 2017, the strongest limits on neutrinoless double beta decay have come from GERDA in 76Ge, CUORE in 130Te, and EXO-200 and KamLAND-Zen in 136Xe.

For mass numbers with more than two beta-stable isobars, quadruple beta decay and its inverse, quadruple electron capture, have been proposed as alternatives to double beta decay in the isobars with the greatest energy excess.

In theory, quadruple beta decay may be experimentally observable in three of these nuclei – 96Zr, 136Xe, and 150Nd – with the most promising candidate being 150Nd.

[38] Neutrinoless quadruple beta decay would violate lepton number in 4 units, as opposed to a lepton number breaking of two units in the case of neutrinoless double beta decay.

Therefore, there is no 'black-box theorem'[definition needed] and neutrinos could be Dirac particles while allowing these type of processes.