Double beta decay

thumb|300px|Leading order Feynman diagram for ordinary double beta decay (<math>\beta^- \beta^-</math> mode)

In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. 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.

The literature distinguishes between two types of double beta decay: ordinary double beta decay and neutrinoless double beta decay. In ordinary double beta decay, which has been observed in several isotopes, two electrons and two electron antineutrinos are emitted from the decaying nucleus. In neutrinoless double beta decay, a hypothesized process that has never been observed, only electrons would be emitted.

History

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

In 1937, Ettore Majorana demonstrated that all results of beta decay theory remain unchanged if the neutrino were its own antiparticle, now known as a Majorana particle.

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.

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

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. The predicted half-lives were on the order of ~&nbsp;years.

Radiometric experiments through about 1960 produced negative results or false positives, not confirmed by later experiments. In 1950, for the first time the double beta decay half-life of was measured by geochemical methods to be 1.4×&nbsp;years,

reasonably close to the modern value. This involved detecting the concentration in minerals of the xenon produced by the decay.

In 1956, after the V − A nature of weak interactions was established, it became clear that the half-life of neutrinoless double beta decay would significantly exceed that of ordinary double beta decay. Despite significant progress in experimental techniques in 1960–1970s, 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 &nbsp;years. At the same time, geochemical experiments detected the double beta decay of and .

Since then, many experiments have observed ordinary double beta decay in other isotopes. None of those experiments have produced positive results for the neutrinoless process, raising the half-life lower bound to approximately &nbsp;years. Geochemical experiments continued through the 1990s, producing positive results for several isotopes. Similar suppression of energetically barely possible single beta decay occurs for ¹⁴⁸Gd and ²²²Rn, but both these nuclides are rather short-lived alpha emitters.

Fourteen isotopes have been experimentally observed undergoing two-neutrino double beta decay (β^–β^–) or double electron capture (εε). The table below contains those nuclides with the latest experimentally measured half-lives for them. Where two uncertainties are specified, the first one is statistical uncertainty and the second is systematic.

{| class="wikitable" style="text-align:center;"

!Nuclide!!Half-life, 10²¹ years

!Mode!!Transition!!Method!!Experiment

|-

|||0.064 ±

|β^–β^–|| || direct || NEMO-3

|-

|-

||| 1.926

|β^–β^–

| || direct || GERDA

|-

||| 11 ± 2 ± 1

|εε

| || direct || XENON1T

|-

|||2.165 ± 0.016 ± 0.059

|β^–β^–

| || direct || EXO-200

|-

| rowspan=2||| 0.00911 ± 0.00063

|β^–β^–

| ||rowspan=2| direct || NEMO-3

The following known beta-stable (or almost beta-stable in the cases of ⁴⁸Ca, ⁹⁶Zr, and ²²²Rn 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, , ⁷⁰Zn, , ⁸⁰Se, , ⁸⁶Kr, ⁹⁴Zr, , ⁹⁸Mo, , ¹⁰⁴Ru, ¹¹⁰Pd, ¹¹⁴Cd, , ¹²²Sn, ¹²⁴Sn, , , ¹³⁴Xe, , ¹⁴²Ce, ¹⁴⁶Nd, ¹⁴⁸Nd, , ¹⁵⁴Sm, ¹⁶⁰Gd, ¹⁷⁰Er, ¹⁷⁶Yb, ¹⁸⁶W, ¹⁹²Os, ¹⁹⁸Pt, ²⁰⁴Hg, ²¹⁶Po, ²²⁰Rn, ²²²Rn, ²²⁶Ra, ²³²Th, , ²⁴⁴Pu, ²⁴⁸Cm, ²⁵⁴Cf, ²⁵⁶Cf, and ²⁶⁰Fm.

The following known beta-stable (or almost beta-stable in the case of 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, , ⁸⁴Sr, ⁹²Mo, ⁹⁶Ru, ¹⁰²Pd, ¹⁰⁶Cd, ¹⁰⁸Cd, ¹¹²Sn, ¹²⁰Te, , ¹²⁶Xe, , 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.

The observation of neutrinoless double beta decay would require that at least one neutrino is a Majorana particle, irrespective of whether the process is engendered by neutrino exchange.

Experiments

Numerous experiments have searched for neutrinoless double beta decay. The best-performing experiments have a high mass of the decaying isotope and low backgrounds, with some experiments able to perform particle discrimination and electron tracking. In order to remove backgrounds from cosmic rays, most experiments are located in underground laboratories around the world.

Recent and proposed experiments include:

• Completed experiments:
• Gotthard TPC
• Heidelberg-Moscow, ⁷⁶Ge detectors (1997–2001)
• IGEX, ⁷⁶Ge detectors (1999–2002)
• NEMO, various isotopes using tracking calorimeters (2003–2011)
• Cuoricino, ¹³⁰Te in ultracold TeO₂ crystals (2003–2008)
• Experiments taking data as of November 2017:
• AMoRE, ¹⁰⁰Mo enriched CaMoO₄ crystals at YangYang underground laboratory
• COBRA, ¹¹⁶Cd in room temperature CdZnTe crystals
• CUORE, ¹³⁰Te in ultracold TeO₂ crystals
• EXO, a ¹³⁶Xe and ¹³⁴Xe search
• GERDA, a ⁷⁶Ge detector
• KamLAND-Zen, a ¹³⁶Xe search. Data collection from 2011.
• XMASS using liquid Xe
• Proposed/future experiments:
• CUPID, neutrinoless double-beta decay of ¹⁰⁰Mo
• CANDLES, ⁴⁸Ca in CaF_2, at Kamioka Observatory
• MOON, developing ¹⁰⁰Mo detectors
• nEXO, using liquid ¹³⁶Xe in a time projection chamber
• LEGEND, Neutrinoless Double-beta Decay of ⁷⁶Ge.
• LUMINEU, exploring ¹⁰⁰Mo enriched ZnMoO₄ crystals at LSM, France.
• NEXT, a Xenon TPC. NEXT-DEMO ran and NEXT-100 will run in 2016.
• SNO+, a liquid scintillator, will study ¹³⁰Te
• SuperNEMO, a NEMO upgrade, will study ⁸²Se
• TIN.TIN, a ¹²⁴Sn detector at INO
• PandaX-III, an experiment with 200&nbsp;kg to 1000&nbsp;kg of 90% enriched ¹³⁶Xe
• DUNE, a TPC filled with liquid Argon doped with ¹³⁶Xe.
• NuDoubt⁺⁺ will study double beta plus decays of ⁷⁸Kr in a pressurized hybrid-opaque liquid scintillation detector

Status

While some experiments have claimed a discovery of neutrinoless double beta decay, modern searches have found no evidence for the decay.

Heidelberg-Moscow controversy

Some members of the Heidelberg-Moscow collaboration claimed a detection of neutrinoless beta decay in ⁷⁶Ge in 2001. This claim was criticized by outside physicists as well as other members of the collaboration. In 2006, a refined estimate by the same authors stated the half-life was 2.3&nbsp;years. This half-life has been excluded at high confidence by other experiments, including in ⁷⁶Ge by GERDA.

Current results

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.

Higher order simultaneous beta decay

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. These decays are energetically possible in eight nuclei, though partial half-lives compared to single or double beta decay are predicted to be very long; hence, quadruple beta decay is unlikely to be observed. The seven candidate nuclei for quadruple beta decay include ⁹⁶Zr, ¹³⁶Xe, and ¹⁵⁰Nd capable of quadruple beta-minus decay, and ¹²⁴Xe, ¹³⁰Ba, ¹⁴⁸Gd, and ¹⁵⁴Dy capable of quadruple beta-plus decay or electron capture (though ¹⁴⁸Gd and ¹⁵⁴Dy are non-primordial alpha-emitters with geologically short half-lives). 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. Triple beta-minus decay is also possible for ⁴⁸Ca, ⁹⁶Zr, and ¹⁵⁰Nd; 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' and neutrinos could be Dirac particles while allowing these type of processes. In particular, if neutrinoless quadruple beta decay is found before neutrinoless double beta decay then the expectation is that neutrinos will be Dirac particles.

, searches for triple and quadruple beta decay in ¹⁵⁰Nd have not been successful.

See also

• Double electron capture
• Beta decay
• Neutrino
• Particle radiation
• Radioactive isotope

References

External links

• Double beta decay on arxiv.org