Double beta decay

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. 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. 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 exists in nature. In 1930–40s, parity violation in weak interactions was not known, and consequently 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 1015–16 years. Efforts to observe the process in laboratory date back to at least 1948 when Edward L. Fireman made the first attempt to directly measure the half-life of the 124Sn isotope with a Geiger counter. 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 130Te was measured by geochemical methods to be 1.4×1021 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 1960–70s, 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 1021 years. At the same time, geochemical experiments detected the double beta decay of 82Se and 128Te. Double beta decay was first observed in a laboratory in 1987 by the group of Michael Moe at UC Irvine in 82Se. 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 1025 years. Geochemical experiments continued through the 1990s, producing positive results for several isotopes. Double beta decay is the rarest known kind of radioactive decay; as of 2019 it has been observed in only 14 isotopes (including double electron capture in 130Ba observed in 2001, 78Kr observed in 2013, and 124Xe observed in 2019), and all have a mean lifetime over 1018 yr (table below). In a typical double beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted. The process can be thought as two simultaneous beta minus decays. In order for (double) 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 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. The emission spectrum of the two electrons can be computed in a similar way to beta emission spectrum using Fermi's Golden Rule. The differential rate is given by where the subscripts refer to each electron, T is kinetic energy, w is total energy, F(Z, T) is the Fermi Function with Z the charge of the final-state nucleus, p is momentum, v is velocity in units of c, cosθ is the angle between the electrons, and Q is the Q value of the decay.