Double beta decay

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The double beta decay is the simultaneous beta decay of two nucleons in an atomic nucleus . A distinction must be made between the two-neutrino double beta decay (observed) and the hypothetical neutrino-free double beta decay .

requirements

Masses m of different isobaric atomic nuclei as a function of the atomic number Z : energetic impossibility of some simple beta decays (red)

Double beta decay is a second order process. Its decay probability is many orders of magnitude smaller, and its partial half-life is thus many orders of magnitude longer than that of simple beta decay . It can only be observed experimentally in nuclides for which simple beta decay is not possible (“forbidden”), because otherwise it will be masked by this process, which is many orders of magnitude more frequent.

The simple beta decay z. B. for a gg nucleus (even number of protons and even number of neutrons) if it has less energy in its ground state than each of its two uu neighbors (odd number of protons and odd number of neutrons). Based on the Bethe-Weizsäcker formula , the masses of nuclei with the same mass number , i.e. isobars , can be represented as a quadratic function of the atomic number Z (see figure). In the case of uu and gg kernels, the pairing term results in a split into two parabolas, and the parabola of the uu kernels lies above the parabola of the gg kernels. A simple beta-plus or beta-minus decay of a gg nucleus must lead to the corresponding neighboring uu nucleus; if both of these are energetically higher than the gg mother core, a simple beta decay is energetically forbidden. If the observed gg-nucleus is not the most stable isobar of the "mass chain", a double beta decay in the nearest gg-nucleus can take place energetically (see also Mattauch's isobar rule ).

Instead of an energy difference, the spin difference between the mother and daughter nucleus can also hinder a simple beta decay , e.g. B. at 96 Zr . Its beta-minus decay into the ground state of the neighboring uu nucleus ( 96 Nb ) is energetically possible, but is strongly suppressed because of the spin difference between the two nuclei.

Observations

The first proven double beta decay was the transition from 82 Se to 82 Kr . It was detected indirectly in 1967 by geochemical studies ( Till Kirsten et al.) And directly in 1987 ( Michael K. Moe et al.).

A total of 35 naturally occurring nuclides with possible double beta decay are known. Until 2016 it was detected in 12 nuclides ( 48 Ca, 76 Ge, 78 Kr, 82 Se, 96 Zr, 100 Mo, 116 Cd, 128 Te, 136 Xe, 150 Nd, 238 U all and 130 Te). The half-lives are between 10 19 and 10 25 years.

Two neutrino double beta decay

The two neutrino double beta decay (2νββ decay) can be clearly interpreted as the simultaneous beta-minus decay of two neutrons into two protons with the emission of two electrons and two antineutrinos . "Simultaneously" could be understood to mean that the decay takes place via a virtual, sufficiently short-lived intermediate state in the sense of the energy-time uncertainty relation: the initial core passes through β-decay into the intermediate core (energetically forbidden, therefore virtual) and this through another β-decay into the daughter nucleus.

The opposite decay of two protons into two neutrons is also possible and has been demonstrated for the 78 Kr (see above). It can take place in three different ways: two electron capture processes or - if energetically possible - two beta-plus decays or one electron capture and one beta-plus decay.

With every 2νββ decay the number of leptons is retained, which is why this decay mode is “allowed” within the standard model of nuclear and particle physics.

Neutrino-free double beta decay

In the case of neutrino-free double beta decay (0νββ), a conceivable additional decay channel of the 35 nuclides mentioned, the number of leptons would have to change by two units. For this reason it is "forbidden" according to the standard model of nuclear and particle physics. Observation of its occurrence would be evidence of “physics beyond the Standard Model”.

Measurements of such decays would also offer a way to directly measure neutrino masses. So far, the matrix elements that are needed to determine the neutrino mass are not accessible experimentally and can only be determined in theoretical model calculations.

A neutrino-free double beta decay has not been discovered to date (2020) despite extensive experiments.

Web links

Individual evidence

  1. ^ R. Arnold, et al .: Measurement of the double-beta decay half-life and search for the neutrinoless double-beta decay of 48 Ca with the NEMO-3 detector . In: Physical Review D . 93, 2016, p. 112008. arxiv : 1604.01710 . bibcode : 2016PhRvD..93k2008A . doi : 10.1103 / PhysRevD.93.112008 .
  2. ↑ C. Patrignani, et al .: Review of Particle Physics . In: Chinese Physics C . 40, No. 10, 2016, p. 768. bibcode : 2016ChPhC..40j0001P . doi : 10.1088 / 1674-1137 / 40/10/100001 .
  3. ↑ C. Alduino, et al .: Measurement of the Two-neutrino double beta decay half-life of 130 Te with the CUORE-0 experiment . In: The European Physical Journal C . 77, 2016. arxiv : 1609.01666 . bibcode : 2017EPJC ... 77 ... 13A . doi : 10.1140 / epjc / s10052-016-4498-6 .