Double beta decay
The double beta decay refers to the simultaneous beta decay of two nucleons in a nucleus. We discuss two different decay modes: the two- neutrino double beta decay and the experimentally not yet proven neutrino double beta decay.
Requirements
The double beta decay is a " second-order process ", ie its decay probability is generally much smaller (the corresponding half-life so much longer) than for the simple beta decay.
Experimentally it is observed only at nuclides for which the simple beta decay is energetically forbidden. This is (even number of protons and neutrons just number ) of the case, the ground states are lower in energy than their odd-odd neighbors (odd number of protons and odd number of neutrons ) for even-even nuclei. Starting from the Bethe- Weizsäcker formula allows the binding energies of nuclei of the same mass number, so isobars, represented as a quadratic function of the atomic number Z. In the case of odd-odd and even-even nuclei is due to the pairing term splitting into two parabolas, and the Parable of the odd-odd nuclei is above the parable of the even-even nuclei.
The simple beta decay of an even-even nucleus leads to the neighboring odd-odd nucleus; this is energetically higher than the even-even parent nucleus, the simple beta decay is therefore energetically forbidden. Since the observed even-even nucleus but is not the most stable isobar of " mass chain ", a double beta decay in the nearest even-even nucleus can energetically take place (see also Mattauchsche Isobarenregel ).
For some nuclides also prevents angular momentum difference between parent and daughter nucleus a beta decay, although this would be energetically possible. An example of this is 96Zr: its decay to the ground state of the neighboring odd-odd nucleus ( 96Nb ) is possible energetically, but strongly suppressed due to the angular momenta of the states involved.
The first recorded double beta decay was the transition from 82Se in 82KR. It was founded in 1967 indirectly through geochemical experiments (Till Kirsten and others) and 1987 directly (Michael K. Moe and others) observed. A total of about 35 nuclides are known in which the double -decay is expected.
Two - neutrino double beta decay
The two- neutrino double beta decay ( 2νββ decay ) is a second order weak interaction process. Clearly it can be interpreted as the simultaneous beta-minus decay of two neutrons into two protons with the emission of two electrons and two antineutrinos. The opposite dissociation of two protons and two neutrons is also possible and can run in three different ways: two beta-plus decay, two electron capture processes or an electron and a beta-plus decay. When 2νββ decay the lepton number is preserved, so this decay mode is allowed particle physics within the standard model of core and.
Another approach to illustrate the double beta decay is the idea that the decay via a virtual intermediate state expires. Here the output nucleus decays by β -decay in the intermediate core ( energetically forbidden, therefore virtual) and this. Further by a β -decay daughter nucleus into the actual By means of charge exchange reaction, the transition to the intermediate state can be experimentally investigated.
The observed half-lives for 2νββ decays are in the range of about 1019 to 1021 years.
Loser neutrino double - beta decay
When neutrino double beta decay ( 0νββ ), the lepton number changes by two units. For this reason, it is forbidden by the Standard Model of nuclear and particle physics. An observation of its occurrence would be a proof for " physics beyond the standard model ".
Measurements of such decays would also provide an opportunity for direct measurement of neutrino masses. So far, the matrix elements needed for the determination of the neutrino mass, not experimentally accessible and can be determined only in theoretical model calculations. However, these calculations are dependent on the physical model used to a great extent and vary among themselves by a factor of 3
To distinguish the 0νββ - decay of 2νββ one measures the total energy spectrum of the emitted electrons. Since, in contrast to 2νββ case no neutrinos are emitted, this is not continuous, but has a " spectral " revealed a fixed value corresponding to the energy gain of decay.
Also the 0νββ -decay can be clearly understood as a simultaneous decay of two neutrons into two protons. However, in contrast to 2νββ -decay neutrinos does not leave the nucleus, but annihilate, ie " destroy " each other within the core.
Another way to look also offers the decay via virtual intermediate states. A neutron decays by emission of an electron and an antineutrino in a virtual intermediate state; the antineutrino does not leave the nucleus, but is used by another neutron absorbed (as neutrino ), which also then decays by emission of an electron into a proton.
For the occurrence of the 0νββ -decay, two conditions must be met:
Reported in 2006 a working group (part of the collaboration of the Heidelberg - Moscow experiment, spokesman Hans - Klapdor Kleingrothaus ) on an observation of the neutrinoless decay mode with 6.4 standard deviations security; the result is thus significantly different from zero ( the accepted confidence limit is five standard deviations). Nevertheless, it is observed events controversial because of the methods of analysis used and the small number.
The GERDA experiment has in its first measurement phase from 2011 to 2013 no evidence of neutrinoless double beta decay in Ge -76 found ( lower limit of its service life: 2.1 × 1025 years ) it was unable to confirm the above results.