ββ-Decay Reference Paper: Nuclear Data Sheets 120, 102 (2014)
Nuclear data on double beta (ββ) decay values and information for all known nuclei. Database content is based on original data compiled at the National Nuclear Data Center and Kiev Institute for Nuclear Research (Ukraine), Nuclear Wallet Cards/2003 Atomic Mass Evaluation publications and ββ-decay philosophy of Prof. Alexander A. Pomansky (1932-1993).
The main goal of this project is to select the lowest half-life times for ββ-decay transitions: (Z,A) → (Z±2,A) + 2e± + (0 or 2)νe. For all ββ - processes (β-β-, β+β+, Kβ+ (εβ+) and 2K (2ε)) ground state transitions ( 0+ → 0+) should have the priority. Transitions to the excited states of daughter nuclei ( 0+ → 0+1, 0+ → 0+2, 0+ → 2+) have a unique decay event signature that attracts attention of experimentalists. However lower transition energies result in higher life-times. Evaluators can use results for ground to excited state ββ-transitions only if ground state transitions are not measured.
There are two ββ-decay modes: two-neutrino (2ν) and neutrinoless (0ν). Two-neutrino mode is not prohibited by any law of Nature and definitely occurs as a second-order process compare to the regular β-decay. Neutrinoless mode differs from the 2ν mode by the fact that no neutrinos are emitted during the decay. This normally requires that lepton number is not conserved and neutrino should contain a small fraction of massive particles that equal to its anti-particles (Majorana neutrino). Obviously observation of neutrinoless double beta decay will have enormous implications on particle physics and fundamental symmetries. While observation of 2ν mode of double beta decay will provide information on nuclear structure physics and shell-model effects.
Double beta decay has the following observables: half-life time, transition energy, energy/angular distribution of emitted particles and daughter nuclei. Half-life time is the most important observable for ENSDF evaluations. Experimental evidence and theoretical calculations indicate that probability for two-neutrino mode is much higher than for neutrinoless. In fact, 76Ge ββ-decay experiments have demonstrated that half-life time for two-neutrino decay is at least four orders of magnitude lower than neutrinoless and only two-neutrino ββ-decay is definitely observed at the present time. Therefore two-neutrino mode half-life time should be preferably used for ENSDF evaluations. The recommended double beta decay half-life time selection rules are following:
Adopted ββ-decay values and their systematics are described in the Brookhaven National Laboratory Reports BNL-99822-2013-CP & BNL-91299-2010. For additional information on this subject we will recommend book of F. Boehm and P. Vogel Physics of Massive Neutrinos, Cambridge University Press, Cambridge, England 1987.