The first large-scale shell-model calculation of the two-neutrino double beta decay of 76Ge to the excited states in 76Se

Abstract

Large-scale shell-model calculations were carried out for the half-lives and branching ratios of the 2νββ decay of 76Ge to the ground state and the lowest three excited states 2+ 1 , 0+ 2 and 2+ 2 in 76Se. In total, the wave functions of more than 10,000 intermediate 1+ states in 76As were calculated in a three-step procedure allowing an efficient use of the available computer resources. In the first step, 250 lowest states, below some 5 MeV of excitation energy, were calculated without truncations within a full major shell 0 f5/2 − 1p − 0g9/2 for both protons and neutrons. The wave functions of the rest of the states, up to some 30 MeV, were computed in two more steps by introducing two consecutive stages of truncation. The computed magnitudes of the 2νββ nuclear matrix elements (including the value of the axial-vector coupling gA), |M2ν |g2 A, converged to the values 0.168g2 A, 1.2 × 10−3 g2 A, 0.121g2 A, and 3.1 × 10−3 g2 A for the 0+ g.s., 2+1 , 0+2 , and 2+2 states, respectively. Using up-to-date phase-space integrals, the corresponding branching ratios were derived to be 99.926%, 4.4×10−5%, 0.074% and 2.5×10−7%. The experimental half-life (1.926 ± 0.094) × 1021 yr of the ground-state transition was used to derive the value gA = 0.80 ± 0.01 for the axial-vector coupling, which is consistent with other shell-model calculations suggesting a quenched value of gA. Using this value of gA, predictions for the transition half-lives were derived.