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

Large-scale shell-model calculations were carried out for the half-lives and branching ratios of the 2 nu beta beta decay of Ge-76 to the ground state and the lowest three excited states 2(1)(+), 0(2)(+) and 2(2)(+) in Se-76. In total, the wave functions of more than 10,000 intermediate 1(+) states in As-76 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 0f(5/2) - 1p - Og(9/2) 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 2 nu beta beta nuclear matrix elements (including the value of the axial-vector coupling gA), vertical bar M-2 nu vertical bar g(A)(2) converged to the values 0.168g(A)(2), 1.2 x 10(-3)g(A)(2), 0.1214g(A)(2) and 3.1 x 10(-3)g(A)(2) 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%, 4.4x10(-5)%, 0.074% and 2.5x10(-7)%. The experimental half-life (1.926 +/- 0.094) x 10(21) yr of the ground-state transition was used to derive the value g(A) = 0.80 +/- 0.01 for the axial-vector coupling, which is consistent with other shell-model calculations suggesting a quenched value of g(A). Using this value of g(A), predictions for the transition half-lives were derived. (C) 2022 The Author(s). Published by Elsevier B.V.

Automated summary

Large-scale shell-model calculations were conducted to determine the half-lives and branching ratios of the 2 nu beta beta decay of Ge-76 to different states in Se-76. The study involved calculating the wave functions of intermediate 1(+) states in As-76 and computing the nuclear matrix elements and branching ratios. The results provided insights into the behavior of the decay process and the value of the axial-vector coupling gA.