Single and double beta decays in the A=100A=100, A=116A=116 and A=128A=128 triplets of isobars

In this paper we analyze the ground-state-to-ground-state two-neutrino double beta (2νββ2νββ) decays and single EC and β−β− decays for the A=100A=100 (100Mo–100Tc–100Ru), A=116A=116 (116Cd–116In–116Sn) and A=128A=128 (128Te–128I–128Xe) triplets of isobars. We use the proton–neutron quasiparticle random-phase approximation (pnQRPA) with realistic G-matrix-derived effective interactions in very large single-particle bases. The purpose is to access the effective value of the axial-vector coupling constant gAgA in the pnQRPA calculations. We show that the three triplets of isobars represent systems with different characteristics of orbital occupancies and cumulative 2νββ2νββ nuclear matrix elements. Our analysis points to a considerably quenched averaged effective value of 〈gA〉≈0.6±0.2〈gA〉≈0.6±0.2 in the pnQRPA calculations.