DOUBLE-BETA-DECAY RATES WITHIN A 2ND QUASI-RANDOM-PHASE APPROXIMATION

A second QRPA procedure is employed to perform calculations of the M(G T)2nu matrix elements for several double-beta transitions. It is found that higher-order QRPA corrections display a weak dependence on parti cle-particle strength g(pp) and, in the physical region (around g(pp) = 1.0), they become important by comparison with QRPA predictions. It turns out that further investigations of the higher QRPA corrections c ould be illuminating to obtain more stable and reliable values for M(G T)2nu matrix elements in this region, where they are very sensitive to the particle-particle strength. Further, we have studied the influenc e that higher-order QRPA corrections have on single-beta transitions w hich enter the double-beta-decay rates and we have also computed Gamow -Teller matrix elements for double-beta-transitions 0i+ --> --> 2f+ in the cases when these processes are allowed.