NUCLEAR-STRUCTURE CALCULATIONS OF 2-NEUTRINO DOUBLE-BETA DECAY TRANSITIONS TO EXCITED FINAL-STATES

The half-lives for two-neutrino double-beta decay transitions to the e xcited 2(1)(+) and 0(1)(+) states for several nuclei are calculated in the framework of a second quasi-random-phase approximation (QRPA). In addition, other single-beta and B(E2) transitions are evaluated and c ompared with experimental data. We find satisfactory agreement with da ta when we tried to describe unitarily, with the same set of parameter s, all the computed physical quantities. The sum rule, which is fulfil led accurately in the framework of the proton-neutron QRPA, is violate d insignificantly by our method. The higher-order QRPA terms only infl uence the beta(+) strengths which enter the double-beta decay rate for mula, shifting them to higher QRPA energies.