Low-lying intruder and tensor-driven structures in As-82 revealed by beta decay at a new movable-tape-based experimental setup
Résumé
The beta decay of Ge-82 was re-investigated using the newly commissioned tape station BEDO at the electron-driven ISOL (isotope separation on line) facility ALTO operated by the Institut de Physique Nucleaire, Orsay. The original motivation of this work was focused on the sudden occurrence in the light N = 49 odd-odd isotonic chain of a large number of J <= 1 states (positive or negative parity) in Ga-80 by providing a reliable intermediate example, viz., As-82. The extension of the As-82 level scheme towards higher energies from the present work has revealed three potential 1(+) states above the already known one at 1092 keV. In addition our data allow ruling out the hypothesis that the 843 keV level could be a 1(+) state. A detailed analysis of the level scheme using both an empirical core-particle coupling model and a fully microscopic treatment within a Skyrme-QRPA (quasiparticle random-phase approximation) approach using the finite-rank separable approximation was performed. From this analysis two conclusions can be drawn: (i) the presence of a large number of low-lying low-spin negative parity states is due to intruder states stemming from above the N = 50 shell closure, and (ii) the sudden increase, from As-82 to Ga-80, of the number of low-lying 1(+) states and the corresponding Gamow-Teller fragmentation are naturally reproduced by the inclusion of tensor correlations and couplings to 2p-2h excitations.