LABORATORI NAZIONALI DI LEGNARO
Nowadays much eort is devoted to understand the role of neutron excess and its influence on nuclei in the vicinity of closed neutron shells whose structure is not yet fully studied. According to available schemes the nuclei in the vicinity of N=82 shells undergo Gamow Taylor (allowed) decays. In contrast, for nuclei crossing the N=82 shell, the first Forbidden (FF) transitions are expected to give a noticeable contribution [1]. With new experimental data on -decay properties of more neutron rich species already (or shortly) available at new facilities the relative contribution of the Gamma-Teller and First Forbidden decays can be understood more. Furthermore, for an r-process site ,waiting points" (nuclei on closed neutron shells) have signicant eects on the r-process dynamics and the abundance distribution [2, 5]. One of the means to investigate nuclear structure is in -decay. Since -delayed neutron emission becomes signicant if not the dominating decaying channel for neutron-rich nuclei far from stability, usage of a proper neutron detector to reveal their properties is indispensable. To conduct the appropriate investigations, in the frame of collaboration JINR (Dubna) and IPN Orsay a new detection system consisting of 80 3He-lled counters (TETRA neutron detector [3]), 4 detector and a HPGe in order to measure simultaneously, and neutron activities was constructed [4]. The eciency of single neutron registration is 60% and is almost at up to 1MeV neutron energy range. We highlight the strong necessity of the detection system created for investigations of neutron rich nuclei. Thus, we present the half-live of neutron rich silver 123􀀀125Ag measured presently as compared to existing QRPA calculations in the allowed approximation as well as taking into account the forbidden decays. We also report the rst measurements of -delayed neutron emission for the neutron rich 126Cd and the rst measurements of its neutron emission probability. The presently observed 126Cd neutron precursor has 4 neutrons less than the lightest Cd precursor known experimentally before [6] (which is, in fact, 130Cd - the r-process nucleus). The impact on understanding of nuclear the shell gap in the vicinity of 132Sn introducing the upper Z-limit for isotones that may be a delayed-neutron precursor [7] is pointed out.

DATA: 25-07-2014