Exploring life-time of low-lying states in neutron rich nuclei towards 78Ni with the plunger technique at GANIL
Presented by Batipste MOUGINOT on 7 Jun 2010 from 12:20 to 12:40
Type: oral with financial aid
Track: At and beyond the dripline
One of the most critical ingredients in determining the disappearance or appearance of magicity in nuclei far from stability is the evolution of single-particle energies with increasing neutron or proton numbers when moving away from the valley of stability. The three known cases of disappearance of shell effects at N=8, 20 and 28 in neutron rich nuclei are understood as due to the effect of the tensor part of the nucleon-nucleon interaction. The tensor force is held responsible for the strong attraction between a proton and a neutron in spin-flip partner orbits. A recent generalization of such mechanism foresees a similar behaviour also for orbitals with non-identical orbital angular momenta. It is expected that orbitals with anti-parallel angular momenta attract each other and orbitals with parallel angular momenta repulse each other. The change in shell structures based on this mechanism has recently been discussed for different mass regions of the nuclear chart. In this context neutron-rich nuclei close to shell gaps are particularly interesting since they allow to search for anomalies when compared with shell-model predictions. It is predicted, for example, that the Z=28 gap for protons in the pf-shell becomes smaller when moving from 68Ni to 78Ni as a consequence of the attraction between the proton f5/2 and neutron g9/2 orbits and the repulsion between the proton f7/2 and the neutron g9/2 states. The same argument would also predict a weakening of the N=50 shell gap when depleting the proton f5/2 state upon approaching the 78Ni nucleus, due to the diminished attraction between the neutron g9/2 and the proton f5/2 orbits and the reduced repulsion between the neutron g7/2 and the proton f5/2 states. Recently an experiment aiming at the study of the evolution of the structure of neutron rich Cu and Zn isotopes has been performed. This experiment uses the plunger technique in order to measure life time of gamma transition involved in the decay of excited states in these exotic nuclei. The studied nuclei have been populated using reactions induced by a cocktail beam composed of 73,74Zn RIB’s of 34MeV/u in a CD2 target. The cocktail beam is produced by the in-flight technique using the first half of the LISE separator. The second half is used for the selection and identification of the final products after interaction in the secondary CD2 target. The EXOGAM array and the differential Plunger technique provide information on the in-beam gamma spectroscopy and life time of the excited states in the picoseconds to tens of picoseconds The first results obtained on the life time of excited states in 72,73,74Zn will be reported together with the comparison to results from Coulomb excitation experiment at REX-ISOLDE. A picture of the low-energy structure in these isotopes towards the middle of the g9/2 orbital will be given via: i) identification of the levels populated with inelastic scattering reaction and ii) determination, in a model-independent way, of the transition probabilities of those levels towards the ground state.