Abstract:
A new experimental method, exploiting the analysis of gamma-gamma coincidences
measured with large germanium multidetector arrays in deep-inelastic heavy ion collisions, has been used to extend the in-beam gamma-ray spectroscopic studies towards previously inaccessible neutron-rich nuclei. In particular, the neutron-rich Ti isotopes have been accessed using deepinelastic processes occurring during the bombardments of a thick 208Pb target with a 305-MeV 48Ca beam. Yrast structures in 53Ti and 54Ti nuclei have been identified for the first time using cross coincidences with known gamma rays from complementary mercury products. Also, spectroscopic information has been extended for 52Ti. The experimental findings have been compared with shell model calculations performed in the full fp shell with the FPD6 and GXPF1 effective interactions. The results fully confirm the existence of a subshell closure at N=32 in Ti neutron-rich isotopes. This new subshell closure has been attributed to a decreased pf7/2 - nf5/2 monopole interaction as protons are removed from the pf7/2 orbital: the migration of the nf5/2 orbital to higher energies with the removal of protons from the f7/2 orbital, in combination with the large spin-orbit splitting for np3/2-np1/2, gives rise to an energy gap at N=32 for nuclides having Z≤24. Data from the same reaction 48Ca+208Pb provided also new spectroscopic information on 208Bi and 210Bi nuclei. Yrast and near-yrast levels up to ~5.6 MeV in 208Bi and up to 4.6 MeV in 210Bi have been located - they are interpreted in light of earlier charged particle spectroscopy results, and with the help of shell model calculations in a large configuration space extending from 132Sn to 310 126X184. Another hard-to-reach nucleus, 206Hg, was accessed using 208Pb(1360 MeV)+238U collisions. The yrast states found include a T1/2=92(8) ns 10+ isomer located above the known 5- isomer. The B(E2;10+→8+) reduced transition probability was used to derive the quadrupole polarization charge epol=0.60(7)e induced by the h11/2 proton hole on the doubly magic 208Pb core. Comparison of epol(ph11/2 -1) with the quadrupole polarization charges known for other high spin orbitals has revealed the constancy of the polarization charges induced by high j protons or neutrons on doubly magic cores. Theoretical considerations, based on the coupling between vibrational degrees of freedom and those of the single particles, reproduce the main features of experimental data regarding epol.
