Nuclear structure studies at high angular momentum

The planning for a multi-detector array for high spin state spectroscopy has taken an increasing amount of time and effort during the past 3 years, and during the past year the testing of the first BGO-shielded Ge detectors has taken place. First five, then nine, and, by the end of this report period, fifteen such units have been operated at the 88-Inch Cyclotron. The first experiments concentrated on nuclei just above the N = 82 closed shell and the Z = 64 sub-shell. The study furthest along is of 156Er, but work on 158Er, 150Dy, and 148Dy is also currently underway. The investigation of the continuum γ-ray spectra of the even-even Er nuclei described last year has been extended to the even-even Yb nuclei and to lighter nuclei 84Sr, 108Pd, 118Te and 136Nd. These studies show that with increasing spin the high-j orbitals from the shell above the valence shell come down to the Fermi surface and are occupied, a condition that results in a large increase in aligned angular momentum at that spin. This phenomenon is a general one throughout the periodic table. A set of experiments on heavy-ion transfer reactions has also been completed and is in press. This is a study of 1- and 2-neutron transfer in reactions of 132Xe projectiles with three rare earth targets. Simultaneous with the transfer process, there is a very large amount of Coulomb excitation. The experiments were done at an energy approx.10% above the Coulomb barrier, and for the 2n process it appears that intermediate states with approx.6 MeV of excitation are usually involved. If true, this shows that the enhancement of 2n yields relative to 1n yields at these bombarding energies is not a measure of the pairing correlations in nuclei, as has been generally assumed