Competing single-particle and collective behavior in71Se

Abstract

The high-spin decay of 71 Se was studied using the 54 Fe( 23 Na,αpn) reaction at 80 MeV and the Florida State University Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. Based on prompt γ -γ coincidences measured in the experiment, the known level scheme was enhanced and extended to higher spin with 19 new transitions. A band that was previously suggested to have positive parity was reassigned as the "missing" signature partner of an existing negative-parity band. Spins were assigned based on directional correlation of oriented nuclei ratios. Lifetimes of 17 excited states were measured using the Doppler-shift attenuation method. Experimental Q t values imply an intermediate degree of collective behavior for 71 Se at high spin. Theoretical Q t values determined from cranked Woods-Saxon (CWS) calculations show better agreement with the experimental ones for the positive-parity states than the negative-parity states. Shape competition and γ softness characterize the low-spin states of the lowest positive-and negative-parity bands based on the CWS calculations. At high spin, triaxial shapes with γ > 0 • are predicted.

Systematical shell-model calculation in the pairing-plus-multipole Hamiltonian with a monopole interaction for thepf5/2g9/2shell

“…21. The calculation describes correctly the change of the ground state from the 5/2 − state in 69 Ge to the 1/2 − state in 71 Ge. The calculated level sequences are essentially similar to those of 69 Ge, although there is no spinparity assigned levels above 3 MeV in the experimental data.…”

“…Odd-mass Se nuclei in this mass region exhibit also rapidlychanging structures [70][71][72]. The study of odd-mass nuclei completes the information on the structural properties of shape-coexisting nuclei, and thus can be helpful in under- 69 Se.…”

“…The configurations of the ground state in 75,77 Ga have a dominant π(p 3/2 f 2 5/2 ) configuration. As one can see, the JJ4B calculations for quadrupole moment fail to reproduce experimental data for 69,71 Ga.…”

“…) values for the positive-parity yrast states and some collective states of 69 Ge and 71 Ge. The experimental data are taken from [63].…”

“…The subscript denotes a serial number for each spin J. 73 systematically seen in 69 Ge and 71 Ge, which could be due to the missing d 5/2 orbit in the present model space. The higherlying yrast states 11/2 + , 13/2 + , and 17/2 + are reasonably reproduced, while there is no observed spectrum above 2 MeV for this isotope.…”