Two-quasiparticle and collective excitations in transitional 108,110 Pd nuclei

Abstract

High-spin states in 108,110Pd isotopes are studied by $\gamma$ -ray spectroscopy of heavy-ion-induced fission. The Pd isotopes are produced in the fission channel of the 31P + 176Yb reaction at beam energy 152 MeV. The prompt $\gamma$ -rays are detected with the EUROBALL4 multidetector array. The yrast states of 108,110Pd have been observed above the region of the first backbend. The level scheme of 108Pd was extended with a new negative-parity band. The yrast sequence in 110Pd is observed up to spin I = 14 + and negative-parity bands have also been identified. The backbending in these even-mass Pd isotopes is associated with the alignment of the neutron (h 11/2)2 pair. The negative-parity states arise from two neutron configurations $\nu h_{11/2} \otimes \nu g_{7/2}$ and $\nu h_{11/2} \otimes \nu d_{5/2}$ and they are interpreted in the frame of two-quasiparticle + rotor model as semidecoupled bands. The observed experimental staggering in the $\gamma$ -bands of 108,110Pd supports the theoretical predictions for $\gamma$ -instability of their shapes.