Single-particle and collective structures in Cr55 and V55

Excited states in V55 and Cr55 have been populated via pn and 2n evaporation channels, respectively, following the fusion of a Ca48 beam at 172 MeV with a Be9 target. Level schemes have been deduced for the two nuclides to excitation energies of 7467 (V55) and 12226 keV (Cr55), with spins of 27/2 + and 33/2+, respectively. Negative-parity states are compared with shell-model calculations using three different effective interactions in the full fp model space. Negative-parity levels of Cr55 are explained in terms of single-particle fp-shell configurations outside N=28 and N=32 cores. Positive-parity states in both isotopes show evidence for the involvement of neutron g9/2 configurations. In the case of Cr55, a quasirotational structure based on the 1/2+[440] Nilsson orbital is observed up to the terminating state. In V55, positive-parity states do not exhibit well-developed collective features, and the observation of octupole decays is an indication of their importance in transitions from neutron g 9/2 configurations to the fp shell. Experimental results are compared with the predictions of a traditional shell model, the projected shell model, and total-Routhian-surface calculations. © 2011 American Physical Society.