Changes in νg9/2 shape polarisation across the odd neutron-rich Cr isotopes

Excited states in 57Cr have been populated to high spin in the C14(Ca48,αn) reaction at a beam energy of 130 MeV. A regular sequence of stretched quadrupole transitions has been established above the yrast 9/2+ level. This sequence is interpreted as a rotational band associated with prolate deformation induced by the excitation of the odd neutron into the 1/2+[440] orbital. Total Routhian surface calculations, which follow this configuration to high spin, reproduce the observed features of the band. They are also able to account for a similar, but less well developed structure in 55Cr. In contrast, the isomeric yrast 9/2+ state in 59Cr appears to be a band-head state dominated by the 9/2+[404] configuration, which is favoured at oblate deformations. Such a marked difference presents a significant challenge for theoretical descriptions, but is consistent with features exhibited by the low-lying negative-parity states in 59Cr.