Yrast transition strengths and band structure of 75Br.

Abstract

High spin states in $^{75}\mathrm{Br}$ were investigated in the reactions $^{62}\mathrm{Ni}$${(}^{16}$O,p2n) and $^{66}\mathrm{Zn}$${(}^{12}$C,p2n). By means of \ensuremath{\gamma}\ensuremath{\gamma}- and \ensuremath{\gamma}-neutron multiplicity coincidence measurements, rotational bands of both parities were extended up to 7 MeV excitation energy and spin I=(33/2). The lifetimes of 17 yrast states were determined with the recoil distance and Doppler shift attenuation methods; the spin dependence of the side feeding time was also measured. The data have been analyzed in terms of the Strutinsky-Bogolyubov cranking model with a Woods-Saxon average potential. The first crossing observed in the negative parity bands is attributed to the alignment of a ${g}_{9/2}$ proton pair; the next irregularity which is also observed in the positive parity band is interpreted as the alignment of ${g}_{9/2}$ neutrons. The coexistence of prolate and oblate configurations at low spin and the change of nonaxial deformation with rotational frequency due to the polarization of the even-even core by the odd particle are discussed. The signature inversion visible in the negative parity bands as well as the changes in the signature splitting and reduction of B(E2) values at high spins may be related to a transition from \ensuremath{\gamma}<0\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{\gamma}>0\ifmmode^\circ\else\textdegree\fi{} triaxial shapes.