Transition strengths and degree of deformation inSr79

Abstract

High-spin states in $^{79}\mathrm{Sr}$ were studied using the $^{54}\mathrm{Fe}$($^{28}\mathrm{Si}$,$2p$$n$) reaction at 90 MeV, with a thick 14-mg/${\mathrm{cm}}^{2}$ $^{54}\mathrm{Fe}$ target used to stop all recoils. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were detected using the Florida State University Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. The most recent $^{79}\mathrm{Sr}$ level scheme has been confirmed in three separate band structures up to a spin as high as the $({\frac{37}{2}}^{+})$ yrast state based on \ensuremath{\gamma}-ray coincidence relations, intensity and effective lifetime measurements, and directional correlation of oriented nuclei ratios. Lifetimes of 33 excited states were measured using the Doppler-shift attenuation method, with the experimental line shapes obtained at two separate observation angles and by gating from above the transitions of interest whenever possible. Transition quadrupole moments ${Q}_{t}$ inferred from the lifetimes indicate a high degree of collectivity and deformation over a rather wide range of spins in all three observed bands, with evidence for modest reductions in the values with increasing spin. The changes in ${Q}_{t}$ are attributed to the onset of quasiproton alignment and are supported qualitatively by the predictions of the projected shell model and cranked Woods-Saxon calculations in conjunction with the cranked shell model. Lifetimes measured in a band based on the [431]${\frac{1}{2}}^{+}$ intrinsic Nilsson configuration suggest a large quadrupole deformation (${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.41$) associated with this band, providing another example of the strong deformation-driving properties of the ${d}_{5/2}$ intruder orbital in the mass 80 region.