Identical bands in 77Sr, 78Sr, and 78Rb: Evidence for a very good spectator orbital.

Abstract

The highest spin states in a ${\mathit{T}}_{\mathit{z}}$=1/2 nucleus have been observed in $^{77}\mathrm{Sr}$: ${\mathit{I}}^{\mathrm{\ensuremath{\pi}}}$=(49/${2}^{+}$) and (29/${2}^{\mathrm{\ensuremath{-}}}$). In addition, states up to ${\mathit{I}}^{\mathrm{\ensuremath{\pi}}}$=(${22}^{+}$) have been seen in $^{78}\mathrm{Sr}$. The \ensuremath{\alpha}=1/2, ${\mathit{g}}_{9/2}$ band and both signature partners of the negative parity band in $^{77}\mathrm{Sr}$ were found to be identical to the yrast band in $^{78}\mathrm{Sr}$ with relative alignments of 1.22(7)\ensuremath{\Elzxh}, 0.52(5)\ensuremath{\Elzxh}(\ensuremath{\alpha}=1/2), and 0.42(4)\ensuremath{\Elzxh}(\ensuremath{\alpha}=-1/2), respectively. In one of the negative parity bands, the identicality persists through the first proton band crossing. The odd spin decay sequence of the ${4}^{\mathrm{\ensuremath{-}}}$ band in $^{78}\mathrm{Rb}$ is identical to the ground band of $^{78}\mathrm{Sr}$ [3.02(3)\ensuremath{\Elzxh}] at low spin and to the \ensuremath{\alpha}=1/2, ${\mathit{g}}_{9/2}$ band in $^{77}\mathrm{Rb}$ [1.11(3)\ensuremath{\Elzxh}] at higher spins. These are the only known examples of identical bands in the neutron deficient A=80 region. The analysis indicates that the [301]3/2 Nilsson orbital is a very good spectator orbital.