Decay properties of high-spin isomers and other structures inSb121andSb123

Abstract

High-spin states populated in the decay of microsecond isomers in the transitional nuclei $^{121}\mathrm{Sb}$ and $^{123}\mathrm{Sb}$ have been investigated in detail in several experiments using $\ensuremath{\gamma}$-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with $^{136}\mathrm{Xe}$ beams and also using the $^{120}\mathrm{Sn}(^{7}\mathrm{Li},\ensuremath{\alpha}2n)^{121}\mathrm{Sb}$ and $^{122}\mathrm{Sn}(^{7}\mathrm{Li},\ensuremath{\alpha}2n)^{123}\mathrm{Sb}$ incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers (${T}_{1/2}~1$ ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the $19/{2}^{+}$ member of the $9/{2}^{+}$ deformed band in $^{121}\mathrm{Sb}$, in competition with the main decay path through the rotational band. This is attributed to mixing between the $19/{2}^{+}$ band member and a $19/{2}^{+}$ spherical state. Both levels are predicted to coincide approximately in energy in $^{121}\mathrm{Sb}$. The fact that a $25/{2}^{+}$ isomer occurs for $A=121$ and the lighter isotopes, while a $23/{2}^{+}$ isomer is observed for $A=123\text{\ensuremath{-}}131$ is explained through a multistate mixing calculation, taking into account the gradual shift of the $2{d}_{5/2}$ and $1{g}_{7/2}$ proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the $N=82$ shell closure. The observed enhancement of the $B(E2;19/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}15/{2}^{\ensuremath{-}})$ values in $^{121}\mathrm{Sb}$ and $^{123}\mathrm{Sb}$ over the $B(E2;{7}^{\ensuremath{-}}\ensuremath{\rightarrow}{5}^{\ensuremath{-}})$ values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.