Competing single-particle and collective states in the low-energy structure of113I

Abstract

To understand the low-energy structure of the neutron deficient iodine isotopes, lifetimes for the low-lying $9/{2}^{+}$ and $11/{2}^{+}$ positive-parity states in ${}^{113}$I have been measured as $\ensuremath{\tau}=28(4)$ ps and $\ensuremath{\tau}=3.7(7)$ ps, respectively. The lifetime for the $11/{2}^{\ensuremath{-}}$ state, which feeds the $9/{2}^{+}$ and $11/{2}^{+}$ states, was remeasured with improved accuracy as $\ensuremath{\tau}=216(7)$ ps. The reduced transition probability, $B(E2)=32(5)$ W.u., for the $9/{2}^{+}\ensuremath{\rightarrow}5/{2}^{+}$ transition agrees with that calculated within the shell model using a Hamiltonian based on the charge-dependent Bonn nucleon-nucleon interaction. In contrast, the much larger transition probability, $B(E2)=209(39)$ W.u., measured for the $11/{2}^{+}\ensuremath{\rightarrow}7/{2}^{+}$ transition has been interpreted, with the aid of configuration-constrained total Routhian surface calculations, as resulting from a slightly $\ensuremath{\gamma}$-soft rotor with an associated quadrupole deformation of ${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.18$. Remarkably similar reduced $E1$ transition probabilities of $5.5(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and $4.9(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ W.u. were deduced for the $11/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}9/{2}^{+}$ and $11/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}11/{2}^{+}$ transitions, respectively, which feed apparently dissimilar but competing structures.