Investigation of signature inversion in I126 through lifetime measurements

Abstract

The reduced transition probabilities $B(E2)$ and $B(M1)$ of the negative-parity yrast states of $^{126}\mathrm{I}$ were studied by measuring the lifetimes in picoseconds using the Doppler-shift attenuation method. We investigated the phenomena of signature splitting and inversion using the two quasiparticle-plus triaxial rotor model (PRM). The experimental result---a sharp decrease in the $B(E2)$ values at the signature inversion---indicated a shape change with a possible shift in the axis of rotation. We interpreted this observation as a change in the triaxiality parameter, with values $\ensuremath{\gamma}=\ensuremath{-}{10}^{\ensuremath{\circ}}$ (Lund convention) below the inversion and $\ensuremath{\gamma}={25}^{\ensuremath{\circ}}$ above it and a constant value of axial deformation of $\ensuremath{\beta}\ensuremath{\approx}0.15$. We also assigned the valence particle configuration---neutron in the ${h}_{11/2}$ orbital and proton predominantly in the ${d}_{5/2}$ orbital mixed with the ${g}_{7/2}$ orbital---from the PRM calculation. Furthermore, the calculation reproduced the overall observed behavior of the signature splitting and inversion reasonably well. The observed backbending at the rotational frequency of 0.47 MeV was due to neutron alignment consistent with the second band crossing from the quasiparticle Routhian diagram.