Successive neutron alignments in the yrast, negative-parity band of oblate-deformed Tl199

Abstract

The negative-parity band structure built on the proton $h{}_{9/2}$ state in $^{199}\mathrm{Tl}$ has been established up to an excitation energy of 7 MeV and spin of (51/2)$\ensuremath{\hbar}$. The level scheme has been extended with the inclusion of fourteen new transitions de-exciting levels at high spin. The $\mathrm{\ensuremath{\Delta}}\mathit{I}=1$ $\ensuremath{\gamma}$ rays are found to be more prominently visible in the spectra when comparing to $\mathrm{\ensuremath{\Delta}}\mathit{I}=2$ transitions. Rotation alignments are evident at frequencies of 0.22 and 0.30 MeV with both being attributed to the breaking of pairs of neutrons in the $i{}_{13/2}$ subshell. Since $^{199}\mathrm{Tl}$ is expected to have weak oblate deformation, and lies in a transitional region where competing contributions to the spin are expected from collective rotation and the angular momentum of high-$j$ nucleons, calculations using both the principal axis cranking (PAC) and tilted axis cranking (TAC) formalisms have been performed to understand its structure. Both the PAC and TAC calculations are found to provide a satisfactory description of the evolution of excitation energies with spin. The PAC calculations give a good account of the experimental crossing frequencies and associated spins. The structure of the yrast, negative-parity band in $^{199}\mathrm{Tl}$ may be primarily understood in terms of the collective rotation of a moderately deformed oblate nucleus, along with contributions to the angular momentum from two pairs of rotation-aligned $i{}_{13/2}$ neutrons.