Effect of neutron alignments on the structure of Tl197

Abstract

The excited states of $^{197}\mathrm{Tl}$ have been studied via $^{197}\mathrm{Au}(^{4}\mathrm{He}$, $4\mathrm{n})^{197}\mathrm{Tl}$ reaction at a beam energy of 50 MeV from the K-130 cyclotron at the Variable Energy Cyclotron Centre (VECC). The $\ensuremath{\gamma}$ rays were detected using the VECC array for Nuclear Spectroscopy (VENUS) with six Compton-suppressed clover HPGe detectors. An improved level scheme of $^{197}\mathrm{Tl}$ has been proposed from this work, which has been extended to 5.1 MeV of excitation energy and 39/2 $\ensuremath{\hbar}$ of spin from the placement of 28 new $\ensuremath{\gamma}$-ray transitions. Band crossings in the known one- and three-quasiparticle (qp) bands have been identified for the first time in this work. Two new bands, based on 3-qp and 5-qp configurations, have been observed for the first time in this nucleus; both of which are identified as the magnetic rotational (MR) in nature. The excitation energies of these bands are similar to that of the doubly degenerate bands observed for the similar 3-qp and 5-qp configurations in $^{195}\mathrm{Tl}$. These indicate a transition from an aplanar geometry of the neutron, proton, and the core angular momentum vectors to a planar one for neutron number $N\ensuremath{\ge}116$ in Tl isotopes. The total Routhian surface calculations suggest a change in shape from oblate for the 1-qp configuration to a near-spherical one for the 3- and 5-qp configurations. This is consistent with the observed MR nature of the bands with multi-qp configuration. The MR bands are well reproduced by a model calculations in the frame work of the shears mechanism with the principal axis cranking.