Oblate deformed band structures in ¹⁹⁹Tl

Thallium (Z = 81) isotopes, having one proton hole with respect to lead (Z = 82), exhibit a variety of phenomena. These range from well-deformed rotational bands in proton-rich isotopes to superdeformation at high spin and possible chiral bands. In the region of isotopes with A ≈ 200, collective and intrinsic modes of generating angular momentum complete leading to excited states with varying contributions from both mechanisms. The nucleus ¹⁹⁹Tl lies in this region of transition with excited states having been established only up to 33/2 ħ from two recent experiments. It is of interest to explore this competition between intrinsic and collective modes at high spin and also the shape evolution. In the present work, excited states at quite high spin were populated through multi-nucleon transfer using a 1450-MeV ²⁰⁹Bi beam from the ATLAS accelerator at Argonne National Laboratory incident on a ¹⁹⁷Au target