Prolate yrast cascade in ${}^{183}\mathrm{Tl}$

The yrast sequence in ${}^{183}\mathrm{Tl}$ has been studied for the first time in recoil-mass selected γ-ray spectroscopic measurements. A rotational-like cascade of seven transitions is established down to the band head with probable spin and parity ${(13/2\mathrm{}}^{+}).\mathrm{}$ Unlike in the adjacent odd-mass Tl nuclei, prompt γ decay from the yrast band to a lower lying weakly deformed (oblate) structure is not observed. These features are consistent with the predicted drop of the prolate band head in ${}^{183}\mathrm{Tl}$ compared to ${}^{185}\mathrm{Tl}.$ The implications for the prolate energy minimum in odd-mass Tl nuclei at the neutron $i_{13/2}$ midshell $\mathrm{}(N=103\mathrm{})\mathrm{}$ are discussed.