Prolate yrast cascade in {sup 183}Tl

The yrast sequence in {sup 183}Tl has been studied for the first time in recoil-mass selected {gamma}-ray spectroscopic measurements. A rotational-like cascade of seven transitions is established down to the band head with probable spin and parity (13/2{sup +}). Unlike in the adjacent odd-mass Tl nuclei, prompt {gamma} 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 {sup 183}Tl compared to {sup 185}Tl. The implications for the prolate energy minimum in odd-mass Tl nuclei at the neutron i{sub 13/2} midshell (N=103) are discussed. (c) 2000 The American Physical Society.