Shape evolution of hot, rotating Tl nuclei

In order to investigate the role played by the Z = 82 shell closure in determining the shape of the nuclei in this mass region we have investigated the shapes of excited 197Tl nuclei populated in the reaction 181Ta + 16O by measuring the high energy gamma rays emitted in the reaction. The following experiments have been carried out: i) gamma-ray spectra were measured for three beam energies, ii) multiplicity dependent gamma-ray spectra and iii) gamma-ray angular distribution measurements. These measurements could give unambiguous information about the nuclear shapes. The 16O beam from the Pelletron at TIFR was used in these experiments. The observed data do not point to a strong dependence of the nuclear shape on the spin and temperature effect seems to be important. Our data thus indicate either a temperature induced shape change or, more likely, the need to consider shape fluctuations of the nuclei at non-zero temperature. (author). 5 refs., 2 figs., 1 tab