Study of nuclear shapes in Tl isotopes via excited state giant dipole resonance studies in the reaction 16O+181Ta.

The present paper describes the measurement of high energy gamma rays ({approximately}5{endash}22 MeV) in the reaction {sup 16}O+{sup 181}Ta. Singles measurements were performed at {ital E}({sup 16}O)=84, 89, and 107 MeV and the multiplicity gated measurements along with the angular distributions were made at {ital E}({sup 16}O)=94 MeV. It is seen that the Tl nuclei are nonspherical at the excitation energies and spins relevant for the present work. The angular distribution data can be explained only by incorporating large orientation fluctuations. An attempt has been made to extract information on shape fluctuations directly from the {gamma} spectra. Under the restriction of using a common {open_quote}{open_quote}intrinsic{close_quote}{close_quote} strength function characterized by {ital E}{sub 0} and {Gamma}{sub 0} for the multiplicity gated data sets, a small increase of the effective mean deformation and a drive towards triaxiality ({gamma}{approximately} 30{degree}) with angular momentum is brought out. If this restriction is relaxed, however, the data can be understood also with a smaller value of {gamma}. The singles data can be understood within the same framework with a higher value of {Gamma}{sub 0}. {copyright} {ital 1996 The American Physical Society.}