Large deformation in A ~ 170 nuclei at high excitation energies

The "/-ray decay of the giant dipole resonance ( G D R ) buil t on highly excited states has not only become a very impor tan t tool for the study o f nuclear shape effects as a function o f tempera ture and angular m o m e n t u m [ 1 ], but is also sensit ive to react ion dynamical effects [2 ]. For example, a reduct ion of the fission probabi l i ty at the early stages of the decay of a highly excited compound nucleus was observed as an enhancement of G D R y rays preceeding fission [ 3,2]. This enhancement o f pre-fission "/rays by dynamical effects provides a means to study such nuclear structure effects as the evolut ion o f the nuclear shape at high tempera tures which is otherwise not accessible. Especially interest ing are mass regions and excitat ion energies where only a part of the total fusion cross section leads to fission, so that b y m e a s u r ing y-ray-f iss ion coincidences one probes the small range of angular momen ta where fission is dominan t [4] . In this let ter we present y-ray-f iss ion coincidence measurements in the mass region A ~ 170. Our mot iva t ion is to provide informat ion on the shape of highly excited nuclei with very large angular momenta. The analysis reported here suggests very highly deformed shapes.