Observation of asymmetric fission inPb204at low excitation energies

The fragment mass and total kinetic energy distributions were measured in fission of ${}^{204}\mathrm{Pb}$, populated in the ${}^7\mathrm{Li}+{}^{197}\mathrm{Au}$ reaction at excitation energies down to 15.3 MeV above the fission barrier. Fits considering admixture of symmetric and asymmetric fission indicate the presence of substantial asymmetric fission at all measured energies. The measured mass distributions are in good agreement with the model predictions. The observed excitation energy dependence of asymmetric fission fractions for the present system is compared with those for near$-$by nuclei and predictions of theoretical models to understand the damping of shell correction at large deformation.