Dr K. Ramachandran
Nuclear Physics Department

Nuclear fission is a unique process in which the shape of a nearly equilibrated system evolves continuously until it splits into two fragments. Fission dynamics is one of the research area which is still not understood completely. I will be presenting about a couple of aspects in the topic of fission dynamics namely fission time scale and fission fragment mass distribution in the mass 180-200 region.

I. Fission time scale: It is found that fission takes more time than the prediction of statistical model calculations. This delay in the fission process might be due to the viscous nature of the nuclear medium. To get a deeper insight into this effect, fission life times have been measured using various techniques. I will present the result of a simultaneous measurement of pre-scission multiplicities and evaporation residue cross-sections and analysis using a statistical model which includes deformation effects.

II. Fission fragment mass distributions have been measured for many systems and understood within the liquid drop model with fragment shell effects incorporated in the model. However, a recent experiment showed that fission of 180Hg following electron capture on 180Tl leads to asymmetric mass distribution. From fragment shell structure point of view, the nuclei in these mass region were expected to have symmetric fission. Recent calculations indicate that the mechanism of asymmetric fission must be very different in these mass region compared to the actinide region. We have measured the fission fragment mass distribution for 13C+182W,176Yb systems forming compound nucleus of 195Hg and 189Os respectively at various energies using the ANU Heavy Ion accelerator. The analysis indicates that, contrary to 180Hg, there is no asymmetric mass distribution for the systems studied. The results of the current study will be presented.