Fission fragment mass distributions from Po210 and At213

Background: The influence of shell effect on the dynamics of the fusion fission process and its evolution with excitation energy in the preactinide Hg-Pb region in general is a matter of intense research in recent years. In particular, a strong ambiguity remains for the neutron shell closed Po-210 nucleus regarding the role of shell effect in fission around approximate to 30-40 MeV of excitation energy. Purpose: We havemeasured the fission fragment mass distribution of Po-210 populated using fusion of He-4 + Pb-206 at different excitation energies and compare the result with recent theoretical predictions as well as with our previous measurement for the same nucleus populated through a different entrance channel. Mass distribution in the fission of the neighboring nuclei At-213 is also studied for comparison. Methods: Two large area multiwire proportional counters (MWPC) were used for complete kinematical measurement of the coincident fission fragments. The time of flight differences of the coincident fission fragments were used to directly extract the fission fragment mass distributions. Results: The measured fragment mass distribution for the reactions He-4 + Pb-206 and He-4 + Bi-209 were symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36.7 MeV and 32.9 MeV, respectively, indicating the absence of shell effects at the saddle. However, in the fission of Po-210, we find minor deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV). Conclusion: Persistence of shell effect in fission fragment mass distribution of Po-210 was observed at the excitation energy approximate to 31 MeV as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of Po-210.