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Background: The influence of shell effect on the dynamics of the fusion fission process and it's evolution with excitation energy in the pre-actinide 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 210 Po nucleus regarding the role of shell effect in fission around ≈ 30 -40 MeV of excitation energy.Purpose: We have measured the fission fragment mass distribution of 210 Po populated using fusion of 4 He + 206 Pb 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 neighbouring nuclei 213 At is also studied for comparison.Methods: Two large area Multi-wire 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 4 He + 206 Pb and 4 He + 209 Bi 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 210 Po, 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 210 Po was observed at the excitation energy ≈ 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 210 Po.

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