  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 $\approx$ 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 $\approx$ 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.
