Fission fragment mass distributions from Po210 and At213

Abstract

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.

Into the Fission Valley of 'magic Nucleus' Polonium

“…The heavier mass side of the asymmetric distribution was observed to peak around ≈132 (doubly magic 132 Sn), a clear indication of the persistence of the shell effect of the nascent fragments. VECC experimental data [10] could reaffirm the theory [22]. It is clear from the above discussions that shell effect on the evolution of fission dynamics is still not fully understood.…”

“…The measured fragment mass distributions [10] for the reaction 4 He + 206 Pb populating 210 Po were found to be symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36 MeV, indicating the absence of shell effect at the saddle. However, deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV) was indeed observed as shown in Figure 4.…”

“…Since currently it is not possible to produce the element with N=184 (as that would require highly neutron rich targets and projectiles which are not available in nature), 210 Po with magic number N=126 has been studied extensively to explore the survival of nuclear shell effects around the excitation energy around 40 MeV, beyond which one would expect the shell effects to vanish. Researchers of our Variable Energy Cyclotron Centre, Kolkata carried out several experiments recently [9,10] to explore the existence of shell effects at fission saddle at moderate excitation energy for 210 Po.…”

Investigating the fission dynamics of the following neutron shell closed nuclei within a stochastic dynamical approach: 210Po, 212Rn, and 213Fr*

“…[63] can be attributed to the remnants of the ground state shells, and this is hence a consequence of not using a pure macroscopic potential energy surface as suggested in Ref. [64]. Nonetheless, the predictions of the multi-dimensional Langevin model for the data of Po as suggested by Karpov .…”

Microscopic self-consistent description of induced fission dynamics: Finite-temperature effects

“…The charge and mass distributions remain asymmetric up to Cf [3]. The probability of symmetric fission increases with excitation energy because of the weakening of shell effects [4][5][6][7][8][9][10][11][12].…”