Fission fragment mass distributions from ${}^{210}\mathrm{Po}$ and ${}^{213}\mathrm{At}$

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 ${}^{210}\mathrm{Po}$ nucleus regarding the role of shell effect in fission around $\approx 30–40\mathrm{MeV}$ of excitation energy.

Purpose: We have measured the fission fragment mass distribution of ${}^{210}\mathrm{Po}$ populated using fusion of ${}^4\mathrm{He}+{}^{206}\mathrm{Pb}$ 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 neighboring nuclei ${}^{213}\mathrm{At}$ 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 ${}^4\mathrm{He}+{}^{206}\mathrm{Pb}$ and ${}^4\mathrm{He}+{}^{209}\mathrm{Bi}$ 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 ${}^{210}\mathrm{Po}$, 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 ${}^{210}\mathrm{Po}$ was observed at the excitation energy $\approx 31\mathrm{MeV}$ as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of ${}^{210}\mathrm{Po}$.

Figure 1

Measured fission cross section for the reaction ${}^4\mathrm{He}+{}^{206}\mathrm{Pb}$. The red points correspond to the presently measured values, and the black points correspond to previously measured values of Moretto et al. [16]. The uncertainties in the cross sections are smaller than the points.

Figure 2

Fission fragment mass distributions for ${}^{210}\mathrm{Po}$. The red line indicates the single Gaussian fit.

Figure 3

Fission fragment mass distribution of ${}^{210}\mathrm{Po}$ at an excitation energy of 30.8 MeV. The red dashed line indicates the Gaussian fit corresponding to the symmetric fragments, while the blue solid lines show the asymmetric components. The overall fitting is shown by the solid black line.

Figure 4

Fission fragment mass distribution for ${}^{213}\mathrm{At}$ at various excitation energies. The red lines show the single Gaussian fit.

Figure 5

The variation of the width of the mass distribution with excitation energy. The shaded region shows the uncertainty of the statistical model values.