Absence of entrance channel effects in fission fragment anisotropies of theFr215compound nucleus
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Abstract
Fission fragment angular distributions have been measured for the reactions $^{11}\mathrm{B}+^{204}\mathrm{Pb}$ and $^{18}\mathrm{O}+^{197}\mathrm{Au}$, both leading to the same compound nucleus $^{215}\mathrm{Fr}$ at near barrier energies. The measured fission fragment anisotropies as a function of ${E}_{\mathrm{c}.\mathrm{m}.}/{V}_{B}$ are found to be consistent with the predictions of the standard saddle point statistical model (SSPM) for both the systems, suggesting the absence of entrance channel effects on fission fragment anisotropies even though the entrance channel mass asymmetries for both these systems fall on either side of the Bussinaro-Gallone critical mass asymmetry. The consistency of the present results with SSPM predictions can be understood within the framework of the pre-equilibrium fission model where fission before $K$ equilibration is severely inhibited by the high values of ratios of fission barrier height to nuclear temperature.