Fission fragment mass distribution in the S32+Sm144 reaction

Background: An asymmetric fission was reported by Andreyev et al. in \ensuremath{\beta}-delayed fission of $^{180}\mathrm{Tl}$ [Phys. Rev. Lett. 105, 252502 (2010)]. Subsequent theoretical calculations suggested that the asymmetric nature of the mass distribution is not restricted to the $^{180}\mathrm{Hg}$ only but is also expected for many other nuclei in the mass region $A\ensuremath{\approx}180$. Thus, it is important to investigate fission fragment mass distribution for different fissioning systems over a wide range of excitation energy in the mass region $A\ensuremath{\approx}180$.Purpose: Present measurements have been carried out to study the nature of the fission fragment mass distribution in the $^{32}\mathrm{S}+^{144}\mathrm{Sm}\ensuremath{\rightarrow}^{176}\mathrm{Pt}$ reaction in the compound nucleus excitation energy range of 38.7--47.5 MeV and investigate the role of multimodal fission.Method: Mass distributions have been determined from the time of flight (TOF) of the fission fragments, which was measured with respect to the beam pulse. Two multiwire proportional counters were placed at the folding angle to detect the fission fragments. Measured TOF of the fission fragments was used to obtain their velocities, which were further used to obtain the fission fragment mass distribution.Results: The fission fragment mass distributions at all three beam energies were observed to have flattop natures, which could not be fitted well by a one-Gaussian function. A fit using a two-Gaussian function significantly improved the ${\ensuremath{\chi}}^{2}$ values. The ratio of the most probable heavy to light fragment mass (${A}_{\mathrm{H}}/{A}_{\mathrm{L}}$) was observed as $\ensuremath{\approx}99.4/76.6$. A systematic study of the centroid values of asymmetric peaks in the mass distribution for different fissioning systems around mass region $A\ensuremath{\approx}180$ showed that heavier mass peak is centered around ${A}_{\mathrm{H}}\ensuremath{\approx}100$. Further analysis of ${A}_{\mathrm{H}}$ and ${A}_{\mathrm{L}}$ to obtain the corresponding neutron and proton numbers gave heavy fragment neutron number (${N}_{\mathrm{H}}$) around $\ensuremath{\approx}56$ and light fragment proton number (${Z}_{\mathrm{L}}$) in the range of $\ensuremath{\approx}34--36$.Conclusions: Observation of the flattop nature indicated the contributions from multimodal fission having both symmetric and asymmetric fission components, similar to those reported earlier in the mass region $A\ensuremath{\approx}180$, although it did not show a clear dip in symmetry as observed in some of the studies. The observed values of the neutron number in the heavy fragment (${N}_{\mathrm{H}}$) and proton number in the light fragment (${Z}_{\mathrm{L}}$) are consistent with the values recently proposed in Phys. Rev. C 100, 041602(R) (2019) and arXiv:2007.16184] for fissioning systems in the similar mass range. Comparison of the data on the width of the mass distribution from the present paper with those from similar fissioning systems with different ${Z}_{P}{Z}_{T}$ values suggested that the observed width of the mass distribution has a dependence on the neutron-proton configuration along with the entrance channel dynamics.