Search for possible fission modes at high excitation energies in Fm254

Background: Shell effects have been found to influence both the compound nuclear fission (CNF) and quasifission processes. Besides quasifission processes, which fission modes remain active at excitation energy (${E}^{*}$) as high as 56 MeV should be investigated.Purpose: We investigate the signatures of fission modes in $^{254}\mathrm{Fm}$ populated by $^{16}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$ through the mass distribution (MD) and total kinetic energy distribution (TKED).Method: The mass--total kinetic energy distributions (M-TKED) of fission fragments of the reaction $^{16}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$ have been measured at two laboratory energies ${E}_{\text{lab}}=89$ and 101 MeV. The spontaneous fission (SF) of $^{254}\mathrm{Fm}$, one-dimensional (1D) fragment MD, and two-dimensional (2D) M-TKEDs of $^{16}\mathrm{O}\phantom{\rule{0.28em}{0ex}}+\phantom{\rule{0.28em}{0ex}}^{238}\mathrm{U}$ have been described by the multimodal random neck rupture (MM-RNR) model.Results: Channel probabilities and the characteristics of different fission modes are obtained and discussed in detail. The enhancement observed in the mass yield ($\ensuremath{\approx}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}%$) in the region 60--70 u for the light fragments at ${E}^{*}\ensuremath{\approx}45$ MeV goes away at the higher ${E}^{*}\ensuremath{\approx}56$ MeV. The heavy fragments of S1 and S2 modes are found to be associated with $Z\ensuremath{\approx}53$ and $Z\ensuremath{\approx}56$ shells, respectively. The slope of asymmetric to symmetric fission yields (when plotted against ${E}^{*}$) of $^{16}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$ is found to be similar to that of previously reported $^{18}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{208}\mathrm{Pb}$.Conclusions: Analysis of 2D M-TKED data by the MM-RNR model reveals the possible presence of fission modes in $^{16}\mathrm{O}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U}$. The liquid-drop-like broad symmetric SL mode is found to peak at a lower energy than predicted by the Viola systematic, which matches mostly with that of Standard 2 mode. No signature of asymmetric quasifission is observed. The MD widths show a linear dependence with the measured energies.