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 ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ 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 ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ 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 ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ 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 ($\approx {10}^{-2}\%$) in the region 60–70 u for the light fragments at $E^{*}\approx 45$ MeV goes away at the higher $E^{*}\approx 56$ MeV. The heavy fragments of S1 and S2 modes are found to be associated with $Z\approx 53$ and $Z\approx 56$ shells, respectively. The slope of asymmetric to symmetric fission yields (when plotted against $E^{*}$) of ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ is found to be similar to that of previously reported ${}^{18}\mathrm{O}+{}^{208}\mathrm{Pb}$.

Conclusions: Analysis of 2D M-TKED data by the MM-RNR model reveals the possible presence of fission modes in ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$. 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.

• Received 23 February 2022
• Accepted 1 April 2022

DOI:https://doi.org/10.1103/PhysRevC.105.044614