Background: For several decades, it was believed that while the fission fragment mass distributions of actinides nuclei are asymmetric, the distributions gradually become symmetric for pre-actinide nuclei. Earlier measurements suggested that the transition from asymmetric to symmetric mass distributions occurs for ${}^{201}\mathrm{Tl}$, where the contribution of the asymmetric component appears to vanish. However, a recent reanalysis of the previously measured fission data has indicated the possible presence of asymmetric fission modes in ${}^{201}\mathrm{Tl}$ as well.

Purpose: To search for the fission modes in ${}^{201}\mathrm{Tl}$, the measurement of the fission fragment mass and total kinetic energy distribution (TKE) in the reaction ${}^4\mathrm{He}+{}^{197}\mathrm{Au}$ was carried out at various excitation energies. To check the predictive powers of theoretical models, the measured mass distributions were compared with the general description of fission observables (GEF) model. To investigate the origin of fission modes, state-of-the-art density functional theory (DFT) calculations were performed to generate the potential energy surface.

Methods: Two multiwire proportional counter detectors were employed to detect fission fragments. These detectors were positioned at the folding angles calculated using Viola's systematic. The mass of the fission fragments was determined using the time-of-flight difference method. By measuring the flight time, the velocities of the fission fragments were calculated, and these velocities were subsequently used to determine the total kinetic energy (TKE).

Results: The mass distributions analyzed across all excitation energies exhibited asymmetry. This asymmetry was evident in the mass-mean TKE correlations, which deviated from the parabolic pattern predicted by the liquid drop model (LDM). To account for the unique features of the mass and energy distributions, two asymmetric modes and one symmetric mode were required. The asymmetric modes are associated with the stabilizing effects of proton shells at $Z_L\approx 36$ (A1) and $Z_H\approx 52$ (A2), whereas the symmetric mode aligns with the predictions of the LDM.

Conclusion: The decomposition of mass and energy distributions suggests multimodal fission in the pre-actnide nucleus ${}^{201}\mathrm{Tl}$, which was earlier envisaged as a transitional nucleus with unimodal fission. DFT calculations also support the existence of asymmetric modes in this nucleus. The experimental findings align qualitatively with the semi-empirical GEF code, although they were unable to reproduce the experimental data accurately.

Close

PrevNext

View figure in article

• Nuclear reactions
• 190 ≤ A ≤ 219
• Radiation detectors