Speaker
Description
The observation of asymmetric fission in the pre-actinide nucleus $^{180}$Hg at low excitation energies has renewed considerable interest in the role of shell effects in the fission process. At the same time, understanding the competition between quasi-fission (QF) and fusion–fission (FF) remains crucial for elucidating the formation mechanism of super heavy elements (SHEs). Since the predictive power of current theoretical models for quasi-fission is limited, systematic experimental investigations are essential for constraining reaction dynamics.
We have carried out a series of experiments [1–7] to measure fission-fragment mass and total kinetic energy (TKE) distributions for a variety of projectile–target combinations. The measurements were performed using large-area multi-wire proportional counters (MWPCs) at VECC Kolkata, the BARC-TIFR Pelletron facilities, and JINR Dubna. The combined analysis of mass and TKE observables provides a sensitive probe of shell effects and reaction mechanisms. In this contribution, we present recent results for the reactions $^{4}\mathrm{He} + ^{197}\mathrm{Au}$,$^{35}\mathrm{Cl} + ^{181}\mathrm{Ta}$, and $^{35}\mathrm{Cl} + ^{205}\mathrm{Tl}$.
For the $^{4}\mathrm{He} + ^{197}\mathrm{Au}$ reaction forming $^{201}$Tl, the measured mass distributions reveal clear evidence of multimodal fission over a broad range of excitation energies [2]. Analysis of mass–energy correlations identifies three distinct fission modes, highlighting the stabilizing influence of shell closures associated with (Z=36) and (Z=52). The asymmetric component weakens with increasing excitation energy, and the (Z=52)-driven mode disappears around E*=45.5 MeV. These observations are supported by density-functional-theory-based potential-energy-surface calculations [2].
The reactions $^{35}\mathrm{Cl} + ^{181}\mathrm{Ta}$ and $^{35}\mathrm{Cl} + ^{205}\mathrm{Tl}$, leading to the compound nuclei 216Th and 240Cf, respectively, exhibit pronounced asymmetric mass distributions and deviations of the mean TKE from Viola systematics, while showing no significant mass–angle correlations. Despite these similarities, the two systems are governed by distinct reaction mechanisms. For $^{35}\mathrm{Cl} + ^{181}\mathrm{Ta}$, the combined analysis of mass distributions and TKE observables indicates a significant contribution from slow quasi-fission near the Coulomb barrier. The quasi-fission fractions extracted from the experimental mass distributions are found to be in good agreement with dinuclear system (DNS) model calculations [1]. In contrast, for $^{35}\mathrm{Cl} + ^{205}\mathrm{Tl}$, the asymmetric mass distributions, characterized by a heavy-fragment peak around A≈ 132 together with the corresponding TKE observables, indicate that the asymmetry originates predominantly from fragment shell effects. A comparison with other fissioning systems in the Cf [9] region further shows that the asymmetric fission fraction decreases with increasing excitation energy while remaining largely insensitive to entrance-channel properties, highlighting the dominant role of shell effects in driving asymmetric fission in this mass region.
We acknowledge all our collaborators and the staff of the accelerator facilities for their support.
[1] A. Sultana, A. Sen, T.K. Ghosh, et al; Phys. Rev. C. (accepted).
[2] A. Sultana, A. Sen, T.K. Ghosh, et al; Phys. Rev. C. 96, 064609 (2025).
[3] K. Atreya, A. Sen, T. K. Ghosh et al; Phys. Rev. C 108, 034615 (2023).
[4] D. Paul, A. Sen, T. K. Ghosh et al; Phys Rev C 104, 024604 (2021).
[5] Kozulin, Knyazheva, T.K. Ghosh, A. Sen et al; Phys. Rev. C. 99, 014616 (2019).
[6] A. Sen, T.K. Ghosh, Kozulin et al; Phys. Rev. C. 105, 014627 (2022).
[7] K. Banerjee, T.K. Ghosh et al. Phys. Rev. C 93, 064602 (2016).
[8] K.-H. Schmidt, et.al., Nucl. Phys. A 665, 221 (2000)
[9] J. Khuyagbaatar. et al; Phys. Rev. C. 91, 054608 (2015)