Background: Mass-asymmetric fission in the actinide nuclei is dominated by shell gaps in the heavy fragment, proposed to be due to octupole deformed shell gaps between $Z=52$ and 56. The mass-asymmetric fission of nuclei lighter than lead shows the influence of different shell gaps, centered around $Z=34,36$ and $Z=44,46$. There is indirect evidence that the $Z=44$ shell gap may influence fission in isotopes as heavy as ${}^{240}\mathrm{Pu}$.

Purpose: Examine the fission modes present in ${}^{216–221}\mathrm{Ra}$ (proton number $Z=88$) where the $Z=44$ shell gap may be expected to influence fission at mass symmetry, and investigate their evolution along the Ra chain.

Method: Fission of ${}^{216–221}\mathrm{Ra}$ was induced in the heavy-ion fusion of ${}^{12,13}\mathrm{C}$ with ${}^{204,206,208}\mathrm{Pb}$ at excitation energies between 24 and 28 MeV. Fission fragments were detected in coincidence and their mass ratios and total kinetic energies (TKEs) reconstructed.

Results: The presence of the asymmetric modes of fission near $Z=52,56$ and the presence of a mode located at symmetry ($Z=44$) with TKE higher than the wide mass-symmetric (liquid-drop) fission is established over the entire ${}^{216–221}\mathrm{Ra}$ chain. In ${}^{216–219}\mathrm{Ra}$, two-dimensional mass-ratio-TKE fits suggest the additional influence of fission driven by $Z=46$ and/or its complementary fragment at $Z=42$.

Conclusions: High-statistics measurements of heavy-ion fusion-fission, combined with statistically rigorous analysis, enables high-fidelity decomposition into Gaussian fission modes, even at relatively high excitation energies. Consistent evidence for a high-TKE shell-driven mode at $Z=44$ contributes to a growing body of evidence that the $Z=44$ mode shell gap is important both in the preactinides and in the actinides, and to the idea that bimodal symmetric fission may occur whenever a favorable shell gap coincides with mass symmetry.

Close

PrevNext

View figure in article

• Fission
• Low & intermediate energy heavy-ion reactions
• Nuclear reactions
• 190 ≤ A ≤ 219
• A ≥ 220
• Radiation detectors