Signature of bi-modal fission in Uranium nuclei

TL;DR

This paper identifies bi-modal fission in Uranium nuclei, revealing both asymmetric and symmetric modes, with the asymmetric component being dominant, based on analysis of experimental mass distributions across various excitation energies.

Contribution

It provides the first model-independent evidence of bi-modal fission in Uranium nuclei and quantifies the asymmetric and symmetric components at different excitation energies.

Findings

Bi-modal fission observed in Uranium nuclei with asymmetric (~85%) and symmetric (~15%) components.

Symmetric fission occurs from a highly deformed saddle configuration with a well-developed neck.

Shell effects are washed out beyond 25 MeV, leading to predominantly symmetric fission.

Abstract

We report here the signature of bi-modal fission, one asymmetric and the other symmetric, in Uranium nuclei in the mass range A = 230 to 236. The finding is unexpected and striking and is based on a model independent analysis of experimental mass distributions (cumulative yields) at various excitations from about 23 to 66 MeV in the alpha induced fission of 232Th. It has been found that the observed asymmetry in the mass distributions and the unusually narrow peak in the symmetry region, can both be explained in a consistent manner if one assumes: a) multi-chance fission, b) bi-modal fission at lower excitations (9 < E* < 25 MeV) for all the Uranium nuclei in the range A = 230 to 236, and c) that the shell effects get washed out completely beyond about 25 MeV of excitation resulting in symmetric fission. The analysis has allowed a quantitative estimation of the percentages of the asymmetric and the symmetric component in the bi-modal fission. It has been found that the bi-modal fission in Uranium nuclei is predominantly asymmetric (~ 85%), which contributes in a major way to the observed asymmetric peaks, while the ~15% bi-modal symmetric fission is primarily responsible for the observed narrow symmetric peak in the mass distributions. The unusually narrow symmetry peak in the mass distributions indicates that the symmetric bi-modal fission in Uranium nuclei must have proceeded from a configuration at the bi-modal symmetric saddle that is highly deformed with a well-developed neck.