Signature of bi-modal fission in uranium nuclei

We report here the signature of bi-modal fission, one asymmetric and the other symmetric, in uranium nuclei in the mass range A = 230–236. 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 α 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 = 230–236, 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. (paper)