Signature of bi-modal fission in uranium nuclei

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 α induced fission of 232 Th. 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. Corresponding author email: alok@vecc.gov.inIt is well known that the liquid drop model [1 -3] only leads to symmetric division of the fissioning nucleus into two fragments of equal mass. To explain the observed highly asymmetric mass distribution in fission of actinides, especially in thermal neutron induced fission of 235 U, the liquid drop potential energy needs to be corrected for the shell and pairing effects [4]. The spherical shell corresponding to major shell closure at N=82 (doubly magic 132 Sn) and deformed shell corresponding to N=88 appear to play a major role in the asymmetric split. However, it has