  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.
