The assymetric mass yield distribution is due to the lowering of the fission barrier by formation of a doubly magic Sn-132 core in the heavy fragment. Symmetric fission fragments increase in yield as neutron energy increases. This observation led to the two-mode fission hypothesis. The neutron energy dependence of the ratio of symmetric to asymmetric fission is surprisingly linear up to the onset of second chance fission. At low neutron energy, i.e., thermal and epithermal, nuclear structure affects the ratio of symmetric to asymmetric fission. The J = 3- resonances favor assymmetric fission. This is because the 3- state in the U-236 compound nucleus belongs to an octopole vibration, whereas the 4- state belongs to a two quasi-particle quadrupole state. We arrive at this conclusion using the mass yields from the "wheel" experiment and the ENDFB VII.1 fission cross sections. Our measured Cd-115g and Ag-111 to Mo-99 ratios can only be fit by assigning the J = 3- resonances to the more assymetric group from the "wheel" experiment.

U-235 fission mass yield dependence on resonance and neutron energy

253rd National Meeting (2017)

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