Pb *

Fission mass distributions for ^205,207,209Bi nuclei at excitation energies close to their fission barriers were measured to determine how the mass distributions change with excitation energy and the neutron number of the compound nucleus¹. Proton beams at various energies were delivered by the 14UD Tandem Electrostatic Accelerator at the Australian National University Heavy Ion Accelerator Facility. Fission fragments were measured using the CUBE fission spectrometer and fission fragment mass distributions were determined using a newly developed time difference analysis method. Measured mass distributions of all three Bi isotopes exhibit a component of mass-asymmetric fission at all energies studied. Comparisons between the three Bi isotopes at similar excitation energies hint at an increase in the mass-symmetric fission yield with increasing neutron number, which could be due to a decrease in the difference between the symmetric and asymmetric fission barriers.  The probability of mass-asymmetric fission decreases significantly with increasing excitation energy, from ~ 70% to ~ 40% over a 10 MeV range. The mass distributions were compared with GEF² and PES³ semiempirical models.

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