Observation of mass-asymmetric fission of mercury nuclei in heavy ion fusion

Background: Mass-asymmetric fission has been observed in low energy fission of Hg-180. Calculations predicted the persistence of asymmetric fission in this region even at excitation energies of 30-40 MeV. Purpose: To investigate fission mass distributions by populating different isotopes of Hg using heavy ion fusion reactions. Methods: Fission fragment mass-angle distributions have been measured for two reactions, Ca-40+Nd-142 and C-13+W-182, populating Hg-182 and Hg-195, respectively, using the Heavy Ion Accelerator Facility and CUBE spectrometer at the Australian National University. Measurements were made at beam energies around the capture barrier for the two reactions and mass ratio distributions were obtained using the kinematic reconstruction method. Results: Asymmetric fission has been observed following the population of Hg-182 at an excitation energy of 22.8 MeV above the saddle point. A symmetric peaked mass ratio distribution was observed for Hg-195 nuclei at a similar excitation energy above the saddle point. Conclusions: Mass-asymmetric fission has been observed in neutron deficient Hg nuclei populated via heavy ion fusion for the first time. The results are consistent with observations from beta-delayed fission measurements and provide a proof-of-principle for expanding experimental studies of the influence of shell effects on the fission processes.