Studying Fission Cross Sections near ¹⁹⁸Pb with AT-TPC at FRIB

In the region of the neutron deficient pre-actinides, around ¹⁹⁸Pb and ¹⁸⁰Hg, there is an unexpected island of asymmetric fission. To develop a method to determine fission barriers for rare isotopes and to study the transition between symmetric and asymmetric fission near ¹⁹⁸Pb, rare isotopes were produced in the A1900 separator at the NSCL and fused with ⁴He target nuclei in the Active Target Time Projection Chamber (AT-TPC). In the experiment, the incident rare isotope beam particles were isotopically identified by the HEavy ISotope Tagger (HEIST), which allowed for the identification of the fissioning nucleus on an event by event basis. A combination of analytical and machine learning methods provided the identification of fission events. The stopping power of the beam particles in the helium gas has been measured by analyzing the Z and velocity of fission fragments at large folding angles. These experimental techniques and the energy dependence of isotopically resolved fusion fission cross-sections will be presented. Preliminary fission barriers extracted from the cross sections will also be presented.