Studying Fission Cross Sections near ¹⁹⁸Pb

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 in this region, a cocktail beam of heavy isotopes, including ¹⁹⁰Au, ¹⁹²Hg, ¹⁹⁸Pb, and ²⁰⁰Bi, was produced at the National Superconducting Cyclotron Laboratory at Michigan State University and fused with ⁴He target nuclei in the Active Target Time Projection Chamber (AT-TPC). Analysis presented several challenges that have been overcome. In the experiment, the incident rare isotope beam particles were isotopically identified by the HEavy ISotope Tagger (HEIST), which allows for the identification of the fissioning nucleus on an event-by-event basis. To identify fission events, a combination of analytical and machine learning methods are used. The energy of the beam particles at the reaction has been measured by analyzing the Z and velocity of fission fragments at large folding angles. Space charge corrections have been found with a combination of theoretical models and experimentally determined parameters. These experimental techniques and the energy dependence of isotopically resolved fusion fission cross-sections will be presented together with recently extracted fission barriers from the cross sections and charge distributions of the fission of ²⁰⁴At.