Measurements of isomeric yield ratios of fission products from proton-induced fission on U-nat and Th-232 via direct ion counting

Independent isomeric yield ratios (IYR) of Ge-81, Y-96, Y-97, Nb-97, Sn-128 and Sn-130 have been determined in the 25MeV proton-induced fission of U-nat and Th-232. The measurements were performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyvaskyla. A direct ion counting measurement of the isomeric fission yield ratios was accomplished for the first time, registering the fission products in less than a second after their production. In addition, the IYRs of U-nat were measured by means of gamma-spectroscopy in order to verify the consistency of the recently upgraded experimental setup. From the obtained results, indications of a dependence of the production rate on the fissioning system can be noticed. These data were compared with data available in the literature, whenever possible. Using the TALYS code and the experimentally obtained IYRs, we also deduced the average angular momentum of the fission fragments after scission.