In the last Uppmax project (2022-22-154), the isomeric yield ratios were calculated by TALYS with different initial populations of the primary fission fragments. The initial population was reconstructed on a basis of the fission model GEF. One parameter, which represents the root-mean-square angular momentum (Jrms), is used in the reconstruction of the initial excitation matrix.
By varying the Jrms, the dependency of the population on the isomer and the ground state was obtained for each fission fragment.
For a given element, one method could be used to derive the Jrms of several fission fragments based on the measured ratios of the fission products, assuming the Jrms of every fission fragment doesn’t deviate a lot from the Jrms of each other. The Jrms of all fission fragments is sampled from a short interval, which would be tested. For those sampled Jrms, one could calculate the isomeric yield ratio of all measured fission products. The likelihood would be calculated between the calculated ratios and the experimental values.
This sampling would be conducted at least 100 000 times. According to the ranks of the likelihoods, the Jrms will be presented as a function of the mass number. If the Jrms converge from the random intervals to a narrow range, the most probable Jrms for each fission fragment is obtained.
This method was tested in a case. 100 000 runs of calculations cost 170 hours approximately.
A significant convergence was observed. In total, 9 elements would be calculated with this method. Each element means that up to 12 fission fragments should be taken into account.
To test this method in other fission systems, more cases would be calculated.