Isomeric yield ratios in proton-induced fission of 238U

Abstract

Background: Isomeric yield ratios are an important observable in nuclear fission as they can guide model development by providing insight into the angular momentum generation. Furthermore, isomeric yield ratios are important in applications for nuclear energy, as well as in the study of the r process in stellar nucleosynthesis, and in the antineutrino mixing angle from reactor spectra. In nuclear data evaluations, the Madland-England model is commonly used to estimate isomeric yield ratios that have not been measured. Purpose: To measure isomeric yield ratios in 25-MeV proton-induced fission of 238U, and to compare the result with the values obtained from the Madland-England model and the fission model code GEF. Furthermore, to evaluate whether the predictions of GEF can be improved by coupling it to the nuclear reaction code TALYS. Methods: Isomeric yield ratios in 25-MeV proton-induced fission of 238U have been measured at the Ion Guide Isotope Separate On-Line facility. The excited state and the ground state were separated by mass using the phase-imaging ion-cyclotron-resonance technique in the double Penning trap JYFLTRAP. The number of counts of each state was extracted from the phase-images using a Bayesian Gaussian mixture model and, after corrections for detector efficiency and decay, the isomeric yield ratios were derived. The experimental values have been compared with the calculated results from the Madland-England model and the GEF code. Furthermore, GEF has been combined with the nuclear reaction code TALYS, in order to take advantage of the latter codes' implementation of the Hauser-Feshbach formalism, and the results have been compared with the experimental values. Results: From the measurements, 19 new isomeric yield ratios in 25-MeV proton-induced fission of 238U are reported and are, together with another 12 isomeric yield ratios (IYRs) from a previous campaign, compared with the model calculations. It is shown that, though the models manage to capture some of the features observed, there is room for improvement. Conclusions: As predicted by the Madland-England model, a strong correlation between the measured IYRs and the spins of the long-lived states of the fission products is confirmed. However, the IYRs also vary between nuclides with the same spin-parity of the two states, and systematic trends in the IYRs of close-lying isotopes and isotones with similar nuclear configurations are observed. From the comparison of the experimental data with the prediction of GEF it is concluded that more data from proton-induced fission are needed to optimize the internal parameters of GEF. Furthermore, using a combination of GEF and TALYS in most cases results in an underestimation of the yield ratios. This might be explained by an underestimation of the angular momentum on the initial fission fragments by GEF. Altogether, these results highlight the possibility to use measurements of IYRs to improve model predictions and to study the angular momentum generation in nuclear fission.