Systematical nucleon-induced optical model analysis for medium and heavy nuclei with coupled-channel framework

Abstract

Optical model analyses have been carried out for nucleon-induced reactions on medium and heavy nuclei from 1 keV up to 200 MeV. The coupled-channels method based on the rigid-rotator model has been employed as an initial approach. It has been found that the coupled-channels analyses are indispensable to obtain a global formulation of optical potential. In addition, the coupled-channels calculation based on the soft-rotator model has been applied to analyses for various even-even fission product isotopes. They reproduced measured cross sections better than the rigid-rotator model in many cases. Though the latter is our ongoing study, we show the advantage of coupled-channels calculations and the applicability of the soft-rotator model. There is no doubt that the optical model plays an important role in evaluating/predicting scattering and absorption cross sections above the resolved resonance region. Since it also provides transmission coefficients required in the statistical model, the optical model potential must be determined pre- cisely in the nuclear data evaluation. The optical potential varies with projectiles, target nuclei and collision energies and it is generally optimized on experimental data of total, absorption and scattering cross sections. However, measured data are very scarce in general, except for some special nuclei, namely, they are sparse over wide nuclear-mass and energy ranges. Therefore, a problem arises, e.g., in doing nuclear data evaluations for the bulk of fission products. Therefore, it might be required to derive an optical potential which predicts cross sections with high accuracy for various nuclei over a wide energy range. Until now, a lot of studies have been devoted in order to obtain such potential. The most useful one was done by Koning and Delaroche (1). However, the spherical shape was assumed in their analyses, where a consideration of deformed shapes and the coupled-channels calculation should give more actual answers. In fact, it is widely known that there exist many deformed nuclei in their ground and/or excited states. In this study, nucleon-induced optical model analyses are carried out in the coupled-channels frameworks for various nuclei (26 ≤ Z ≤ 92) in the energy range from 1 keV up to 200 MeV. The coupled-channels method based on the rigid- rotator model (RRM-CC) is employed as an initial approach, in order to survey global tendencies of optical potential parameters. Then, we apply the soft-rotator model (SRM) (2) to the coupled-channels calculations for various even-even fission product nuclei. We show the advantage of the coupled- channels calculation and applicability of the SRM in this work. We also would like to discuss about systematics for ground- state deformation within the framework of the SRM-CC.