Systematic study of the radiative proton capture including the compound, pre-equilibrium, and direct mechanisms

Abstract

A comprehensive study of the proton capture reaction is performed. The three major reaction mechanisms, namely, compound-nucleus capture (CNC), pre-equilibrium capture (PEC), and direct capture (DIC), are simultaneously considered on the basis of the Hauser-Feshbach model, the exciton model, and potential model, respectively. The same nuclear ingredients are consistently used in the three models, and especially the same nucleon-nucleus optical model potential (OMP) ensures that the three components are calculated on the same footing and represents partial fluxes of the same total reaction cross section. For about 2700 nuclei with 8 Z 100 lying between the proton drip line and the valley of β stability, the proton capture cross sections and astrophysical reaction rates corresponding to the CNC, PEC, DIC, and total (CNC + PEC + DIC) contributions are computed. The specific nuclear structure ingredients involved in the calculation, namely the nuclear mass, electromagnetic multipole moments, γ -ray strength function, excited-level scheme, spectroscopic factor, and proton-nucleus OMP, are determined from experimental data whenever available and, if not, from global nuclear models. For the reactions involving the targets with mass number A 48, fair agreements between the calculated proton total capture cross sections and the experimental data are found. For the lightest nuclei (A < 28), however, it is found that only the predicted DIC cross sections reproduce the experimental results well, and the total CNC + PEC + DIC contributions tend to overestimate the experimental cross sections. Furthermore, the systematic analysis for the nuclei beyond 40 Ca shows that the relative contribution of the proton DIC reaction rate to the total reaction rate increases with increasing temperatures. In particular, the proton DIC reaction rates are comparable to the CNC + PEC reaction rates in the temperature range of the astrophysical p process (2-3 GK) for some open-shell nuclei in the mass region of 70 A 160 around the valley of β stability. Such a high DIC contribution stems from the large number of direct transitions to final excited states. In order to better understand the DIC contribution, further experiment and theoretical analyses need to be performed. The impact of the proton DIC reaction rates on the r p-and p-process nucleosynthesis remains to be investigated.