Primary γ-ray intensities and γ-strength functions from discrete two-step γ-ray cascades in radiative proton-capture experiments

Background: Reaction rates of radiative capture reactions can play a crucial role in the nucleosynthesis of heavy nuclei in explosive stellar environments. These reaction rates depend strongly on gamma-ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the gamma-ray strength function and the nuclear level density. Recently, the ratio method was applied to primary gamma rays observed from (d, p) reactions and nuclear resonance fluorescence measurements to extract the dipole strength in atomic nuclei and to test the generalized Brink-Axel hypothesis. Purpose: The purpose of this work is to apply the ratio method to primary y -ray intensities of the Cu-63,Cu-65(p,gamma) reactions to extract gamma-ray strength information on the nuclei Zn-64,Zn-66. The impact of spin distribution, total gamma-ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative gamma-ray strength at different excitation energies, conclusions on the validity of the generalized Brink-Axel hypothesis can be made. Method: The radiative proton capture reaction measurements have been performed at the HORUS gamma-ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary gamma-ray intensities have been determined by normalizing secondary gamma-ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary gamma-ray intensities as well as to previous measurements by Erlandsson et al. at different excitation energies. Results: The relative strength function curve for Zn-64 from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for Zn-66 where both measurements were at almost the same excitation energy. Absolute gamma-strength function values have been obtained by normalizing the relative curves to quasiparticle random phase approximation calculations because of the absence of experimental data in the respective energy region. Conclusion: The generalized Brink-Axel hypothesis, i.e., the independence of the strength function on the excitation energy, seems to hold in the studied energy region and nuclei. The method to obtain primary gamma-ray intensities from two-step cascade spectra was shown to be a valuable and sensitive tool although its uncertainties are connected to the knowledge of the low-energy level scheme of the investigated nucleus. The scaling in the ratio method should be taken with care, because the relative strength is not a simple sum of f(E1) and f(M1) but a somewhat complex linear combination dependent on the excitation energy of the nucleus.