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

Abstract

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 γ -ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the γ -ray strength function and the nuclear level density. Recently, the ratio method was applied to primary γ 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 γ -ray intensities of the 63,65 Cu(p,γ ) reactions to extract γ -ray strength information on the nuclei 64,66 Zn. The impact of spin distribution, total γ -ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative γ -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 γ -ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary γ -ray intensities have been determined by normalizing secondary γ -ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary γ -ray intensities as well as to previous measurements by Erlandsson et al. at different excitation energies. Results: The relative strength function curve for 64 Zn from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for 66 Zn where both measurements were at almost the same excitation energy. Absolute γ -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 γ -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 E 1 and f M1 but a somewhat complex linear combination dependent on the excitation energy of the nucleus.

Independent normalization for γ -ray strength functions: The shape method

“…When the ratios from different excitation energies are compared, information on the energy dependence of the γ SF is obtained as demonstrated from (d, pγ γ ) [12], (p,p'γ γ ) [14], ( γ , γ γ ) [49], and (p, γ γ ) [50] reactions.…”

Comprehensive Test of the Brink-Axel Hypothesis in the Energy Region of the Pygmy Dipole Resonance

“…Because of the importance for astrophysical applications, there are many recent experimental studies in the low-energy regime with controversial results, claiming either confirmation [18][19][20][21] or violation [22][23][24][25] of the BA hypothesis. Possible nonstatistical γ-width distributions observed in sand p-wave neutron capture experiments [26] would also represent proof against the BA hypothesis [27].…”

Independent Normalization for $γ$-ray Strength Functions: The Shape Method

“…When the ratios from different excitation energies are compared, information on the energy dependence of the γSF is obtained as demonstrated from (d,pγγ) [12], (p,p'γγ) [14], ( γ, γγ) [45], and (p,γ) [46] reactions.…”