Primary $$\gamma$$-ray intensities and $$\gamma$$-strength functions from discrete two-step $$\gamma$$-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 ($$\textit{d, p}$$) 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 $$\gamma$$-ray intensities of the 63,65Cu($$\textit{p},\gamma$$) reactions to extract $$\gamma$$-ray strength information on the nuclei 64,66Zn. Here, 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 64Zn from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for 66Zn 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.
- Research Organization:
- Ohio Univ., Athens, OH (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); German Research Foundation (DFG); European Research Council (ERC)
- Grant/Contract Number:
- NA0002905; ZI 510/8-1; 637686
- OSTI ID:
- 1801134
- Journal Information:
- Physical Review C, Vol. 101, Issue 4; ISSN 2469-9985
- Publisher:
- American Physical Society (APS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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