Title: Primary $$\gamma$$-ray intensities and $$\gamma$$-strength functions from discrete two-step $$\gamma$$-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 $$\gamma$$-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.

Authors:

Scholz, P.  ^[1];

• Search DOE PAGES for author "Scholz, P."
• Search DOE PAGES for ORCID "0000-0003-1011-0791"

Guttormsen, M. ^[2]; Heim, F. ^[1]; Larsen, A. C. ^[2]; Mayer, J. ^[1]; Savran, D.  ^[3];

• Search DOE PAGES for author "Savran, D."
• Search DOE PAGES for ORCID "0000-0002-2685-5600"

Spieker, M. ^[1]; Tveten, G. M. ^[2]; Voinov, A. V. ^[4]; Wilhelmy, J. ^[1]; Zeiser, F. ^[2]; Zilges, A.  ^[1]

• Search DOE PAGES for author "Zilges, A."
• Search DOE PAGES for ORCID "0000-0002-9328-799X"

---

+ Show Author Affiliations

1. Univ. of Cologne (Germany)
2. Univ. of Oslo (Norway)
3. GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt (Germany)
4. Ohio Univ., Athens, OH (United States)

Publication Date:

Mon Apr 27 00:00:00 EDT 2020

Research Org.:

Ohio Univ., Athens, OH (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA); German Research Foundation (DFG); European Research Council (ERC)

OSTI Identifier:

1801134

Grant/Contract Number:  

NA0002905; ZI 510/8-1; 637686

Resource Type:

Accepted Manuscript

Journal Name:

Physical Review C

Additional Journal Information:

Journal Volume: 101; Journal Issue: 4; Journal ID: ISSN 2469-9985

Publisher:

American Physical Society (APS)

Country of Publication:

United States

Language:

English

Subject:

73 NUCLEAR PHYSICS AND RADIATION PHYSICS; Physics