Probing astrophysically important states in the ²⁶Mg nucleus to study neutron sources for the s process

Talwar, R.; Adachi, T.; Berg, G. P. A.; Bin, L.; Bisterzo, S.; Couder, M.; deBoer, R. J.; Fang, X.; Fujita, H.; Fujita, Y.; Görres, J.; Hatanaka, K.; Itoh, T.; Kadoya, T.; Long, A.; Miki, K.; Patel, D.; Pignatari, M.; Shimbara, Y.; Tamii, A.; Wiescher, M.; Yamamoto, T.; Yosoi, M.

Abstract

Background: The ²²Ne(α,n) ²⁵Mg reaction is the dominant neutron source for the slow neutron capture process (s process) in massive stars, and contributes, together with C¹³(α,n)O¹⁶, to the production of neutrons for the s process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with ²²Ne(α,γ)²⁶Mg radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of ²⁶Mg near the α and neutron separation energies. These uncertainties affect the s-process nucleosynthesis calculations in theoretical stellar models. Purpose: Indirect studies in the past have been successful in determining the energies and the γ-ray and neutron widths of the Mg26 states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the α widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the α widths by α-transfer techniques. Methods: The α-inelastic scattering and α-transfer measurements were performed on a solid ²⁶Mg target and a ²²Ne gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The (α,α′) measurements were performed at 0.45°, 4.1°, 8.6°, and 11.1° and the (⁶Li,d) measurements at 0° and 10°. The scattered α particles and deuterons were detected by the focal plane detection system consisting of multiwire drift chambers and plastic scintillators. The focal plane energy calibration allowed the study of ²⁶Mg levels from Eₓ = 7.69–12.06 MeV in the (α,α′) measurement and Eₓ = 7.36–11.32 MeV in the (⁶Li,d) measurement. Results: Six levels (Eₓ = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the α threshold in the region of interest (10.61–11.32 MeV). The α widths were calculated for these states from the experimental data. The results were used to determine the α-capture induced reaction rates. Conclusion: The energy range above the α threshold in ²⁶Mg was investigated using a high resolution spectrometer. A number of states were observed for the first time in α-scattering and α-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in ²⁶Mg. From the observed resonance levels the Eₓ = 10717 keV state has a negligible contribution to the α-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at Ex = 10951, 11167, and 11317 keV. The Eₓ = 11167 keV state has the most appreciable impact on the (α,γ) rate and therefore plays an important role in the prediction of the neutron production in s-process environments.

Citation

Talwar, R., Adachi, T., Berg, G. P. A., Bin, L., Bisterzo, S., Couder, M., deBoer, R. J., Fang, X., Fujita, H., Fujita, Y., Görres, J., Hatanaka, K., Itoh, T., Kadoya, T., Long, A., Miki, K., Patel, D., Pignatari, M., Shimbara, Y., Tamii, A., …Yosoi, M. (2016). Probing astrophysically important states in the ²⁶Mg nucleus to study neutron sources for the s process. Physical Review C, 93(5), https://doi.org/10.1103/physrevc.93.055803

• Journal Article Type: Article
• Acceptance Date: Mar 14, 2016
• Online Publication Date: May 10, 2016
• Publication Date: May 10, 2016
• Deposit Date: May 31, 2016
• Publicly Available Date: May 31, 2016
• Journal: Physical review. C, Nuclear physics
• Print ISSN: 2469-9985
• Publisher: American Physical Society
• Peer Reviewed: Peer Reviewed
• Volume: 93
• Issue: 5
• DOI: https://doi.org/10.1103/physrevc.93.055803
• Keywords: Mg²⁶ nucleus, S process, Neutron sources
• Public URL: https://hull-repository.worktribe.com/output/438974
• Publisher URL: http://journals.aps.org/prc/abstract/10.1103/PhysRevC.93.055803
• Additional Information: Copy of article first published in: Physical review. C, Nuclear physics, 2016, v.93, issue 5
• Contract Date: May 31, 2016