Nuclear Physics Seminar: Shahina, University of Notre Dame


Location: 127 Nieuwland Science Hall (View on map )

Stellar Neutron sources for the s-process nucleosynthesis

PhD Candidate
Department of Physics & Astronomy
University of Notre Dame

About half of the heavy elements beyond Iron are created in stars via the slow-neutron capture process. The s-process, in turn is divided into the weak and main branches. The main branch produces primarily the heavy s-process nuclei A > 90, while the weak component is  responsible for the element production over the range 60 ≤ A ≤ 90. The weak s-process occurs in massive stars and the neutron source is primarily the 22Ne(α, n) 25Mg reaction. Determining the overall neutron flux available is one of the main challenges in modeling this weak s-process nucleosynthesis. The interplay and correlation between the 22Ne(α, γ)26Mg and the competing 22Ne(α, n)25Mg reaction determines the efficiency of the 22Ne(α, n)25Mg reaction as a neutron source for the weak s-process. In both cases, the reaction rates are dominated by the strength of the α cluster resonance at 830 keV. This plays a particularly important role in determining the strength of the neutron flux for weak and main s-process environments. We performed the measurement of the 830 keV resonance in 22Ne(α, γ)26Mg at the Sanford Underground Research Facility using a γ-summing detector. We confirmed the previous studies of the resonance strength and obtained a strength of ωγ = 35 ± 4 μeV, however, the strength of the corresponding resonance in the 22Ne(α, n)25Mg still carries large uncertainties. In a new and independent study performed at Notre Dame using a stilbene crystal detector, we confirmed previous results and demonstrate that the resonance strength in the competing 22Ne(α, n)25Mg reaction channel is significantly higher.

We also studied, the 25Mg(α, n)28Si reaction, which acts as a potential neutron source for weak s-process and destroys the strongest neutron poison 25Mg. Previous measurements for this reaction suffered from two shortcomings: they were not performed at low enough energies relevant for weak s-process in massive stars and measurements using neutron counters were hindered by the contamination of targets with lower Z material. In this work, we used two different setups consisting of deuterated liquid scintillator detectors for neutrons and LaBr3 for γ-rays in order to measure the 25Mg(α, n)28Si cross-section in the Gamow range 1.4-2.6 MeV. The neutron spectroscopy was performed via  neutron spectrum unfolding technique which allows for a clear separation of the signal and the background. Preliminary results, including cross-sections determined from gamma-ray and neutron spectroscopy will be presented. 

Shahina1,2, R.J. deBoer1,2, M. Febbraro5, J. G ̈orres1,2, D. Robertson1,2, M. Couder1,2, O. Gomez1,2, A. Gula1,2, M. Hanhardt3,4, T. Kadlecek3, R. Kelmar1,2, J.T. Nattress5, P. Scholz1,2, A. Simon1,2, E. Stech1,2, F. Strieder3, M. Wiescher1,2

1 Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana 46556, USA
2 The Joint Institution of Nuclear Astrophysics-Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, Indiana 46556, USA
3 Department of Physics, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, USA
4 South Dakota Science and Technology Authority, Sanford Underground Research Facility, Lead, South Dakota 57754, USA
5 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA

Hosted by Prof. Wiescher