Stellar Neutron sources for the s-process nucleosynthesis

Shahina
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.

Shahina^1,2, R.J. deBoer^1,2, M. Febbraro⁵, J. G ̈orres^1,2, D. Robertson^1,2, M. Couder^1,2, O. Gomez^1,2, A. Gula^1,2, M. Hanhardt^3,4, T. Kadlecek³, R. Kelmar^1,2, J.T. Nattress⁵, P. Scholz^1,2, A. Simon^1,2, E. Stech^1,2, F. Strieder³, M. Wiescher^1,2

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