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Low-energy R-matrix fits for the 6Li(d,a)4He S factor

Background: The information about the 6Li(d,a)4He reaction rates of the astrophysical interest can be obtained by extrapolating direct data to the lower energies, or by indirect methods. The indirect Trojan Horse method, as well as various R-matrix and polynomial fits to direct data, estimate the electron screening energies much larger than the adiabatic limit. Calculations that include the subthreshold resonance estimate smaller screening energies. Purpose: Obtain the 6Li(d,a)4He reaction R-matrix parameters and the astrophysical S factor for the energies relevant to the stellar plasmas by fitting the R-matrix formulas for the subthreshold resonances to the S factor data above 60 keV. Methods: The bare S factor is calculated using the single and the two-level R-matrix formulas for the closest to the threshold 0+ and 2+ subthreshold states at 22.2, 20.2 and 20.1 MeV. The electron screening potential Ue is then obtained by fitting it as a single parameter to the low energy data. Results: The low energy S factor is dominated by the 2+ subthreshold resonance at 22.2 MeV. The influence of the other two subthreshold states is small. R-matrix fits result in the electron screening that is smaller than the adiabatic value. Neglecting the electron screening above 60 keV reduces the electron screening potential significantly. Calculations show a large ambiguity associated with a choice of the initial channel radius. Conclusions: The R matrix fits do not show a significantly larger Ue than predicted by the atomic physics models. The R-matrix best fit produces Ue=149.5 eV and Sb(0)=21.7 MeV b.