Astrophysical S factor for the 15 N(p,γ)16 O reaction from R-matrix analysis and asymptotic normalization coefficient for 16 O 15 N + p. Is any fit acceptable?
Abstract
The 15 N(p,γ)16 O reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong Jπ=1- resonances at ER= 312 and 962 keV and direct capture to the ground state. Recently, a new measurement of the astrophysical factor for the 15 N(p,γ)16 O reaction has been published [P. J. LeBlanc et al., Phys. Rev. C 82, 055804 (2010)]. The analysis has been done using the R-matrix approach with unconstrained variation of all parameters including the asymptotic normalization coefficient (ANC). The best fit has been obtained for the square of the ANC C2= 539.2 fm-1, which exceeds the previously measured value by a factor of ≈ 3. Here we present a new R-matrix analysis of the Notre Dame-LUNA data with the fixed within the experimental uncertainties square of the ANC C2=200.34 fm-1. Rather than varying the ANC we add the contribution from a background resonance that effectively takes into account contributions from higher levels. Altogether we present 8 fits, five unconstrained and three constrained. In all the fits the ANC is fixed at the previously determined experimental value C2=200.34 fm-1. For the unconstrained fit with the boundary condition Bc=Sc(E2), where E2 is the energy of the second level, we get S(0)=39.0 1.1 keVb and normalized 2=1.84, i.e. the result which is similar to [P. J. LeBlanc et al., Phys. Rev. C 82, 055804 (2010)]. From all our fits we get the range 33.1 ≤ S(0) ≤ 40.1 keVb which overlaps with the result of [P. J. LeBlanc et al., Phys. Rev. C 82, 055804 (2010)]. We address also physical interpretation of the fitting parameters.
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