Low energy scattering cross section ratios ofN14(p,p)N14
Physical Review C
The slowest reaction in the first CNO cycle is 14 N(p, γ) 15 O, therefore its rate determines the overall energy production efficiency of the entire cycle. The cross section presents several strong resonance contributions, especially for the ground state transition. Some of the properties of the corresponding levels in the 15 O compound nucleus remain uncertain, which affects the uncertainty in extrapolating the capture cross section to the low energy range of astrophysical interest. Purpose
... 14 N(p, γ) 15 O cross section can be described using phenomenological R-matrix. Over the energy range of interest, only the proton and γ-ray channels are open. Since resonance capture makes significant contributions to the 14 N(p, γ) 15 O cross section, resonant proton scattering data can be used to provide additional constraints on the R-matrix fit of the capture data. Methods A 4 MV KN Van de Graaff accelerator was used to bombard protons onto a windowless gas target containing enriched 14 N gas over the proton energy range from Ep = 1.0 to 3.0 MeV. Scattered protons were detected at θ lab = 90, 120, 135, 150, and 160 • using ruggedized silicon detectors. In addition, a 10 MV FN Tandem Van de Graaff accelerator was used to accelerate protons onto a solid Adenine target, of natural isotopic abundance, evaporated onto a thin self-supporting carbon backing, over the energy range from Ep = 1.8 to 4.0 MeV. Scattered protons were detected at 28 angles between θ lab = 30.4 and 167.7 • using silicon photodiode detectors. Results Relative cross sections were extracted from both measurements. While the relative cross sections do not provide as much constraint as absolute measurements, they greatly reduce the dependence of the data on otherwise significant systematic uncertainties, which are more difficult to quantify. The data are fit simultaneously using an R-matrix analysis and level energies and proton widths are extracted. Even with relative measurements, the statistics and large angular coverage of the measurements result in more confident values for the energies and proton widths of several levels in particular the broad resonance at Ecm = 2.21 MeV, which corresponds to the 3/2 + level at Ex = 9.51 MeV in 15 O. In particular the s and d wave angular momentum channels are separated. Conclusion The relative cross sections provide a consistent set of data that can be used to better constrain a full multichannel R-matrix extrapolation of the capture data. It has been demonstrated how the scattering data reduce the uncertainty through a preliminary Monte Carlo uncertainty analysis, but several other issues remain that make large contributions to the uncertainty, which must be addressed by further capture and lifetime measurements.