Indirect approaches to constraining the best estimate of the astrophysical S-factor for proton radiative capture on nitrogen-14 Public Deposited

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  • March 21, 2019
  • Bertone, Peter Felix
    • Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
  • Achieving a sufficiently precise experimental determination of the astrophysical S-factor for proton radiative capture on nitrogen-14 remains one of the most enduring and important problems in the field of experimental nuclear astrophysics. This reaction regulates the rate of energy generation during the main-sequence phase in stars greater than approximately 1.5 solar masses, for all stars at the termination of the main sequence, and for all stars during the red-giant phase. This fact has numerous ramifications in other aspects of stellar evolution including globular cluster age estimation, solar neutrino production, and carbon star evolution. The first attempt to directly measure the cross section for the 14N(p,γ)15O reaction took place in 1951. After more than five decades and several attempts, a sufficiently precise direct measurement of this reaction still eludes researchers. Given the vanishingly small cross section at stellar energies and the inherent limitations of detection technology, there is some reason to suspect that the direct approach to measuring this reaction will likely not markedly improve the situation in the future, thereby necessitating the exploration of indirect approaches to constraining the best estimate of the 14N(p,γ)15O astrophysical S-factor. These were the considerations that motivated this dissertation. Prior to this study, there were several unresolved questions concerning the nuclear structure of 15O which, if measured precisely enough, could substantially improve the best estimate of the S-factor for the 14N(p,γ)15O reaction. In particular, it was thought that the width of the Ex=6793 keV sub-threshold level in 15O resulted in a large contribution from this state to the 14N(p,γ)15O S-factor at stellar energies, but this width had never been measured. Also, accepted best practice in the analysis of experimental S-factor data involves inclusion of elastic scattering data for higher lying states in the compound nucleus. Proton elastic scattering data on 15O exist, but nearly all are more than five decades old. In addition, such scattering data have never been included in the analysis of the radiative capture data. Therefore, the results reported here are the new measurements of proton elastic scattering on 15O, the width of the Ex=6793 keV sub-threshold level in 15O, the effect of these quantities on the best estimate for the 14N(p,γ)15O S-factor, and the resulting astrophysical consequences.
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  • Champagne, Arthur
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  • University of North Carolina at Chapel Hill
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