Determination of Ne20(p,γ)Na21 cross sections from Ep=500−2000keV

Abstract

Background: The reaction $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ influences the nucleosynthesis of Ne, Na, and Mg isotopes while contributing to hydrogen burning in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars, and oxygen-neon (ONe) novae. In the relevant temperature range for these environments ($T$ = 0.05--0.5 GK), the main contributions to this reaction rate are from the direct capture process as well as the high-energy tail of a subthreshold resonance in the ground-state transition at ${E}_{x}$ = 2425 keV in the $^{21}\mathrm{Na}$ compound nucleus.Purpose: The previous measurement of this reaction reports cross sections with large uncertainties for the ground-state transition. At higher energies, where the subthreshold resonance makes a smaller contribution to the total cross section, only upper limits are provided. This work aims to reduce the uncertainty in the cross section where direct capture dominates, as well as provide cross-section data in previously unmeasured regions.Method: The $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ reaction was measured over a wide proton energy range (${E}_{p}$ = 0.5--2.0 MeV) at ${\ensuremath{\theta}}_{\text{lab}}$ = ${90}^{\ensuremath{\circ}}$. Transitions to the ground state and to the 332 and 2425 keV excited states were observed. The primary transitions to these three bound states were utilized in an $R$-matrix analysis to determine the contributions of the direct capture and the subthreshold resonance to the total cross section.Results: The cross sections of the present measurements have been found to be in good agreement with the previous data at low energy. Significantly improved cross-section measurements have been obtained over the ${E}_{p}$ = 1300--1900 keV region. The narrow resonance at ${E}_{\text{c.m.}}$ = 1113 $\mathrm{keV}({E}_{x}$ = 3544.3 keV) has also been remeasured and its strength has been found to be in good agreement with previous measurements.Conclusions: An extrapolation of the $S$ factor of $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ has been made to low energies using the $R$-matrix fit. The reaction rate from the subthreshold resonance was found to be the main contributor to the reaction rate at temperatures below about 0.1 GK. The present rate is lower in the temperature range of interest than those presented in current reaction rate libraries by up to 20%.