Direct capture cross section and resonances in the 22Ne(p, γ) 23Na reaction at low energy

Background: Among the several inhomogeneities in the composition of globular cluster stars, an overabundance of 23Na is interpreted as the signature of the operation of the neon-sodium (NeNa) cycle. One of the hypothesis to explain the observed O-Na anticorrelation invokes massive asymptotic giant branch stars as the main agents. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars the 22Ne(p, γ ) 23Na reaction rate has been the most uncertain so far, giving rise to considerable experimental efforts in recent years. While overall there is a good agreement between reported cross section results, some tensions still remain on the branching ratios of resonance γ -ray modes and direct capture to excited sates. Purpose: The present paper offers full details and a partial analysis of the high sensitivity study, of both direct capture and low-energy resonances in the 22Ne(p, γ ) 23Na reaction, performed at LUNA, and whose results were previously published in abbreviated form [F. Ferraro et al., Phys. Rev. Lett. 121, 172701 (2018)]. Methods: During the LUNA measurement an intense proton beam was delivered to a 22Ne gas target. The γ rays from the 22Ne(p, γ ) 23Na reaction were detected by a high efficiency 4π, sixfold segmented bismuth germanate (BGO) detector. In the present paper the data from individual detector segments were combined with simulated detector responses to obtain cascade branching ratios. Results: For the three resonances at Ep = 156.2 and 259.7 keV new γ -decay branchings are provided. Moreover, partial cross sections for the direct capture to different states of 23Na are reported down to Ep = 188 keV, the lowest energy measured to date. Conclusions: A revised reaction rate has been calculated based on a new R-matrix fit of the recent 22Ne(p, γ ) 23Na S-factor data and results for the resonances. The thermonuclear reaction rate is provided in tabular form to be used in stellar models