Direct Capture Cross Section and Resonances in the Ne22(p,γ)Na23 Reaction at Low Energy

Background: Among the several inhomogeneities in the composition of globular cluster stars, an overabundance of 23 Na 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 22 Ne( p , gamma ) 23 Na 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 gamma-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 22 Ne( p , gamma ) 23 Na 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 22 Ne gas target. The gamma rays from the 22 Ne( p , gamma ) 23 Na reaction were detected by a high efficiency 4 pi , 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 E p = 156 . 2 and 259.7 keV new gamma-decay branchings are provided. Moreover, partial cross sections for the direct capture to different states of 23 Na are reported down to E p = 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 22 Ne( p , gamma ) 23 Na S-factor data and results for the resonances. The thermonuclear reaction rate is provided in tabular form to be used in stellar models.