R-matrix analysis of ²²Ne(α, n)²⁵Mg and ²²Ne(α, γ)²⁶Mg reactions

Abstract The reaction rates of 22 Ne( α , n) 25 Mg and its competing channel 22 Ne( α , γ ) 26 Mg control the production of neutron flux for weak s -process nucleosynthesis in low mass asymptotic giant branch stars and in massive stars with M ≥ 10 M ⊙ . The temperature range of interest for these reactions lies between 0.2 and 0.4 GK. However, the rates of these reactions are poorly constrained at these temperatures due to uncertainties in the nuclear properties of several resonance states in the compound nucleus 26 Mg, lying within the Gamow window. The present work reports a full R -matrix evaluation of the 22 Ne( α , n) 25 Mg and 22 Ne( α , γ ) 26 Mg reaction rates using updated nuclear data of 26 Mg states. Previous rate evaluation by Adsley et al and R -matrix calculations of Wiescher et al were limited by using narrow resonance approximations and omission of the resonances below E r = 705 keV, respectively. In this work, the R -matrix fit to the available 22 Ne( α , n) 25 Mg reaction data is performed by including the contributions of previously neglected resonances below E r = 705 keV and considering the interference effects. The ( α , n) reaction rate from the present R -matrix evaluations is noticeably higher than the narrow resonance approximation calculations in the temperature range 0.1−0.3 GK. In particular, the present ( α , n) reaction rate is significantly higher (7.5 − 4.5 times) compared to Adsley et al at 0.2−0.3 GK and ≈2 times greater than Wiescher et al at 0.3 GK. The estimated reaction rate ratio of ( α , n) to ( α , γ ) in the relevant temperature window 0.2−0.8 GK indicates that the production of neutrons for the s -process is more likely than the radiative alpha capture reaction, compared to the previous estimate by Adsley et al .