Measurement of the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ Reaction Rate at JUNA and Its Impact on Probing the Origin of SiC Grains

The ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction is critical for AGB star nucleosynthesis due to its connection to the abundances of several key isotopes, such as ${}^{21}\mathrm{Ne}$ and ${}^{22}\mathrm{Ne}$. However, the ambiguous resonance energy and spin-parity of the dominant 470 keV resonance leads to substantial uncertainty in the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rate for the temperature of interest. We have measured the resonance energies and strengths of the low-energy resonances in ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ at the Jinping Underground Nuclear Astrophysics experimental facility (JUNA) with improved precision. The key 470 keV resonance energy has been measured to be $E_{\alpha }=474.0\pm 1.1\mathrm{keV}$, with such high precision achieved for the first time. The spin-parity of this resonance state is determined to be $1^{-}$, removing discrepancies in the resonance strengths in earlier studies. The results significantly improve the precision of the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates by up to about 10 times compared with the previous data at typical AGB temperatures of 0.1–0.3 GK. We demonstrate that such improvement leads to precise ${}^{21}\mathrm{Ne}$ abundance predictions, with an impact on probing the origin of meteoritic stardust SiC grains from AGB stars.

Figure 1

Thick target yield curve of the 470 keV resonance in ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$. The solid red line is the fitting curve, the red shadow represents the uncertainty of the fitting curve, and the dashed red line marks the position of resonance energy. All yields are normalized to the maximum yield of the fitting curve.

Figure 2

(a) Sum spectra of the 470 keV resonance after subtracting the background. (b) Single spectra produced by putting a 9550–10 550 keV gate on the sum peak shown in (a). The different color areas represent the fractional contribution.

Figure 3

Comparison of the present ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates (JUNA) with previous values reported by Angulo et al. [26] (NACRE) and Iliadis et al. [36] (Iliadis10).

Figure 4

Ne isotopic ratios predicted in the intershell of different AGB models (filled symbols) using the JUNA ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates and those observed in meteoritic stardust SiC grains of different sizes Lewis et al. [39] (open symbols), in which error bars only represent the contribution from the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates. The symbol legend is at the bottom-right side of the plot. (See Fig. 8 of Lewis et al. [39] and Fig. 2 of Karakas et al. [7] from similar figures). The three regression lines are fits to the KJB, KJC, and KJD samples representing the mixing between material of normal composition ($\simeq$ solar, located at the top-right outside the plot boundaries) and of AGB He-shell composition (located at the bottom left). When comparing the observed and predicted AGB He-shell composition with find an agreement of the models with the smallest size samples. The top-left inset shows the ${}^{21}\mathrm{Ne}/{}^{22}\mathrm{Ne}$ ratios calculated with different ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates.