Study of the ${}^{22}\mathrm{Mg}$ Waiting Point Relevant for X-Ray Burst Nucleosynthesis via the ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ Reaction

The ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts. We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3–6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ reaction rate is a factor of $\sim 4$ higher than the previous direct measurement of this reaction within temperatures relevant for x-ray bursts, resulting in the ${}^{22}\mathrm{Mg}$ waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the $(\alpha ,p)$ reaction.

Figure 1

Experimental cross sections for ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ from the present measurement (red triangles) and the data from Ref. [18] (blue squares). Also shown are the theoretical HF cross section calculations using $\text{TALYS}\times 0.93$ for $(\alpha ,p)$ only (dash black line), along with $\mathrm{NON}\text{−}\mathrm{SMOKER}/8$ (dot-dash blue line) and $\mathrm{NON}\text{−}\mathrm{SMOKER}/2$ (dot-dot-dot-dash black line), which includes both $(\alpha ,p)$ and $(\alpha ,2p)$.

Figure 2

Upper panel: The ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ reaction rate based on this work in comparison to the rates from Randhawa et al. [18] and Hu et al. [19]. Lower panel: The same reaction rates as a ratio to the rate by Randhawa et al. [18].

Figure 3

The upper and lower bounds of the flow into the $\alpha \mathrm{p}$ process calculated using the ignition conditions of Merz and Meisel [42] ($X_{\mathrm{H}}=0.06$, $X_{\mathrm{He}}=0.19$) using the ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ rates from the present work (red solid lines), Hu et al. (black dashed line), and Randhawa et al. (blue dot-dashed line). The vertical line denotes the peak temperature for the model by Ref. [42].