Studying the $\alpha p$-process waiting points using Radioactive Ion Beams

The nucleosynthetic flow in type I X-ray Bursts (XRBs) is driven by the triple-$\alpha$, $rp$ and $\alpha p$ processes. Several intermediate mass nuclei, $^{22}$Mg, $^{26}$Si, $^{30}$S, and $^{34}$Ar, have been identified as possible candidates for waiting points in XRBs. When such a nucleus is reached, the flow stalls due to ($p,\gamma$)-($\gamma,p$) equilibrium and must await $\beta$ decay unless the ($\alpha,p$) reaction is fast enough to break out of the waiting point first. A method to study these $\alpha p$-process reactions has been developed whereby the time- inverse reaction is studied in inverse kinematics using radioactive ion beams produced by the in-flight method at the Argonne National Laboratory ATLAS facility. The reactions $p$($^{29}$P,$^{26}$Si)$\alpha$, $p$($^{33}$Cl,$^{30}$S)$\alpha$, and $p$($^{37}$K,$^{34}$Ar)$\alpha$ have been studied to determine reaction rates for $^{26}$Si($\alpha,p$)$^{29}$P, $^{30}$S($\alpha,p$)$^{33}$Cl, and $^{34}$Ar($\alpha,p$)$^{37}$K, respectively. The results and possible implications for nucleosynthesis in XRBs will be discussed.