Prospects for Improved Measurements of the S-Process Neutron Source Reactions

The ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$ reaction is thought to be the main s-process neutron source, taking place in AGB starts at temperatures around $10^8$~K. The ${}^{22}{\rm Ne}(\alpha,n){}^{25}{\rm Mg}$ reaction is also thought to be an important neutron source and takes place in more massive stars at somewhat higher temperatures of $(2-3)\times 10^8$~K. Both reaction rates are uncertain at astrophysical temperatures due to the difficulty of measuring the low cross sections. In the case of ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$, measurements exist down to $E_{c.m.}=300$~keV but the extrapolation to the needed range of 150-200 keV is complicated by subthreshold resonances. The ${}^{22}{\rm Ne}(\alpha,n){}^{25}{\rm Mg}$ reaction rate is dominated by narrow resonances -- the possibility of as-yet-unobserved resonances near threshold leads to significant uncertainty in this reaction rate. The prospects for improved data using high-intensity beams, inverse kinematics, and background reduction techniques will be discussed.