Design of the recoil mass separator St. George

A recoil mass separator has been designed for the purpose of studying low energy (α,γ)(α,γ) reactions in inverse kinematics for beam masses up to about A=40A=40. Their reaction rates are of importance for our understanding of energy production and nucleosynthesis during stellar and explosive helium burning. The reactions take place in a windowless He gas target at the beginning of the separator which consists of three sections. The first section selects the most abundant charge state. The Wien filter in the second section efficiently separates the intense beam from the few reaction products. In the last section the reaction products are focused into a detector system consisting of time pickup and energy detectors. In order to accept the complete kinematic cone of recoil particles for energies as low as reasonably possible we specified a large circular angular acceptance of ±40mrad. This requires a careful minimization of higher-order aberrations. The present system has been designed to allow for a future upgrade to extend the experimental program to the analysis of (p,γ)(p,γ) reactions.