Recoil separator ERNA: ion beam specifications

For improved measurements of the key astrophysical reaction ^(12)C(α,γ)^(16)O in inverted kinematics, a recoil separator ERNA is being developed at the 4 MV Dynamitron tandem accelerator in Bochum to detect directly the ^(16)O recoils with about 50% efficiency. Calculations of the ion beam optics including all filtering and focusing elements of ERNA are presented. Since the ^(12)C projectiles and the ^(16)O recoils have essentially the same momentum, and since the ^(12)C ion beam emerging from the accelerator passes through a momentum filter (analysing magnet), the ^(12)C ion beam must be as free as possible from ^(16)O contamination for ERNA to succeed. In the present work, the 16O contamination was reduced from a level of 1 × 10^(−11) to a level below 2 × 10^(−29) by the installation of Wien filters both before and after the analysing magnet. The measurement of these and other beam specifications involved other parts of the final ERNA layout – sequentially a Wien filter, a 60˚ dipole magnet, another Wien filter, and a ΔE-E telescope. The setup led to a measured suppression factor of 5 × 10^(−18) for the ^(12)C ion beam. The experiments also indicate that an almost free choice of the charge state for the ^(16)O recoils is possible in the separator