Recoil separator ERNA: ion beam specifications

Abstract.

For improved measurements of the key astrophysical reaction ¹²C(α,γ)¹⁶O in inverted kinematics, a recoil separator ERNA is being developed at the 4 MV Dynamitron tandem accelerator in Bochum to detect directly the ¹⁶O recoils with about 50% efficiency. Calculations of the ion beam optics including all filtering and focusing elements of ERNA are presented. Since the ¹²C projectiles and the ¹⁶O recoils have essentially the same momentum, and since the ¹²C ion beam emerging from the accelerator passes through a momentum filter (analysing magnet), the ¹²C ion beam must be as free as possible from ¹⁶O 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 ¹²C ion beam. The experiments also indicate that an almost free choice of the charge state for the ¹⁶O recoils is possible in the separator.