/He collisions*

20 years ago, single ionization of Helium induced by 100 MeV/u C⁶⁺ projectiles was investigated in a kinematically complete experiment. The experimentally obtained results were in strong contrast to state of the art theories at this time and even most recent calculations. While the electron angular distribution should exhibit two well separated lobes, the so-called binary- and recoil-lobe, the node between them was mostly filled. This launched an avalanche of controversial discussions, which are still ongoing today. The most heavily debated explanations are a) experimental issues/limited resolution and b) transversal coherence of the projectile, a concept introduced in 2011 by Schulz et al.

 

In order to solve the “C⁶⁺-mystery”, we used a state-of-the-art COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) Reaction-Microscope and redid the initial experiment in Cave D4 of GANIL. As insufficient momentum resolution might have been an issue in the original experiment, the spectrometer was built in a time-focusing geometry; the ion arm additional in a space-focussing geometry. On both ends of the spectrometer, hexagonal delay-line-detectors were used, with an overall spatial resolution <100 µm. The gas jet was precooled to 80 K. Separate calibrations for electrons and ions using a 25 keV ion source were performed. With this, the ion momenta were calibrated, focusing on discrete structures in momentum space as a result of single electron capture using He²⁺ and He⁺ projectiles. A momentum resolution of Δp<0.1 au for He⁺ ions was achieved. The electron arm was calibrated via autoionizing states of a Neon target (He²⁺ + Ne → He⁺ + Ne²⁺ + e^-), which create various, energetically well-defined continuum electrons.