/He Collisions*

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

 

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, ion arm of the spectrometer was build in a time- and space-focussing geometry in order to reduce the diminishing influence of the extended target size. The electron arm was build in a time-focussing geometry. On both ends of the spectrometer, hexagonal delay-line-detectors were used, which have an overall non-linearity <100 µm.

 

Also the gas jet was precooled to 80 K and for both, electrons and ions, separate calibrations, using an additional 25 keV ion source were used. With this we achieved a He⁺ momentum resolution of Δp<0.1 au was achieved. The electron arm of the spectrometer was calibrated via autoionizing states of a Neon, which create numerous energetically sharp isotropically emitted electrons. Results of the experiment will be presented and discussed.