Atomic collisions

Accurate knowledge of the atomic interaction or energy loss of heavy ions penetrating through matter is essential for the successful operation of the in-flight separator Super-FRS. An uncertainty of one percent in the momentum prediction would already cause difficulties for the identification of rare isotopes at the outskirt of the chart of nuclides where no guidance of re ference isotopes is comprised in the spectrum.
The operating domain with the Super-FRS extends more than 500 A MeV higher than the present FRS. In this new energy range no data for stopping powers, energy and angular straggling and charge-state distributions of heavy ions exist. 
An additional challenge comes from the predicted large mean free path lengths for charge-changing collisions. This will cause that the resulting contribution to the energy-loss straggling will become larger than the collisional part.
The theoretical uncertainties are presently too large. Therefore, basic atomic collision processes have to be measured in an early stage of the Super-FRS operation. The complete slowing down in a gas-filled stopping cell (for instance at the LEB of the Super-FRS) is another challenging task because of the large momentum spread of the fragments emerging from the production target. In this case, adequate stopping efficiencies can be achieved only after reduction of the energy spread in a dispersive magnetic stage combined with a mono-energetic degrader system. In experiments with such a gas cell the atomic interaction of heavy ions has to be known with high accuracy over the full energy range down to thermalization. Especially the different contributions to the range straggling have to be correctly predicted to perform efficient experiments with very exotic nuclei stopped in such a gas cell.
Besides amorphous solids, crystals will be used which will enable for the first time to observe the nuclear Okorokov effect of resonant coherent excitation (RCE).
The feasibility of channeling experiments has been demonstrated with pilot experiments at the FRS many years ago. At Super-FRS, RCE experiments will be performed with exotic nuclei. Already the first measurements of the Cherenkov radiation from relativistic heavy ions with the present FRS have revealed that the well-known Tamm-Frank theory cannot describe the observed results.
It was already investigated in new models that slowing-down in the radiator leads to additional broadening and appearing of the complex diffraction-like structures of both spectral and angular distributions. Experiments with the Super-FRS will contribute to the detailed understanding of Cherenkov radiation of heavy ions and will lay the grounds for improved and novel detector developments.