Investigation of the influence of impact parameter selection on energy loss measurements in gases

Abstract

For ion-atom collisions the mean energy loss per collision and hence the stopping cross section depends on the state of aggregation (phase effect). The stopping cross section of gases is usually measured by transmission experiments where ions penetrate through a gas cell. This contribution deals with the question whether the observed phase effect might be partially due to the inevitable particle selection when the ions leave the gas cell through a small aperture. If this were true it would imply systematic errors in many energy loss measurements in gases. We present Monte Carlo calculations of the statistical distribution of the impact parameters in ion-atom collisions when the ions penetrate a gas cell of 30 cm length and an exit aperture with a diameter of 1 mm and exit with a direction of flight to reach the detector (exit angle below 1°). The calculations are done for hydrogen ions with impact energies from 40 to 1000 keV impinging on zinc vapor with gas densities n of 8.9 × 10 14 to 8.9 × 10 15 atoms/cm 3. We find that at high energies, all impact parameters larger than 0.1 Å contribute, even at the low density. At low energies a noticeable cut-off at small impact parameters appears for the low density, which is quenched by multiple scattering at higher densities. By applying calculations in First Born Approximation, we determine how the cut-off at small impact parameters influences the energy loss of the ions quantitatively for 100 keV protons in zinc. We find a reduction of the stopping cross section by 15% for the low density and by 2% for the high density.