INVESTIGATION OF THE INFLUENCE OF IMPACT PARAMETER SELECTION ON ENERGY-LOSS MEASUREMENTS IN GASES

For ion-atom collisions the mean energy loss per collision and hence t he stopping cross section depends on the state of aggregation (phase e ffect). The stopping cross section of gases is usually measured by tra nsmission experiments where ions penetrate through a gas cell, This co ntribution deals with the question whether the observed phase effect m ight be partially due to the inevitable particle selection when the io ns leave the gas cell through a small aperture, If this were true it w ould 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 penetrat e 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 an gle below 1 degrees), 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 X 10(14) to 8.9 X 10(15) atoms/cm(3). We find that at high energies, all impact parameters larger than 0.1 Angstrom contr ibute, even at the low density. At low energies a noticeable cut-off a t small impact parameters appears for the low density, which is quench ed by multiple scattering at higher densities. By applying calculation s in First Born Approximation, we determine how the cut-off at small i mpact parameters influences the energy loss of the ions quantitatively for 100 keV protons in zinc, We find a reduction of the stopping cros s section by 15% for the low density and by 2% for the high density.