Energy loss of He ions in solidified gases

Energy loss measurements of 0.5-2.0 MeV 4He ions in argon, oxygen and CO2 have been made in their solid phase. The purpose of this experiment was to check for 'solid state' effects on energy loss. The experimental method involves backscattering of 4He ions from a low temperature substrate on which a thin layer of gas has been condensed. The substrate consists of a Be disc with a thin gold layer of measured thickness (atoms/cm2) on its surface. Backscattering measurements of 4He ion from the target before and after gas condensation provide the energy loss measurement. The results indicate that above 1 MeV there is only a small difference in energy loss between gaseous and solid phase. However, the maximum energy loss is seen to occur at somewhat higher energy in the solid phase and in the energy region below 1 MeV our observed 4He stopping cross sections are approximately 5% lower than those reported for the corresponding gas. Thus, the solid/gas difference is far smaller than had been predicted by Ziegler et al. and so only accounts for part of the observed Bragg's Law deviations in oxides and nitrides. (Auth.)