Stopping cross section of bulk materials for alpha particles

The stopping cross sections of Au, Ag, C (graphite), and H2O ice have been measured with a probable error of +-4% by a thick-target elastic scattering method for alpha particles of energy 0.3 to 2.0 MeV. The stopping cross sections of Au and Ag agree within this error to values previously obtained by a thin-target reflection method. The stopping cross section of graphite is 6 to 20% higher than that of amorphous thin-film vapor-deposited carbon. The difference is attributed to an allotropic effect in carbon. The stopping cross section of H2O ice is 6 to 8.4% lower than the value obtained for water vapor, and this difference is less than that found for protons (10 to 14%) in the same velocity interval. It is proposed that the physical state difference in the stopping cross section of water is caused by changes in the modes of electron excitation caused by the state of aggregation