The solid-gas difference in stopping powers, and statistical analysis of stopping power data

For the purpose of treating the difference in stopping power between condensed and gaseous substances, we first discuss our method of statistical analysis of published experimental data. We distinguish between a positive effect where the difference between mass stopping powers Ssolid − Sgas is positive and a negative effect where the opposite holds true. Experimentally, the positive effect has been found so far with heavy ions at high energy, and the negative effect with light ions at low energy. The positive effect is due to the difference in ionic charges and can be described by the CasP program by Grande and Schiwietz. An apparent persistence of the positive effect down to low energy is found to be mainly an artifact inherent in the stopping table of ICRU Report 73. The negative effect can be described by a difference in mean ionization potentials between gases and condensed matter. It is large for metals, much smaller for compounds. It should be possible to see both effects in one projectile - target combination.