Density Effect on the Ionization in Gases by Electrons

The density effect on the ionization produced by electrons has been observed by comparing their specific ionization in He and ${\mathrm{H}}_2$ gases at one and ten atmospheres pressure. The measurements were made by sending primary electrons of energy variable from 2-35 Mev through an ionization chamber into a Faraday cup. No density effect is observed or expected at one atmosphere pressure at these energies, but at ten atmospheres pressure, the results indicate the onset of the density effect at about 10 Mev in ${\mathrm{H}}_2$ and 20 Mev in He. At 35 Mev, the percentage reduction in ionization from that expected without density effect amounts to (8.0±1) in ${\mathrm{H}}_2$ and (3.5±1.3) in He. These results are in near agreement with the calculations of Sternheimer.