The Relative Ionization in Different Gases for Slow-Moving Electrons

Abstract

A uniform beam of electrons was obtained by allowing thermions from a tungsten filament to fall through a given potential difference between the filament and a perforated plate and to pass through the plate into a short region devoid of strong electric fields. There, due to the energy acquired, they ionized the gas present, the positive ions being caught on a concentric ring element and the electrons themselves on a plate opposite the perforated plate. With the pressure low enough to prevent secondary effects the ionization was determined for electron energies ranging from 20 to 300 volts. With increasing electron energies the ionization in each gas was found to reach maximum at the following voltages: for He 140, Ne about 220, A 100, CO 120 and ${\mathrm{N}}_2$ 100 volts. For 200 volts the relative ionizations at 1 mm pressure, with reference to that in Ne taken as 1.0, are: ${\mathrm{H}}_2$.91, He.48, C${\mathrm{H}}_4$ 3.5, ${\mathrm{N}}_2$ 3.2, CO 3.45, A 4.1. The results are in good agreement with those of Hughes and Klein, except that the value for argon is 0.7 higher. A second maximum was observed in each case at 350 to 400 volts which was probably due to secondary rays, but is not completely understood.