Electron Transfer in Irradiated Vacuum Cavity Chambers

GREENING 1 has explained the residual current in an ionization chamber, when the pressure is reduced to very low values, in terms of the transfer of slow electrons between the electrodes. He has proposed a theory for the saturation characteristics of evacuated chambers under irradiation. On the basis of a review of experimental data, he suggested an electron energy spectrum emitted from the walls, namely, the low-energy part of the electron spectrum from 0–41 eV could be represented by KE ½ e − E /8 per unit energy interval and the high-energy part of the electron spectrum from 41–∞eV by per unit energy interval, where E is the electron energy and K is a constant. Generally it is necessary to know the directional distribution of the electrons emerging from the wall before the saturation characteristic of a vacuum chamber can be calculated from Greening's theory. Notwithstanding this, it is possible to calculate the saturation characteristics of a spherical vacuum chamber without any knowledge of the directional distribution of the electron emitted from the walls, because of the threefold symmetry in this case. A spherical vacuum chamber, with the radii of the inner and outer electrodes being 3.13 cm and 5.08 cm respectively, has been constructed and a theoretical saturation characteristic calculated, using the electron energy spectrum suggested by Greening, and is represented by the curve labelled A in Fig. 1.