A Contribution to the Theory of Ionization Chamber Measurements at Low Pressures

Abstract

The ionization chamber measurements of other workers at low pressures have indicated (i) that the measured current is in excess of that to be expected on the Bragg-Gray theory; (ii) that there is a residual current even at sensibly zero pressure; (iii) that the gradient of the ionization-pressure curve at low pressures is less than that at atmospheric pressure, and (iv) that the saturation curves of chambers at low pressures are often grossly asymmetrical. Effects (i) and (ii) are attributed to the deflection by the chamber voltage of slow electrons emitted from the chamber walls. Expressions are derived for te number of electrons deflected between plane, spherical and cylindrical electrodes. The numbers of electrons emitted/cm2/r deduced from ionization chamber measurements are found to agree, within a factor of 2, with the corresponding numbers derived indirectly from other data, and are approximately 5 × 105/cm2/r and 1 × 106/cm2/r for graphite and elektron metal respectively irradiated by radium γ rays. The approximate energy distribution of these slow electrons is indicated. Effect (iii) is attrributed to the difficulty of achieving saturation without producing ionization by collision. At low chamber volts some ions have sufficient energy to reach the electrode of the same polarity. Effect (iv) is attributed to differences in electrode areas and the asymmetry of the electric field between te electrodes. An example of a calculated saturation curve is given for spherical electrodes. Finally it is suggested that similar effects would occur at atmospheric pressure in chambers having extremely small electrode separations.