The Saturation Curve for Ionization Measurements in Pulsed Radiation Beams

Abstract

In a previous paper (Boag, 1950) the saturation curve for an ionization chamber exposed to pulsed radiation was investigated mathematically and some preliminary measurements were presented, but experimental difficulties prevented any great reliance being placed on these measurements, and there was at that time no possibility of repeating them. It has recently been possible to make some further measurements, using as a source of pulsed radiation the 8 MeV linear accelerator, which is under construction for the Medical Research Council at the Trafford Park works of the Metropolitan-Vickers Electrical Company. The ionization chamber used for these measurements is illustrated in Fig. 1. The chamber was constructed from perspex, the electrode surfaces being coated with colloidal graphite. The spacing of the electrodes could be varied by means of interchangeable rings which were made to give air gaps of 0·48 mm., 0·99 mm., 1·47 mm., and 1·98 mm. respectively. A guard ring was provided to ensure that the electric field over the region from which current was collected was accurately planeparallel, and concentric telcothene cable was used for the leads, with zero potential across the insulation. For the measurements, the ionization chamber was set up with the axis of the coin-shaped air gap coincident with that of the X-ray beam. The ionization current was balanced against the current through a standard high resistance in the simple null circuit shown in Fig. 2, a Lindemann electrometer being used as the null instrument.