Insulators for Radiological Instruments

The problem of eliminating induced conductivity from insulators used in ionization chambers has been recognised for a long time (Farmer, 1945), and it is one of the limiting factors in the accurate measurement of X and γ rays. Hitherto it has been general practice to use amber for the parts of an ionization chamber most heavily exposed to radiation. This induced conductivity has a twofold effect. First, it causes a “leakage” current, during the irradiation, in parallel with the ionization current. This leakage current rises to an equilibrium value in a few seconds or minutes. Secondly, after the end of irradiation the recovery of the insulator is slow and this after-effect may give rise to further serious leakage before the conductivity returns to its normal (static) low value. The “recovery time” for polystyrene is of the order of 100 hours, but depends upon the dose-rate and duration of the irradiation and of course on the level below which the leakage conductivity can be considered negligible. It appears that this level is at least as low as 10−20 (ohm. cm)−1 and may be 10−21 (ohm. cm)−1 for ordinary condenser chambers. Ambrosen and Kofoed-Hansen (1954) have shown that for short irradiations it is the after-effect which causes the major part of the leakage. In practice this means that the reading of an ionization chamber will depend upon the length of time it is left standing before measurement, and also upon the number of exposures it has recently received.