The Ionisation produced in Air by X Rays and Gamma Rays

Abstract

A parallel plate chamber and measuring system suitable for use with high-voltage X rays are described. Saturation and plate separation conditions were shown to be satisfactory for X rays and apparently so for gamma rays. Measurements of gamma rays in röntgens gave the equivalent of 1 mgm.-hr. at 1 cm. from a point source, 3·08 r. This result does not agree with previous “air wall” chamber experiments. An investigation of the discrepancy showed that the paths of recoil electrons were greater than the linear dimensions of the measuring system, and that the ionisation observed in an “air wall” chamber is a complex function of conditions unless the walls are of a thickness at least equal to the ranges of electrons produced by the radiation in them. It was found experimentally that the thick wall chamber has an equivalent air volume equal to its real volume for hard X rays. Measurements of gamma rays in röntgens gave as the equivalent of 1 mgm.-hr. at 1 cm. from a point source (0·5 mm. Pt. filter) 8·3 r. It appears that this result is sufficiently accurate for clinical use, and that a suitable “air wall” chamber forms a satisfactory measuring instrument for high voltage X rays or gamma rays, whereas a parallel plate chamber of normal dimensions appears inadequate.