Absorption of X-Rays in Air

The use, in recent years, of beryllium-window x-ray tubes with their high-intensity output of soft x-rays has produced a need for air-absorption corrections where distance is a factor in x-ray exposure measurements. Such is the case, for example, where the inverse-square law is utilized for computing the exposure at a point when the actual measurement is of necessity made at some other point. There is also need for corrections to be applied to the free-air type of ionization chamber due to air absorption between the limiting diaphragm of the chamber and the front edge of the collector plates. Since the exposure rate is specified in the plane of the limiting diaphragm, air-absorption along the path between the diaphragm and the collector plate results in the readings of the chamber being too low. This distance in the case of the guarded-field free-air standard for calibrations up to 200 kv. at the Bureau of Standards is 21.1 cm., and for unfiltered radiations produced at, say, 50 kv. considerable corrections may be necessary. Apparatus and Experimental Procedure A diagram of the experimental arrangement is shown in Figure 1, where a mobile, free-air, guard-plate ionization chamber is indicated. The opening in its lead shield is 10 cm. in diameter. This opening is adequate to accommodate, at any point along the track, the total area of x-rays determined by the fixed, limiting diaphragm, the diameter of which is 1.75 cm. The effective width of the collector plates is 10.2 cm., and the diameter of the beam with the chamber in its farthest position from the tube (Position B, Fig. 1) is 6.5 cm. The inter-electrode separation is a fixed distance of 12.7 cm., and this is insufficient spacing to allow for the production and collection of all secondary electrons associated with a primary beam 6.5 cm. in diameter. Hence, there is a slight loss of ionization, which earlier studies show does not exceed 1 or 2 per cent (1). The charge collected in the ionization chamber at various points along the track is measured with a capacity compensator and string electrometer (2) with an accuracy of ± 1.0 per cent. The exposure of the chamber is controlled by a timer operating a one-half-inch lead shutter. The timer is actuated by an accurately controlled 60-cycle signal from the Radio Section of the Bureau of Standards. Tungsten-target x-ray tubes with low inherent filtration are used in conjunction with various types and thicknesses of initial filtration. The term “initial filtration,” as here used, refers to filters of aluminum, copper, or lead introduced between the x-ray tube and the column of air over which air-absorption measurements are made. The window thickness of the cerium glass in one tube used is 1.29 mm., and of the beryllium in another, 1.5 mm. Power supplied to the x-ray tube is furnished by a kenotron rectifier with a resistance-capacitance filter incorporated to reduce the ripple to 0.05 per cent per milliampere current drain.