MEASUREMENT OF IONIZING RADIATIONS 



151 



1934; Fifth Intern. Congr. Radiology, 1937; Intern. Com. Radiological 

 Units, 1951). 



Standard Air-ionization Chamher. The roentgen may be realized by 

 means of the conventional ionization chamber shown diagrammatically 

 in Fig. 2-2 (the figure is drawn out of proportion in order to emphasize 



h|i[mH1' 



Fig. 2-2. Parallel-plate standard ionization chamber. 



certain essential features). The chamber proper consists of a flat plate 

 H which is maintained at a suitable potential by a battery or its electronic 

 equivalent. The ion collector C is connected to an electrometer of such 

 type that, when in operation, C is maintained at ground potential; G, G 

 are guard plates on each side of C along the direction of the beam. 



The electric field, namely, the lines of force between H and the three 

 plates G, C, G, are roughly as shown, bulging outward at the ends and 

 gradually becoming parallel at the center of the chamber. It has been 

 found experimentally that the electric field between a pair of parallel 

 plates becomes uniform at a distance from the ends about 11 2 times the 

 plate separation. Hence in the figure, if the widths of the guard plates 

 are 13^2 times the spacing CH, then the region between the lines aa' will 

 have a uniform electric field (or parallel lines of force). X rays in passing 

 between the plates produce fast electrons, which in turn produce ions that 

 are drawn to the plates along the paths of the force lines; it is therefore 

 necessary to know exactly which lines terminate on the grounded guard 

 plates and on the insulated plate C, which is connected to the electrometer. 



Since there must be a finite spacing between the edges of plates C and 

 G, and the lines of force are supposed to be parallel in these regions, the 

 plate C has an effective width greater than its actual width by an amount 

 equal to half the gap between them. The effective width of C is also 

 affected by its potential with respect to the guard plates, being that given 

 above only when the potential of C is the same as G. On the other hand, 

 according to whether C is above or below the potential of G, its effective 

 width may be less or greater than the correct value. This is caused by 

 field distortion, as a result of which, owing to the small spacing between 

 C and G, the field strength may be comparatively large. In accurate 

 measurements a potential difference of 0.5 volt between guard and collec- 

 tor spaced 0.5 mm apart may introduce a noticeable error in the results 

 even though the field strength between C and H may be as much as 200 

 volts/cm. 



