MEASUREMENT OF X-RAYS AND RADIUM 



63 



water, and body tissue over a wide range of wave-lengths), and (6) for 

 convenience and ready reproducibility. Therefore, the internationally 

 accepted unit of X-ray quantity is determined by the ionization produced 

 in 0.001293 gm. of air. The definition accepted by the United States 

 (56) differs from the international definition only in being less ambiguous 

 for voltages above 200 kv., and reads as follows: "The roentgen is the 

 quantity of X-radiation which, when the secondary electrons are fully 

 utilized and the effects of all scattered radiation avoided, produces in 

 1 cubic centimeter of atmospheric air at 0°C. and 76 cm. mercury pres- 

 sure such a degree of conductivity that 1 e.s.u. of charge is measured 

 under saturation conditions." 



The problem of realizing the roentgen is then resolved, essentially, 

 into two parts: (a) segregating a known volume of air which is ionized 



|l|-HHli- 



Fig. 12. — Parallel-plate standard ionization chamber. 



by the beam in question, and (6) measuring the number of ions produced 

 in that volume. Actually neither of these quantities is obtainable 

 directly, although the indirect methods to be described below do not 

 introduce any uncertainties, at least up to 200 kv. 



Standard Free-air Ionization Chamber. — The roentgen may be realized 

 by means of the conventional ionization chamber shown diagrammati- 

 cally in Fig. 12 — (the figure is drawn out of proportion in order to empha- 

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

 plate H which is maintained at a suitable potential by a battery. 

 The ion collector C is connected to an electrometer of such type that 

 when in operation, C is maintained at ground potential (cf. page 72). 

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



The electric field (lines of force) between H and the three plates G, C,G 

 are roughly as shown, bulging outward at the ends and gradually becom- 

 ing parallel at the center of the chamber. It has been found experi- 

 mentally that the electric field between a pair of parallel plates becomes 

 uniform at a distance from the ends about 1}^ times the plate separation 

 (11, 17, 42). Hence, in the figure, if the guard-plate widths are 13-^ 

 times the spacing GH, 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 ions which are drawn to the plates along 



