MEASUREMENT OF IONIZING RADIATIONS 153 



tion ; these requirements are met in a statistical sense only when both the 

 flux and the quality of the X-ray beam remain essentially constant 

 throughout the length of the chamber. For the case of low-energy X rays 

 of the so-called "Grenz" (up to about 20 kev) region, these requirements 

 are not met; however, by a shghtly modified design of the chamber and 

 proper experimentation, it is possible to make adequate corrections 

 (Taylor andStoneburner, 1932; Day and Taylor, 1948; Eddy and Farrant, 

 1944). Since the mass of 0.001293 g is the mass of 1 gc of air at 76 cm 

 Hg pressure and 0°C, the significant volume must be reduced to those 

 conditions by Charles' and Boyle's laws. In further satisfaction of the 

 definition of the roentgen, X rays scattered by the air and rescattered by 

 the walls and plate of the chamber are reduced to a minimum by making 

 them of material of low atomic numbers such as carbon or aluminum. 

 Following out the procedure given above, the intensity of an X-ray beam 

 as measured in terms of roentgens is given by the following formula: 



r/sec ^ iT X 7Q0/laP X 273 (6) 



where i is the current to the collector electrode measured in electrostatic 

 units, I the effective length of the collector electrode in centimeters, a the 

 area of the hmiting diaphragm in square centimeters, T the absolute tem- 

 perature, and P the atmospheric pressure in millimeters of mercury. 



As the energy of the photons increases, the standard air-ionization- 

 chamber method of determining the roentgen presents certain difficulties, 

 which can be fully appreciated only by studying the literature accumu- 

 lated as a result of efforts to measure accurately in roentgens the 7 radia- 

 tion emitted by radium (Failla and MarineUi, 1937; Friedrick, 1938; 

 Kaye and Binks, 1938; Taylor and Singer, 1940). It is difficult, for 

 example, to design a diaphragm system which deUmits the beam accu- 

 rately, and it becomes very expensive to construct, and very awkward 

 to operate, a free-air chamber of dimensions adequate to ensure full 

 utilization of associated electrons and collection of all ions produced by 

 them. In contrast to the excellent agreement ( ± 1 per cent) found by 

 intercomparison in the measurements of X-ray radiation in roentgens in 

 several laboratories throughout the world (Taylor, 1932), the experi- 

 mental determination of dosage rate from radium with free-air chambers 

 has been reported only twice (Kaye and Binks, 1938; Taylor and Singer, 

 1940). Although the agreement between them may be considered satis- 

 factory, in view of the many corrections involved, these values are dis- 

 tinctly lower than the majority of the results obtained with the "air- 

 wall" chamber (White, MarineUi, and Failla, 1940; White, 1950) (see 

 p. 154), and the lack of agreement may well be due to the experimental 

 difficulty of assessing beyond dispute the adequate separation of the elec- 

 trodes. Thus Kaye and Binks (1938) report a plate separation of 3 

 meters in atmospheric air as sufficient, Taylor and Singer (1940) find it 



