152 



RADIATION BIOLOGY 



In order to maintain the collector plate at the same potential as the 

 guards (earth) during measurement, it is necessary to employ a null or 

 other equivalent current-measuring device. In practice, quartz-fiber and 

 vacuum-tube electrometers of adequate sensitivity have been found sat- 

 isfactory (Taylor, 1931; Failla, 1930). 



The significant volume of air is determined by the product of the area 

 of D and the length of C provided (1) that the distance between D and the 

 proximal side of C is equal to or longer than the range of the most ener- 

 getic electron liberated by the X ray in use and (2) that the separation of 

 the electrodes C and H is sufficient to permit the recoil electrons e to 

 expend their total energy in producing ions before striking the plates. 

 Since the flux incident on the area of the aperture D traverses the cham- 

 ber, the fraction converted in any one layer dx of air between the plates 

 will be essentially the same throughout the chamber if the primary pho- 

 tons are not sensibly reduced in number in their path through the cham- 

 ber. This will be true irrespective of the relative size of the umbral and 

 penumbral portions w and p of the beam limited by D and caused by the 

 finite size of the source F. 



Under these conditions the equivalent of the total ionization produced 

 by the corpuscular radiation associated with the collecting volume (area 

 of diaphragm X C) is collected by the system. This is schematically 

 shown in Fig. 2-3, where AB and A'B' represent the paths of two second- 



F ' F 



Fig. 2-3. Schematic representation of electronic compensation in the standard open- 

 air chamber. 



ary electrons ejected from the air at points A and A' located, respec- 

 tively, at equal distances from the planes FF and F'F' bounding the col- 

 lecting volume V of the chamber. It is obvious from the figure that the 

 ionization produced along A'B' , but not collected in F, is compensated by 

 that fraction of the ionization along AB which is produced within F. 

 This compensation obtains in an ordinary X-ray beam when the elec- 

 trons released at A and A' are identical as to number and configura- 



