MEASUREMENT OF IONIZING RADIATIONS 



175 



In general, to simplify measurement, scatter should be minimized by 

 removing as much mass as possible from the vicinity of both S and P and 

 by relegating it to distances large compared to the distance SP. In prac- 

 tice this can be achieved by depositing both source and biological speci- 

 mens on very thin films of material such as nylon or aluminum, by secur- 

 ing these to distant supports, by constructing a large enclosure, by setting 

 up the experiment near its center, and by placing a diaphragm between S 

 and P. Materials for diaphragm and shield should be of low atomic 

 number to minimize scatter and production of photons. Some of these 



(A) 



TO 



BATTERY 



■A 



J SOURCE 



v/////////^///y //,'///y//)^ /^ 



J*^\ D 



A 



E 



± 





M 



m 



K 



TO 

 ELECTROMETER 



(B) 



Fig. 2-10. (A) Typical setup for the exposure of biological systems to ionizing particles. 

 (B) Setup for the measurement of dose resulting from irradiation by ionizing particles. 



restrictions, however, are not essential in exposures to a, proton, or 

 deuteron radiation for they are not easily scattered by matter and are 

 not good producers of photons in this range. 



In setups of this type, measurements should be conducted with an 

 ionization chamber of the type illustrated in Fig. 2-lOB, where both the 

 high-voltage electrodes E,E and the collecting electrode E' are of minimal 

 thickness. The supporting structure (represented by cross-hatching) 

 should be placed well within the "shadow" of the circular diaphragm D 

 to avoid scattering and to allow at the same time particles traveling out- 

 side the geometrical beam to produce measurable ionization within the 

 chamber. The average dose in a very light biological medium (such as 



emitting isotopes is illuminating in this respect and should be consulted by the inter- 

 ested reader (National Research Council, 1950). 



