MEASUREMENT OF IONIZING RADIATIONS 



183 



Laurence, 1940) ; the latter should be consulted for details. It will suffice 

 to add here that the dose at a point P, at the surface of the object, will 

 not be the same as the dose measured at P in air. The excess, called 

 "backscatter," has also been investigated under detailed geometrical 

 conditions and radiation hardness of interest in radiation therapy 

 (Quimby, Marinelli, and Farrow, 1938). Whenever confronted by 

 unavoidable conditions of backscatter the investigator may well place his 

 material on Presdwood, or some such light material, of dimensions com- 

 parable to those for which data on backscatter are available. Com- 

 pliance with this suggestion will make detailed dosimetric study of each 

 particular setup unnecessary, and an occasional check on the constancy 

 of the machine will in most cases prove adequate. 



Photon-Electron EquilibHum. It has been already mentioned that 

 measurement of the dose by the ionization method is possible when full 

 utilization of the associated corpuscular radiation within the ionization 

 chamber is attained. In order for these measurements to be representa- 

 tive of the dose in biological materials, the materials should be exposed 

 under conditions abiding by the same principle of full utilization (e„ = 1), 

 which will be called "photon-electron eciuilibrium." In the illustration 

 of the principle (Fig. 2-14) the reader will recall the analogous comments 



DEPTH 



— >- 





Fig. 2-14. Build-up of photon-electronic equilibrium within a biological specimen of 

 thickness greater than the range of secondary electrons. 



on the standard air chamber. Symbolically, the arrows /p represent 

 those primary photons in a beam which interact in a biological system of 

 thickness t. The electrons released by the radiation are shown by the 

 undulating lines of range denoted by r.^^ If the photons are assumed to 



1' The secondary jAotons /,, unless of very low energy, need not be considered 

 because their probability of interacting within I is negligible. 



