MEASUREMENT OF IONIZING RADIATIONS 



181 



Some Details of Practical Interest in Photon Dosimetry. It may prove 

 useful to the reader to ponder briefly on what a radiobiologist with access 

 to a typical photon source and in possession of a typical commercial 

 dosimeter would have to consider in order to measure the dose correctly. 

 The physical setup may be represented 

 symboUcally as in Fig. 2-13, where E' 

 represents the over-all shield of the 

 apparatus, be it the walls of a room 

 or a lead-lined box of smaller dimen- 

 sions. In order to diminish the scatter 

 of radiation, which in the case of pho- 

 tons is composed of both photons (/,) 

 and electrons (cs), the source of X-ray 

 radiation S is usually shielded by an 

 opaque enclosure E, and only the radia- 

 tion emerging through the diaphragm 

 system D1D2 is utilized. This radia- 

 tion passes through filters Fi and F2 

 which modify the quality of the beam, 

 impinges on the biological object of 

 thickness t, penetrates the support, and 

 strikes the shield E'. 



Let us suppose that the dose at point 

 P is desired and that a thimble chamber 

 is placed there for the purpose. In 

 most cases the scattered radiation from 

 E is no cause for concern since the 

 electronic component e^ will not reach 

 the point in question and the scattered 



photons fl, proceeding in the general direction of the primary radiation 

 originating from .S, will be correctly measured atP by the thimble chamber. 

 Usually this "off-focus" radiation does not contribute more than 10 per 

 cent of the dose. The secondary radiation from the filter Fi may reach 

 the object but not be recorded by the chamber in many instances; this 

 applies to both the electron component ep and the photon component fp. 

 At distances greater than 10 to 15 cm the former is ehminated by air 

 absorption, but the latter may be cause of error when the fluorescent 

 component is unduly absorbed by the walls of the chamber and does not, 

 therefore, produce ionization in the cavity of the chamber. ^^ This uncer- 

 tainty is usually avoided by the use of a secondary filter F2 of lower atomic 

 number than Fi, which absorbs strongly the characteristic radiation of 

 Fl but which emits less penetrating radiation easily absorbed in a few 

 centimeters of air. In practice, if Fi is of aluminum, F2 is unnecessary 



" The harder scattered component will be registered along with the primary. 



Fig. 2-13. Schematic representation 

 of typical X-ray irradiation setup. 

 S, source; D1D2, diaphragms; F1F2, 

 filters. 



