188 Applied Biophysics 



but this will not strictly obtain ( 1 ) if the strength of the radia- 

 tion varies appreciably over a distance comparable to the max- 

 imum range of secondary particles reaching the point, or (2) in 

 the region of a boundary between two different media. The 

 question in such cases, therefore, is whether "energy conversion" 

 or "energy communication" is to be adopted as the measure of 

 dose. From the point of view of biological effect, the latter 

 quantity is the important one. while the former is, logically 

 speaking, irrelevant, but far simpler to deal with in practice, 

 and it was adopted at the fifth international congress of radiology 

 in 1937. 



**The rontgen shall l^e the quantity of X- or gamma radiation 

 such that the associated corpuscular emission per 0.001293 gram 

 of air (the mass of 1 cubic centimeter of air at 0° C and 760 

 millimeters of mercury pressure) produces, in air, ions carrying 

 1 electrostatic unit of quantity of electricity of either sign." 



This is effectively the same as the 1928 definition with cer- 

 tain ambiguities removed. 



In a detailed consideration of the biological effects of radia- 

 tion in the borderline cases referred to above, it is necessary 

 to bridge the gap between a knowledge of the energy conversion 

 in air and the energy actually communicated to the medium. 

 For this purpose a very thin-walled chamber is used, the ioniza- 

 tion in which gives an indication of the secondary particles 

 (the "corpuscular emission") effective at the point.* The 

 "extrapolation" chamber introduced by Failla ^ is of this type. 

 The procedure is to take observations with a gradually decreased 

 spacing between the walls of the chamber, and extrapolate the 

 results to obtain the value for a chamber of negligible width. 

 With the very high-energy X-rays that can now be produced by 

 the betatron, studies of this kind, particularly for surface effects, 

 i.e., at the skin of the patient, will become increasingly important. 



* Note that such a chamber gives an indication of the effect of the secondary 

 particles on air (which is normally used in the chamber) and not on the medium. 

 To investigate the latter, it would be necessary to fill the chamber with a gas 

 whose effective atomic number was the same as that of the medium, and to know 

 the energy required to produce a pair of ions in the gas. Alternatively, the energy 

 absorption in the medium could be fairly closely calculated from that in air if 

 the composition of the former is known. 



