172 RADIATION BIOLOGY 



relaxation lengths, 



gl = H[e^" + hEi{-h)] + y2[e-'+' + {h + t)Ei{-h - t)] outside 



the slab 

 and 



gi = l-gl- y2[e+'-' + {t- h)ES - h)] inside the 



slab 



Finally, for an infinite thin plane source of C /xc/cm^ the dose at a distance 

 of h relaxation lengths will be 



Particular difficulties are presented by "spotty" distributions of /3 

 emitters in tissues. In general, even for the case of well-defined point 

 distributions, the computational approach is at best cumbersome and 

 inaccurate. An experimental approach, profusely applied in metabolic 

 problems but insufficiently explored for the purpose of |8-ray dosimetry, is 

 represented by the autoradiographic technique (Pelc, 1951; Lamerton 

 and Harriss, 1951; Dudley and Dobyns, 1949; Miller and Hoecker, 1951). 

 Despite the shortcomings of the development process of photographic 

 emulsions, this technique seems to possess the fundamental prerequisite 

 of completeness as far as geometrical information is concerned. An 

 excellent start in this direction has been made by Dudley and Dobyns 

 (1949) by the use of thick tissue sections on emulsions. In order to cor- 

 relate film opacity to dose, these authors use thick sources of known dis- 

 integration rate containing the radioelement in material of atomic 

 number comparable to that of the tissue. Calculation of the dose 

 effective on the film from the source is carried out by means of formula 

 (16). Another example of the application of autoradiography to dosim- 

 etry is the careful work of Miller and Hoecker (1951) on the a-ray 

 dosimetry of radium deposited in bone. 



Of special interest to the biologist investigating the microdosimetric 

 aspects of radiobiological action will be the volumetric limit beyond 

 which the term "dose" loses its significance. It can be readily verified 

 that the expenditure of 16 ev (roughly the energy required to create an 

 ion pair) within a sphere of 15 m^u diameter (roughly the ion separation) 

 corresponds to a dose of the order of 10^ r within the sphere. It would 

 seem that as the linear dimensions of the region of interest approach this 

 limit, the concept of continuity of dose must be abandoned in favor of a 

 description in terms of discrete localized energy transfers to supposedly 

 significant chemical or biological entities within the region. 



Gamma-ray Emitters. At a given point the total dose Dy due to 7 rays 

 emitted by the complete disintegration of a radioelement biologically 



