184 RADIATION BIOLOGY 



be monoergic, their probability of interaction per unit thickness t will be 

 the same, irrespective of the depth in the material; therefore, if the 

 number of interactions is small compared to the total number of photons 

 crossing the object, the rate of electron release per unit thickness will be 

 sensibly constant at any point in the material. If the additional assump- 

 tion is made that no electrons have been produced by the beam outside 

 the test object, it is easily seen that the ionization produced per unit 

 volume by the particles is not constant — as a count of the electron tracks 

 in each element A will disclose. In fact, the ionization as a function of 

 depth will vary according to the insert on the right-hand corner of the 

 figure. Namely, the dose will increase linearly with depth up to r and 

 remain constant from depth r to depth t. The total energy utilized is 

 proportional to the area OABC, whereas the energy converted will be 

 proportional to the area OA'BC. It is obvious that under these condi- 

 tions e„ < 1. In practice, the presence of heterochromatic radiation, the 

 release of electrons in various directions, the scattering of electrons by the 

 object, etc., will yield an experimental curve of the type represented by 

 the dotted line OA, and the exact magnitude of €„ will be outside the reach 

 of a simple calculation. A practical solution is offered by covering the 

 material in question on both sides with material of low atomic number of 

 thickness equivalent to r. It might be argued that this is unnecessary 

 since the lower ionization within depth r is compensated by electrons 

 released by the air around the material ; this is correct provided the thick- 

 ness of air around is at least of equivalent thickness r and that the flux 

 throughout this air mass is the same as at the surface. This would usu- 

 ally be the case for low-energy photons attenuated by properly placed 

 filters, but it can hardly be achieved in practice with the harder radia- 

 tions, such as those obtained from Ra-'-^ or Co^°, for example, or when 

 filters are grossly misplaced. 



It is obvious that electron equilibrium may be in jeopardy near 

 boundaries of media of different densities and atomic composition, or 

 both. To avoid lengthy experimentation or calculation, small biological 

 organisms or small volumes of solutions should not be irradiated in glass 

 vessels but, in so far as possible, in vessels of materials such as Incite, 

 polystyrene, or polyethylene. The importance of this effect on the local 

 dose is difficult to evaluate, except for order of magnitude; it is an inverse 

 function of the linear dimensions of the object and a direct function of 

 both the range of the released electrons and the atomic number of the 

 container. In the event that the object irradiated is a mammal or a 

 crustacean and the dose received by the mineralized tissues (or soft tissues 

 in the immediate vicinity) is desired, special experimentation or calcula- 

 tion is necessary. The same comment applies to the estimate of dose in 

 fatty tissues. For a thorough understanding of the problem and cogent 

 calculations, a lengthy dissertation is unavoidable; the reader is referred 



