IONIZATION AND BIOLOGICAL EFFECTS 103 



proportion ^3 of the intensity of the incident radiation. For very hard 

 X-rays or gamma rays it is again small because Compton "scattering" 

 in this case takes place largely in the forward direction. It is obvious 

 that the proportion of back-scattered radiation must increase with the 

 thickness of the irradiated body, up to a certam point. When the thick- 

 ness is such that photons hberated at the far side lose all their energy 

 before reaching the near side, there can be no further increase. This 

 critical thickness depends, of course, on the wave-length of the incident 

 radiation. It is e\'ident also that within certain limits, the cross sections 

 of the irradiated object and the beam must influence the proportion of the 

 back-scattered radiation. 



No mention has been made of the secondary electrons liberated by the 

 hypothetical monochromatic beam in its passage through organic matter. 

 From the discussion of this subject already given, and the preceding 

 considerations, it follows that: (a) The maximum length of the ionization 

 loci is set by the wave-length (photon energy) of the primary radiation, 

 and is the same throughout the material. (6) But the relative number 

 of loci of this length decreases with the depth in the material at which 

 they are produced. This is a direct consequence of the softening of the 

 radiation by its passage through organic matter, (c) Near the surface 

 there is a sharper separation in the length of the ionization loci, because 

 those produced by the back-scattered radiation are much shorter than 

 those produced by the primary photons. In the deeper layers, loci 

 of intermediate length increase in number. These three conclusions refer 

 to the relative distribution of loci of different lengths at different levels of 

 the material. In addition, (d) the total number per cubic centimeter 

 decreases with the depth on account of the effect of the inverse square law 

 and the abstraction of energy from the primary beam by the intervening 

 matter. The loci are, therefore, more sparsely distributed in the deeper 

 layers than at the surface. Also, it should be noted that (e) a beam of 

 radiation which has a well-defined cross section before entering the 

 material, produces ionization outside of its geometrical contour on account 

 of the secondary rays which are emitted in all directions. The ionization 

 loci in this peripheral region are always shorter than within the confines 

 of the geometrical beam proper. 



What takes place when a polychromatic or heterogeneous beam of 

 X-rays passes through organic matter may be surmised readily from the 

 preceding discussion. The interaction with matter is essentially the 

 same for the different components, but it varies in degree. It may 



" The back-scattered radiation as ordinarily measured appears to have a high 

 "intensity," even up to 50 per cent of that of the primary beam, in the case of 200-kv. 

 filtered X-rays. This is due to the use of the term "intensity" in a different sense 

 from the one given so far (see later). 



