IONIZATION AND BIOLOGICAL EFFECTS 99 



where much longer distances can be used, the decrease of intensity 

 from one side of the object to the other is much less. Thus, for a target- 

 to-object distance of 30 cm., it amounts to less than 7 per cent. The 

 main thing to remember in this connection is that the controlling factor 

 is the ratio of tissue depth to distance of source. The smaller this ratio, 

 the less variation of intensity there is between the surface and any chosen 

 depth. In the first example the depth, or thickness, of the object, 1 cm., 

 is the same as the distance of the source, and the intensity at the far 

 side is 25 per cent of that on the surface. In the last example the depth 

 is only }ioth. of the distance and the variation of intensity is small. If 

 in this case the depth were increased to 30 cm., the intensity at this point 

 would also be 25 per cent of that at the surface.^ Similarly, if in the case 

 where the radium is placed at a distance of 1 cm. the thickness of the 

 object were }i mm., the difference between the intensities on the two sides 

 would be less than 7 per cent. 



INFLUENCE OF MATTER ON THE INTENSITY OF RADIATION 



We may now proceed to the discussion of the influence of matter on 

 the intensity of radiation. Let us examine first what happens to mono- 

 chromatic radiation when it reacts with matter. For the present we shall 

 neglect the reaction between the radiation and the air which surrounds 

 the source, and we shall concern ourselves solely with liquids or solids of 

 organic constituents. In the first very thin layer of the material some 

 of the photons which make up the beam undergo photoelectric trans- 

 formations whereby most of their energy is transferred to the secondary 

 beta particles. The remainder of the energy reappears subsequently as 

 photons of much lower energy.^ These photons, it will be recalled, are 

 emitted when electrons fall into the spaces in the atoms left vacant by 

 the electrons which were expelled through photoelectric encounters. 

 They are, therefore, the characteristic radiations of the elements in the 

 material and, in the case under discussion, are low-energy photons. 

 (The maximum photon energies of the K regions for the principal ele- 

 ments of organic matter are: H 13.6, C 25.1, N 33.8, 50.2 ev.) Their 

 emission, depending only on the resumption of the normal state of the 

 atoms, is a random phenomenon, and in the aggregate these photons are 

 emitted equally in all directions. Since for each photon of the original 

 beam which undergoes a photoelectric change one low-energy photon and 

 a high-speed electron are liberated, the total number of photons remains 



8 Neglecting all influence of matter on the radiation. 



' It is tacitly assumed in this discussion that the beam of X-rays under considera- 

 tion is of medium hardness such as is ordinarily used at present. As already men- 

 tioned, the energy of the initial photon is subdivided between secondary electron and 

 photon in different proportions, depending on the hardness of the radiation. 



