104 BIOLOGICAL EFFECTS OF RADIATION 



happen, therefore (and in practice this is usually the case), that the 

 hardness of the radiation increases with the depth, up to a certain point, 

 because the soft components are unable to penetrate beyond the super- 

 ficial layers of the material. Obviously, under these conditions the 

 concentration of ionization loci drops very rapidly with increasing depth. 



FILTRATION 



The hardening effect just mentioned, which can be enhanced by suit- 

 able choice of material, is made use of in practice to obtain beams of a 

 more homogeneous character {i.e., less difference in the component 

 wave-lengths) than that of the radiation emitted by an X-ray tube or 

 radium. The process by which this is accomplished is called filtration. 

 The choice of the material to be used as a filter is governed by the follow- 

 ing considerations : The longer wave-length components lose energy more 

 readily through both photoelectric and Compton transformations. But 

 in the case of the photoelectric effect the wave-length (energy) of the 

 secondary photon depends only on the atomic number of the material, 

 whereas in the case of the Compton effect it depends on the wave-length 

 of the incident radiation and the angle at which the secondary photon is 

 emitted. The photoelectric effect is small, or even negligible, when the 

 radiation is hard and the material has a low atomic number. The 

 Compton effect is predominant under these conditions, and vice versa. 

 One may conclude, therefore, that the proper material to use in any 

 given case is one of such atomic number that the photoelectric effect takes 

 a predominant part in removing the softer components of the beam. At 

 the same time, the atomic number must not be so high that the photo- 

 electric effect is also large for the harder components of the beam, since 

 this would diminish unduly the intensity of the desired radiation. Also, 

 it should be noted that, while the characteristic radiation of materials of 

 low atomic number resulting from photoelectric interchanges is negligible, 

 it assumes greater importance with the higher atomic-number elements. 

 In practice it has been found that aluminum is a satisfactory filter for 

 soft X-rays, produced at voltages not exceeding 100 kv. peak. Copper 

 is ordinarily used for voltages in the neighborhood of 200 kv. peak. In 

 this range, when high filtration is desired, say, 2 mm. Cu, Thoraeus (4) 

 has shown that it is more economical to use tin. However, this introduces 

 certain complications because its characteristic radiation (K radiation 

 0.424 A, corresponding to photon energy of 29 kv.) is in the range of 

 wave-lengths which one wishes to eliminate. ^-^ As a result, some fairly 

 long radiation in the primary beam is transmitted readily (anomalously) 

 through the filter, and some soft characteristic radiation is also emitted. 

 To overcome this difficulty a sheet of copper is added to the tin, of 

 " An element is particularly transparent to its own characteristic radiation. 



