PROTONS 31 



With beams of large cross section, on the other hand, diffraction scat- 

 tering produces no effect, for it is predominantly forward and is elastic 

 so that the protons penetrate just as far, ending up with a slight lateral 

 displacement. Even with beams of small cross section diffraction 

 scattering is not too serious. About 10 per cent of the initial protons 

 of a 15-cm beam are scattered out of the beam, and these are spread more 

 or less uniforml}^ over an area of about 10 cm^, so that the density of pro- 



FiG. 5. Photographic plate irradiated under water by a beam of 190-mev deuterons. 

 Note spreading at end of beam and increased ionization. 



tons outside the beam drops to about 1 per cent of its value in the beam. 



Tobias and Auger (6) have made experimental studies using 190-mev 

 deuterons, which are similar to protons. Figure 5 shows a direct picture 

 of the beam taken by allowing the deuteron to pass through a photo- 

 graphic film which had been immersed in water. One can observe the 

 spreading at the end of the beam and also the increased ionization 

 density. Figure 6 shows quantitatively the differences in ionization 

 density or dose characteristics among x-rays, electrons, and protons. 

 Figure 7 is a typical isodose curve for a beam of 190- mev deuterons. 



There are a few things to emphasize in the use of protons. Higher 

 energies than necessary should not be employed. It is true that the 

 protons can be slowed down in some other material outside the tissue, 



