26 



BEAMS OF HIGH-ENERGY PARTICLES 



Photons 



Let me begin with photon beams. You are probably more famihar 

 than I with the low-energj^ x-rays, that is, a few hundred kev. Such 

 radiations can be well collimated using lead slits but are rapidly ab- 

 sorbed in tissue. The x-rays, of course, act in the tissue when they are 

 absorbed to form low-energy electrons, the electrons having a range of 



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 Water, cm 



Fig. 1. Isodose contours in phantom, using 16-mev electrons from betatron. 



much less than 1 mm. As the energy of the x-rays is raised to the order 

 of 1 mev, the character of the absorption process begins to change. The 

 penetration becomes greater and the electrons formed now begin to have 

 a range in tissue of several millimeters. Additionally, these electrons 

 can now radiate part of their energy into secondary x-rays which in turn 

 can be absorbed farther on in the tissue, and we now begin to see an 

 increase in the dosage with depth. As the energy is increased further, 

 this maximum in the depth-dosage curve becomes more pronounced and 

 its position occurs at a depth below the surface which has been found 

 to increase almost linearly with energy: the maximum dosage comes at 



