ELECTRONS 



27 



3-cm depth when 20-mev electrons are used (1). (See Fig. 1.) The 

 peaks are quite broad, however, and the intensity falls off very slowly 

 thereafter. At energies higher than about 20 mev the exit dose will be 

 almost as large as that received at the maximum position. Betatrons 

 and synchrotrons now give energies in excess of 300 mev, but there seems 

 little advantage from a radiological point of view in using such high- 

 energy x-rays; the penetration is far too great. At all such high energies 

 the biological effects of the x-rays should be roughly the same, inasmuch 

 as the specific ionization density is nearly independent of the energj^, 

 for energies higher than 1 mev. 



At very high photon energies, nuclear explosions or stars are induced 

 by the photons. In such stars, several nuclear particles are emitted, 

 thus offering a mechanism for producing high specific ionization density 

 in the tissue. The phenomenon is not well studied as yet, but it does not 

 seem to be large enough to be biologically significant. 



Electrons 



A real step forward was made by Kerst and his group at Illinois when 

 the electrons were brought directly out of the betatron. An exposure 

 to homogeneous high-energy electrons can now be made directly, without 

 the usual transition of the electrons inside the betatron to a degraded 

 x-ray or bremsstrahlung spectrum and then the additional transition 

 and further degrading of this spectrum back to electrons in the tissue. 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 



Depth, cm 



Fig. 2. Depth-dose measurements with electron beam from betatron, in phantom. 



