PROTONS 



29 



range of the electrons. On the other hand, and this is the advantage, 

 if one uses a beam of large cross section, the lateral motion will tend to 

 increase the ionization density near the end of the range, for the scatter- 

 ing gives the electron an oblique and hence longer path in a given incre- 

 ment of depth. 



The measurements of Skaggs (2), using an 1 1-cm-diameter beam of 

 16-mev electrons, show that the latter effect predominates, so that a 

 slight maximum is observed (Figs. 2 and 3) . Skaggs finds also that the 

 extrapolated range in centimeters in H2O is about one-half the energy 

 in mev minus 0.5 cm. A 40-mev betatron would be about right for 

 radiological work. 



Nuclear disintegrations are induced by such electrons, but the number 

 of disintegrations is far too small to be biologically significant. 



Protons 



High-energy proton beams obtainable from synchro-cyclotrons offer 

 considerably higher precision in delivering a large dose to a small volume 

 without overexposing neighboring tissue. Whereas the electron-specific 



1400 

 1200 



o 1000 



sz 



S" 800 

 ■o 



^ 600 



400 



200 







6 8 10 12 14 16 18 

 Depth, cm 



Fig. 4. Calculated depth dose due to protons. The dotted curve shows the effect of 

 a single 140-mev proton in tissue; the full line, the estimated depth dose for a well- 

 coUimated beam. The difference shows the effect of straggling and scatter. Repro- 

 duced by permission from Radiology, 47: 487, 1946. 



ionization is nearly constant for the energies we are considering here, 

 the proton-specific ionization decreases nearly inversely with energy. 

 (See Fig. 4.) The reason is that the electron motion is completely 

 relativistic, that is energy large compared to that of the rest mass 

 (0.5 mev), whereas the proton energy (150 mev) is small compared to 



