HEAVIER PARTICLES 



33 



tion density at the peak also decreases, for the increase in stragghng of 

 the beam spreads the region of high specific ionization over a great 

 volume. Actually nuclear absorption and scattering of the protons 

 reduce the ratio obtainable at high energies. 



Because the protons are charged, it should be easy to pass a divergent 

 or parallel beam through a magnetic lens and so produce a convergent 

 beam whose point of convergence comes in the tissue at a depth equal 

 to the proton range. This is equivalent to cross fire and should produce 



I I I I 



5 10 15 



Centimeters 



Fig. 7. Isodose curves for 190-mev deuterons in water. 



E 



20 



similar spectacular results in reducing the dosage in the surrounding 

 tissue. The same idea could be used with any charged particles, such 

 as electrons. 



Heavier Particles 



Heavier particles such as deuterons and alpha particles, or even nuclei 

 of atoms, have some advantages over protons. Multiple scattering and 

 struggling decrease as the square root of the mass. Hence the extremes 

 of ionization density will more closely resemble the ideal specific ioniza- 

 tion. These extremes are even more emphasized because of the well- 

 known specific ionization dependence on the square of the charge of the 

 particles. The dependence is not quite as good as it first sounds, for, as 

 the particles slow down to velocities approaching those of the atomic 

 electrons, electrons become attached to the particle and so reduce the 

 effective nuclear charge. Eventually the principal loss of energy comes 

 about because of nuclear collisions, and such collisions increase the 

 straggling. 



