34 BEAMS OF HIGH-ENERGY PARTICLES 



Cyclotrons that accelerate deuterons can also accelerate without read- 

 justment heavier particles such as the nuclei of carbon, the energy being 

 greater in proportion to the increase in mass. Thus, in a 200-mev 

 deuteron cyclotron, one could readily get small beams of 12-bev carbon 

 nuclei. Apart from the factors just mentioned, the specific ionization 

 would be increased by more than 36 times that of the deuterons. The 

 range, however, would be reduced from 16 cm to about 2.5 cm — still 

 usable — and the precision better by a factor of 3. Li'^ nuclei would 

 have an energy of 700 mev, a range of about 6 cm, and a specific ioniza- 

 tion greater than protons by a factor of 9. Such beams should be par- 

 ticularly valuable in precision research work on small organisms. The 

 larger accelerators now under construction at Brookhaven National 

 Laboratory and at Berkeley will give greater penetration for these heavy 

 nuclei and make it possible to use even heavier atoms. 



The nuclear effects will be somewhat enhanced, but will probably not 

 be large enough to make the use of such beams impractical. 



Mesons 



Figure 8 shows the track of a negative meson in a photographic plate. 

 At the end of its path, the meson comes to rest and is absorbed by a 

 nucleus. An energy equivalent to the meson rest mass (140 mev) is 

 then liberated, some of which appears in the charged fragments of the 

 nucleus seen in the typical "star" at the end of the track. Thus we 

 have a mechanism for depositing a large amount of ionization at the end 

 of the range of these particles. However, since their mass is nearly one- 

 tenth that of the proton, multiple scattering is very large and may vitiate 

 any concentration of ionization by the star. Our understanding of 

 mesons is still developing. It seems now that, if mesons become plentiful, 

 they may seriously compete with neutrons, but do not seem to have any 

 advantages over, say, a high-energy alpha-particle beam. 



Neutrons 



Solomon will discuss low-energy neutrons. It is possible to make 

 reasonably homogeneous beams of high-energy neutrons by a process 

 called stripping, but there seems to be little radiological interest in them, 

 for the neutrons become effective in tissue by the production of recoil 

 protons which are now more easily and more homogeneously available 

 directly. 



