56 Applied Biophysics 



The cyclotron did not provide high-energy electrons as well 

 as positive ions, because the lightweight electron behaves rela- 

 tivistically when its kinetic energy is still very small. Kerst -^ 

 gives details of the construction of an improved induction accel- 

 erator which gives electrons 20 million electron-volts energy. 

 The accelerator has a 19-inch [about 58 centimeters] diameter 

 pole face and weighs approximately 33^ tons [about 3,050 

 kilograms]. The X-ray output, as measured in a thick-wall 

 ionization chamber, is 16 revolutions per minute at one meter. 

 The most important improvement incorporated in this accelerator 

 is the electromagnetic expansion of the equilibrium orbit, which 

 can be timed to send the electrons against the target at any 

 desired energy up to 20 million electron-volts. 



The high-energy X-rays and electrons which are made avail- 

 able by the betatron can be employed for both physical experi- 

 ments and practical purposes. It is probable that all the elements 

 in the periodic table can be disrupted with the 20 million 

 electron-volts now available by a photonuclear process. The 

 energy of the X-rays or y-rays is used, generally, in ejecting 

 a neutron from the parent nucleus. The electrons of 20 million 

 volts energy are capable of penetrating at least 10 centimeters 

 into the human body. It has been suggested that they could be 

 used therapeutically instead of X-rays, and that they would 

 have the advantage that the ionization produced by them would 

 stop rather abruptly at about the middle of the body, and do 

 no damage beyond. The betatron produces X-rays which have 

 intensities comparable with those produced by commercial 

 machines. The maximum ionization caused by these X-rays 

 occurs at about 4 centimeters beneath the surface, which makes 

 it possible to administer a large dose to the interior of the body 

 without harming the surface. 



In conclusion. I wish to thank Mr. C. Home, of Marconi 

 Instruments ; Metropolitan Vickers of England ; Mr. G. Parr, 

 editor of Electronic Engineering ; Mr. T. J. Shields, librarian 

 of the British Medical Association ; and Mr. G. F. Home, 

 librarian of the Royal Society of Medicine. 



