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Beams of High-Energy Particles 



ROBERT R. WILSON 



Cornell University 

 Ithaca, New York 



Introduction 



Nuclear physicists, in their quest for the ultimate elementary particles 

 of nature, have constructed larger and larger accelerating machines. Of 

 the fruits of this research, perhaps the most important is the availability 

 of radiologically usable beams of nearly all the known particles. Thus, 

 by means of a cyclotron, a betatron, a synchrotron, or a linear accelerator, 

 one can get a well-collimated beam of high-energy protons, electrons, 

 photons, neutrons, alpha particles, and, indeed, even of nuclei such as 

 Be or C. The use of beams of mesons, the new particles intermediate 

 in mass between electrons and protons, may soon be practical. This 

 sudden wealth of unexploited radiological tools should be of considerable 

 usefulness in the research of the radiobiologist, and in this paper will be 

 described the general characteristics of such beams. 



A beam of nuclear particles is characterized by its range, ionization 

 density, and homogeneity. The range of a beam is here given in terms 

 of the distance in centimeters that it can penetrate tissue. The ioniza- 

 tion density is the number of ions per cubic centimeter produced on the 

 average at a given point along the beam. To be distinguished from this 

 is the perhaps more important concept of specific ionization, namely the 

 number of ions per centimeter along the track of a single particle. The 

 statistical nature of the loss of the energy of the beam introduces inhomo- 

 geneities into the beam : inhomogeneities of energy, of direction, and of 

 penetration. This straggling, as it is called, causes a spreading of the 

 beam and a corresponding decrease in ionization density. Nuclear 

 interactions between the particle and then stopping medium can also 

 become important at high energies and also contribute to the inhomo- 

 geneity of the beam. These effects will be discussed in more detail as 

 each particle is taken up in turn. 



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