PRINCIPLES OF RADIOLOGICAL PHYSICS 



13 



OSCILLATOR 



Fig. 1-10. Diagram of the elements 

 of a cyclotron. The particles follow 

 the outward spiraling path under the 

 combined influence of a guiding mag- 

 netic field perpendicular to the draw- 

 ing and of the driving oscillating 

 electric force produced by the os- 

 cillator. {Richtmyer and Kennard, 

 1947.) 



deflecting magnets. Moreover, particles being deflected along a closed 

 path constitute a variable current which emits electromagnetic radiation; 

 the energy loss by radiation is expected to become very serious at energies 

 of the order of 10'^ ev. Finally, the problems of keeping the particle beam 

 focused and of timing the variations of 

 all fields are not trivial. 



Various types of accelerators yielding 

 energies of several hundred million 

 electron volts were in operation in 1950. 

 For further information on high-energy 

 accelerators see HalUday (1950), Chap. 

 9. 



l-2c. Nuclear Sources of Fast Charged 

 Particles. Many substances emit elec- 

 trons or a particles with energies of the 

 order of a few million electron volts as 

 a result of spontaneous transformations 

 of their atomic nuclei. The substances 

 whose nuclei undergo such transforma- 

 tions are called "radioactive." These 

 spontaneous transformations or "disintegrations" occur now and then at 

 random in the nuclei of different atoms of a substance at an average rate 

 which is characteristic for each kind of nucleus. 



The substances found in nature contain between 300 and 400 species of 

 atomic nuclei, among which nearly 50 were known to be unstable, i.e., 



radioactive, by the end of 1950. The 



^^:— g:::;: ^^ best known of these radioactive sub- 



n^^-J— Ti stances are the elements heavier than 



bismuth. A few radioactive isotopes 

 of lighter elements are also found in 

 nature. One of these is an isotope of 

 potassium. 



Nearly 1000 other species of nuclei 



have been produced artificially as the 



result of nuclear collisions. These are 



generally unstable, i.e., radioactive, 



and emit either electrons or positrons ; 



some of them emit a particles, or 



neutrons. 



Natural radioactive substances were for a long time the only sources of 



fast charged particles. Natural and artificial radioactive substances are 



still very useful as steady and finely divisible sources of fast particles. 



An amount of radioactive material whose nuclei disintegrate at a rate 

 of 3.7 X 10^" per second is called a "curie." A radioactive source of 



Fig. 1-11. Diagram of a betatron. 



