24 



RADIATION BIOLOGY 



y .. 



*.. \ 





ELECTRONS 



increased electron bombardment and higher X-ray intensity. (When the anode 

 is cool it releases far fewer electrons than the filament. Therefore electrons can 

 flow easily only from the filament to the anode even if the apj^lied voltage is 

 reversed, and the tube may be employed as a self-rectifying unit with an applied 



I a-c potential.) In X-ray tubes of earlier 



models the anode is not cooled and becomes 

 red or even white hot. For technical data, 

 see, for example, Glasser (1944), pp. 1395 fT, 

 The faster the electrons, the sharper are the 

 variations of their motion and the more intense 

 the emission of X rays. The higher the atomic 

 number of the target material, the stronger is 

 the attraction exerted by the nuclei on the 

 electrons and the more intense the emission of 

 X rays. Under ordinary conditions only a 

 small fraction of the energy of all incident elec- 

 trons is radiated. This fraction y is approxi- 

 mately proportional to the atomic number Z of 

 the target material and to the accelerating 

 voltage T' applied to the electrons in the tube according to the formula 



FILAMENT 



RAYS 



ANODE 



Fig. 1-15. Diagram of a medium- 

 voltage X-ray tube. 



y ~ ZV/10' 



(12) 



12 



10 



CO 

 LlI 



I- 



F.op exapiple, if the anode is of tungsten (Z = 74) and the accelerating voltage 

 is; 100^ 000 volts, the energy fraction radiated is y ~ 0.0074 ~ 0.01 within the 

 Unjlt^pf this approximation. The value of 

 if approaches '% in the case of a tungsten 

 ■feew.'gft^rfer an applied voltage of 10 Mev. 

 fti* foMiuk, no* longer holds at higher elec- 

 tron energies. „ '• 



The.X rays emitted by the electrons as 

 they move jerMly across atoms consist of a 

 ■<. con^nuous spectrum of monochromatic 

 co'nfpdnents whose frequencies range up to 

 a maximum value I'max- The maximum 

 energy hv^nnx. of the X-ray photons equals 

 the kinetic energy of the electrons which 

 strike the anode in the X-ray tube. Figure 

 1-16 shows the intensity of the components 

 of different frequencies in a typical X-ray 

 spectrum. 



Electron accelerators of the types de- 

 scribed in Sect. l-2b (betatrons and synchro- 

 trons) serve to produce X rays of very high 

 frequencies and high photon energies. 

 The X rays are generated when the elec- 

 trons, after acceleration, hit a target, preferably of heavy metal. 



The X rays emitted by electrons in the multimillion-volt range propagate 

 mainly in the original direction of the electrons since they carry most of the 







Fig. 1-16. 



tribution 



operated 



Tungsten 



White.) 



10 



40 50 



20 30 



ENERGY, kev 



Approximate spectral dis- 



of X rays from a tube 



at different voltages. 



target. {Courtesy G. R. 



