PRINCIPLES OF RADIOLOGICAL PHYSICS 



47 



When the absorption of radiation results in the ejection of an internal 

 electron, the place of this electron is usually taken by an outer electron 

 which drops in closer to the nucleus. This internal rearrangement is 

 equivalent to a loss of excitation. The energy released may radiate 

 away as an X-ray photon ('* X-ray fluorescence") or may serve to eject 

 another outer electron ("Auger effect"; see Sect. 2-lc). 



An atomic electron does not respond strongly to high-frequency radia- 

 tion whose photon energy greatly exceeds the energy reciuired to remove 

 the electron from its atom. In other words, the photoelectric effect 

 becomes unlikely when the radiation frequency is far higher than the 

 threshold frequency. The reason is that the own inertia of the electron, 



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 < 



10,000 A 



I.O/i 

 1 



WAVE LENGTH 

 1000 A 

 0.1 ;i 



100 A 

 O.OI;i 



LINES^ 



^CONTINUOUS SPECTRUM- 



10 



14 



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 FREQUENCY 



10 



16 



Fig. 1-34. Diagram of an atomic absorption spectrum. 



rather than its attraction by the nucleus, presents the main obstacle to 

 oscillations with excessive frequency. Therefore the electron behaves 

 practically as a free electron which scatters but does not absorb radiation. 



Figure 1-34 illustrates diagrammatically the absorption of radiation of 

 different frequencies by the atoms of a monoatomic gas or vapor. It 

 represents the so-called "absorption spectrum," i.e., the fraction of the 

 intensity of different spectral components which remains unabsorbed after 

 traversing a certain thickness of vapor. The sharp "lines" correspond 

 to the excitation of particular stationary states of the electrons and con- 

 stitute the characteristic "line spectrum" of the substance. The broad 

 bands on the high-frequency side constitute the "continuous spectrum" 

 and correspond to absorption processes leading to ionization of the atoms. 

 The "edges" of successive broad bands correspond to the onset of the 

 ejection of electrons of higher and higher binding energy. The edges 

 corresponding to the ejection of the innermost electrons lie in the X-ray 

 region, whereas the line spectrum and the beginning of the continuum lie 

 in the visible and ultraviolet region. 



The absorption spectrum of the molecules of diatomic and polyatomic 

 substances appears to consist of a succession of bands of limited width 

 followed, in the ultraviolet range, by a continuous spectrum which cor- 



