38 



RADIATION BIOLOGY 



X RAYS 



When scattered radiation emerges sidewise from an incident beam and 

 a photon of the scattered radiation is detected, the electron that has 



caused the scattering recoils on the oppo- 

 site side of the beam (see Fig. 1-25). The 

 simultaneity of the scattering of a photon 

 with the recoil of an electron has been 

 carefully checked. The kinetic energy of 

 the recoiling electron is supplied by the 

 incident radiation; the scattered photons 

 have actually lower energy than the inci- 

 dent ones. 



The energy loss associated with scatter- 

 ing of a photon by a free electron is borne 

 out by a corresponding decrease of fre- 

 quency and increase of wave length of the 

 scattered radiation with respect to the 

 incident radiation. This change of properties is called " Compton effect," 

 after its discoverer. 



Fig. 1-25. Diagram of the Comp- 

 ton-Simon experiment for the 

 verification of the law of Comp- 

 ton scattering. 



1.0 



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FRACTION TO RECOIL ELECTRON 



0.8 0.6 0.4 0.2 

 1 1 1 1 



0.01. 

 Mev 



1 40 



30 



20 



10 



to Mev 



1.0 Mev 



0.1 Mev 



0.2 0.4 0.6 0.8 1.0 



FRACTION TO SCATTERED PHOTON 

 Fig. 1-26. Statistical distribution of the energy of a photon that is Compton scattered, 

 showing the fraction that is left with the scattered photon and the fraction that goes 

 to the recoil electron. The curves correspond to various energies of the incident 

 photon. Example: The shaded area, which is equal to the product of a 0.05 range of 

 energy fraction and the mean probability per imit scale 1.1, measures the probability 

 0.055 that a 1-Mev photon keeps 40 to 45 per cent of its energy after scattering and 

 transfers 55 to 60 per cent (i.e., 550 to 600 kev) to the recoil electron. {Courtesy 

 G. R. White.) 



The exact sharing of energy between the scattered photon and the 

 recoiling electron depends on the detailed balance of the energy and 



