SCATTERED X-RADIATION 23 



Scattered X-Radiations as Modified by the Compton Effect 



The preceding section discussed the properties of the scattered x-radia- 

 tions which possessed the same wavelength as the incident energy. 

 Coexisting with this phenomenon are found scattered x-radiations 

 having longer wavelengths than the incident energy. 



In 1922, A. H. Compton first demonstrated experimentally that a 

 modification of the wavelength took place as the result of the scattering 

 of the incident x-ray energy by the electrons in the material. He found, 

 for instance, that, when very soft x-radiation having a wavelength 



o 



equal to 0.7078 A was allowed to penetrate a cube of carbon, the scat- 

 tered radiation emitted by the cube at right angles to the incident beam 

 was composed of two groups of wavelengths. The first was a true, or 



o 



regularly scattered wave with unmodified wavelength (X = 0.7078 A); 

 in addition, he found a modified scattered wavelength equal to 0.7320 A, 

 the increase in wavelength amounting to 0.0242 A. 



Therapeutically speaking, the Compton scattered energy is softer 

 than the incident and regularly scattered energy. If the wavelength 



o 



of the primary beam is exactly 1.0 A (very soft radiation), the wave- 

 length of the x-rays emitted at right angles to the incident beam will 

 increase by 0.0242 A, so that its wavelength becomes 1.0242 A. This 

 increase in wavelength of about 2 per cent is not very important. If, 



o 



however, a primary beam whose wavelength is 0.1 A is examined (hard 



o 



radiation), the increase in wavelength is also 0.0242 A, so that the modi- 



o 



fied scattered wavelength is 0.1242 A. This increase is nearly 25 per 

 cent, and if any considerable proportion of the modified x-radiation is 

 present, the properties of this scattered beam will be very different from 

 those of an unmodified beam. Such changes in wavelength imply great 

 changes in the mass absorption coefficient. If, in addition, the intensity 

 of the modified portion of the beam is greater than the intensity of the 

 unmodified wavelength of the beam then a serious error arises by not 

 taking the Compton scattering into consideration in biological absorbing 

 material. Table 1-6 shows to what dimensions the error may rise when 

 elements of small atomic number are used as absorbing material. In 

 deep therapy where radiations comparable to gamma rays are used, the 

 softening of the scattered beam by the tissues is even more pronounced. 

 A. H. Compton has shown that it is possible to calculate the increase 

 in wavelength (AX) as a function of the angle of scattering 0: that is, 

 the angle at which an observer measures the scattered radiation with 

 respect to the direction of the incident ray, from the simple relation 



h ,, 

 AX = — (1 — cos <p) 



m c 



