ABSORPTION OF GAMMA RADIATION BY MATTER 71 



driven at 1.5 million volts difference of potential will emit wavelengths 

 approaching the shortest wavelengths of the gamma radiations emitted 

 by RaC. 



Absorption of Gamma Radiation by Matter 



The reduction in intensity of gamma radiations by an absorbing sub- 

 stance takes place, like the reduction of intensity of x-rays, in two dis- 

 tinct ways : by true absorption and by scattering. In true absorption 

 the energy of the gamma beam is completely absorbed by an atomic 

 structure, and the result is emission of planetar}^ electrons (photoelectric 

 emission). The scattering loss takes place as if the gamma radiation 

 were composed of photons or bundles of energy. 



Photons colliding with electrons in the filter material communicate 

 energy to the electrons, so that the electrons are set in motion. The 

 penetrating photon is deflected by the electron from its path (see Comp- 

 ton effect) after collision, and hence scattered. The scattered photon 

 in this act, having lost some of its energy by the collision, moves off with 

 less energy content. The number of photons emitted by radioactive 

 material used in radiotherapy that suffer no energy loss in this scattering 

 process is inappreciable. 



When gamma radiation of very short wavelength (hard gamma rays) 

 passes through low-atomic-weight filters composed of aluminum, water, 

 or tissue, the reduction in intensity of the beam is almost entirely due 

 to scattering. This explains the comparative loss in different kinds 

 of tissue as shown in Table II— 7. As the atomic number of the absorber 

 increases, the loss due to the photoelectric process becomes more pro- 

 nounced. 



TABLE II-7 

 Absorption of Gamma Rays by Various Kinds of Beef Tissue 



By Courtesy of G. Failla [1921]. 



