178 Applied Biophysics 



individual electron, so that a unidirectional beam becomes diffuse. 

 Unmodified scattering is not of great importance in our present 

 considerations. 



2. "Modified;' or Compton scattering. A quantum '^collides" 

 with an individual electron, projecting it in one direction while 

 itself rebounding in another (and related) direction, with a 

 reduced energy (and. therefore, longer wave length) depending 

 on the direction taken. The detailed theory of the fractions of 

 the energv of an incident beam of quanta imparted to the recoil- 

 ing electrons and scattered quanta and their angular distribution 

 has been given by Klein and Nishina, and is in very good agree- 

 ment with experiment. The phenomenon is only slightly affected 

 by the atomic number of the substance. 



3. Photoelectric absorption. A quantum is absorbed com- 

 pletely by the atom as a whole. Nearly all the energy (a very 

 small fraction is expended in atomic recoil) is expended in 

 extracting an electron from the atom and endowing it with 

 kinetic energy. The phenomenon is practically completely de- 

 scribed by theoretical and empirical relations. The fraction of 

 energy of the incident beam converted, reckoned per electron, 

 is approximately proportional to the cube of the atomic number, 

 i.e., the effect is much more pronounced in "heavy" than in 

 "light'' elements. Apart from certain well-understood discontinui- 

 ties, the energy conversion varies roughly as the cube of the 

 wave length of the radiation, i.e., it becomes less important for 

 higher quantum energies. 



4. Various nuclear effects. Production of electron and posi- 

 tron pairs and nuclear disintegrations becomes of importance 

 only for quanta of high energy. These effects are practically 

 negligible even for radium gamma rays. They vary with the 

 atomic number of the nucleus. 



These processes all contribute to a removal of quanta from 

 a beam ; the fraction of the energy removed is termed an absorp- 

 tion coefficient, and may be reckoned per electron, per unit mass, 

 or per unit volume of the material.* Some of the energy is 



* The absorption coefficients of any one atomic type are practically independent 

 of its state of chemical combination. 



