PRINCIPLES OF RADIOLOGICAL PHYSICS 91 



on the electron energy and also upon the atomic number of the material. 

 The following sections deal first with the various ciualitative aspects of 

 penetration and then give more specific data on the energy distribution. 



4-2a. Moderate Energy: Penetration Limited by Deflection. The relative 

 importance of deflection by elastic collisions against nuclei and of energy 

 losses by collision against atomic electrons is nearly independent of the 

 electron energy for electrons of energy up to approximately 1 Mev. This 

 is understandable since both processes result from impacts between 

 charged particles. 



On the other hand, the relative importance of deflections and collisions 

 varies in proportion to the atomic number of the material for the following 

 reason. The deflections depend mainly on the square of the nuclear 

 charge [Eq. (13), Sect. 2-2c], i.e., on the square of the atomic number, but 

 the energy loss depends mainly on the number of electrons [Eq. (17), 

 Sect. 2-4b], i.e., on the first power of the atomic number. The scattering 

 effects turn out to be more important than energy losses in opposing the 

 penetration, in all materials but those of lightest atomic number. The 

 two effects are of comparable importance in materials like carbon or 

 oxygen. By and large, the deflections effectively stop the initial penetra- 

 tion of an electron before the electron has lost all, or even most, of its 

 energy. 



Once the electron track has been turned off its initial direction, it con- 

 tinues to curve in various directions at random. Sudden kinks appear 

 here and there on the track where a large-angle scattering has taken 

 place. Since large-angle scattering arises mostly from elastic collisions 

 with atomic nuclei, its probability is given approximately by Eq. (13). 



A track may curve all the way back to the surface of the material 

 through which the electron had entered, in which case we speak of a 

 " backscattering " of the electron, or the changing curvature may lead to a 

 deeper penetration. The probability of backscattering depends on the 

 relative importance of scattering and energy dissipation and therefore 

 primarily on the atomic number. Figure l-55a gives data on the prob- 

 ability of backscattering. 



The curvature of the track and the rate of energy dissipation become greater 

 and greater as an electron loses speed (see Fig. 1-47) . The rate of energy dissipa- 

 tion varies along the track according to the Bragg curve of Fig. 1-52, except that it 

 starts at a much lower level than in the case of heavy particles ; therefore the rising 

 portion of the curve is greatly extended. 



Because of the increasing curvature, the eventual depth attained by an electron 

 depends primarily on the depth of initial penetration, i.e., on the distance covered 

 by the electron before being turned off its initial direction. This distance may 

 vary greatly from one electron to another owing to accidental summation or can- 

 cellation of successive minute deflections along each track and to the accidental 

 occurrence of sharp deflections. 



