102 BIOLOGICAL EFFECTS OF RADIATION 



jected most copiously in the forward direction and their energy content 

 is also greatest in this direction. Hence, the radiation which leaves the 

 material body^^ is hardest and most intense in the forward direction and 

 gradually becomes softer and less intense as the angle between the axis 

 of the primary beam and the direction of emission of the secondary 

 radiation increases to 180 deg. 



D. Since the energy carried by all the secondary radiation is derived 

 from the primary beam, and since by the very process which produces it, 

 some energy is transferred to electrons, the energy of the secondary rays 

 is always much less than that of the primary. A fortiori, in any one 

 direction the intensity of the secondary radiation is much lower than that 

 of the primary beam at the level of the material object. This is generally 

 true whether the radiation is hard or soft. For, if the radiation is hard, 

 it has a high penetrating power and it undergoes few photoelectric and 

 Compton transformations by which alone it can transmit energy to 

 secondary rays. If it is soft, it undergoes many transformations in its 

 passage through matter, but the secondary radiation is still softer, and a 

 large proportion is unable to leave the material body.^- 



E. Mention has been made of the fact that some secondary radiation 

 leaves the body in a direction opposite (or nearly so) to that of the 

 primary beam. This constitutes what is usually called the "back- 

 scattered radiation," or simply (and improperly) the "back scatter." It 

 plays an important part in many practical cases, and should always be 

 taken into account in biological work. Considering the process of its 

 formation described in the preceding paragraphs, the following qualitative 

 conclusions may be reached: The contribution of the (characteristic) 

 photons liberated by the photoelectric effect to the back-scattered 

 radiation outside the body is generally negligible. This radiation is 

 extremely soft and can traverse only a small thickness of air. The 

 photons derived from Compton encounters, which constitute the main 

 part of the back-scattered radiation, have all less energy than the primary 

 photons. In addition, they differ in energy content among themselves 

 because they do not all leave the surface (they are not all emitted) in the 

 same direction. Hence, this radiation is very heterogeneous, even when 

 the primary beam is monochromatic. The relative intensity of the back- 

 scattered radiation varies with the wave-length (photon energy) of the 

 primary rays. For soft radiation it is small, since in this case the 

 photoelectric effect predominates. For hard radiation it is an appreciable 



" Obviously the size of the body and the quahty of the primary radiation must 

 also be taken into account in any actual case. 



12 Also, in the case of soft radiation the photoelectric effect predominates and 

 therefore most of the energy is transferred directly to electrons. 



