94 BIOLOGICAL EFFECTS OF RADIATION 



practically no energy from the photon. On the other hand, it should be 

 remembered that the electron which is emitted initially in the general 

 direction of the photon path will deviate from its course by impacts with 

 atoms and may then travel in the opposite direction. The energy of the 

 photon which is not transmitted to the electron by the Compton-effect 

 process appears as a photon of lower energy (longer wave-length) emitted 

 in a direction which is compatible with the laws of elastic impact. As 

 in the case of the photoelectric effect, the energy of the secondary photon 

 is not immediately available to produce ions. But these photons, of 

 course, are identical with primary photons, although their energy is less, 

 and therefore are capable of undergoing exactly the same transformations 

 that we have just discussed (photoelectric effect and Compton effect). 

 When these occur, they will produce beta particles which further contrib- 

 ute to the ionization of the medium. The transformation of photons into 

 high-speed electrons and other photons, goes on indefinitely until prac- 

 tically all the energy of the original photons has been transferred to 

 electrons through repeated steps of this process. Mention might be made 

 here of the converse process, whereby secondary electrons traveling at 

 high speeds may be stopped suddenly, in which case their kinetic energy 

 appears in the resultant photon. (This is how X-rays are produced in the 

 first place, except that the electrons are accelerated artificially in the 

 X-ray tube.) This is not a common occurrence in organic materials, but 

 in any case it simply retards the inevitable disappearance of all photons 

 impinging on matter through the transformation of their energy into 

 ionization, 



INFLUENCE OF THE QUALITY OF RADIATION ON THE DISTRIBUTION OF 



IONS IN LIVING MATTER 



We may now consider the influence of the quality of radiation on the 

 distribution of ions in living matter, on the basis of the processes just 

 described. The quality (wave-length range and energy distribution 

 in this range) varies with the voltage applied to the X-ray tube.^ But 

 in a beam of X-rays produced at a given peak voltage there cannot be 

 wave-lengths present which are shorter than a theoretical minimum, 

 given by the expression 



_ 12,337 

 peak volts 



The upper limit is rather indefinite, but may be taken as that wave-length 

 which is capable of traversing the walls of the X-ray tube. Radiation 

 of the minimum wave-length is actually present in negligible proportion. 



' The influence of filtration on the quality of radiation will be discussed later. 

 See also paper by L. S. Taylor for other factors which affect quality. 



