PRINCIPLES OF RADIOLOGICAL PHYSICS 65 



of "chemical" size through further glancing collisions by secondary 

 particles. 



According to the discussion in Sects. 2-4b and c, the atomic electrons 

 "feel" the glancing passage of a fast charged particle as a quick pulse of 

 electric force. The action of this pulse is equivalent to the action of a 

 complex electromagnetic radiation whose intensity is distributed evenly 

 among all frequencies. 



The external electrons are thus in a position to absorb just those 

 amounts of energy which correspond to their own characteristic fre- 

 quencies of oscillation. There results no preferential or selective absorp- 

 tion of energy by the electrons of any particular group of atoms. In 

 other words, ionizing radiations exert through glancing collisions an 

 entirely nonspecific action. 



Since atomic electrons are not very apt to absorb photons of energy 

 greatly in excess of their own binding energy (see Sect. 2-3), the energy 

 absorbed by external electrons through glancing collisions is generally of 

 chemical or nearly chemical scale. Finally since the glancing passage of 

 a charged particle acts only as a quick electric pulse, the relative fre- 

 quency of different energy absorptions in any particular atom does not 

 depend upon the charge, mass, and velocity of the incident particle (see 

 Sect. 2-4c). These quantities affect equally the probability of each 

 absorption. Here is a further reason that all kinds of ionizing radiations 

 act in the same way, if not to the same extent. 



3-lb. Ionization Yield. Ionization constitutes a particularly drastic 

 form of chemical activation. When an electron is ejected from an atom, 

 the resulting separation of electric charges lasts for a much longer time 

 than the minor dislocations of atomic electrons which accompany simple 

 excitations. It is uncertain whether the somewhat larger energy involved 

 in the ionization processes than in excitations and the greater permanency 

 of charge separation have a particularly great significance with regard to 

 biological effectiveness. 



The separation of charges which results from ionization processes in 

 gases affords a convenient and sensitive method for the physical measure- 

 ment of radiation effects. It is frequently assumed, on somewhat uncer- 

 tain grounds, that a radiation produces essentially equal amounts of 

 ionization in a given amount of matter whether the matter is in gaseous, 

 liquid, or solid state. Present information on the subject of ionization 

 concerns mostly the occurrence of this phenomenon in gases. 



The ratio e of the energy dissipated in a gas to the number of ionizations 

 produced is approximately equal for different ionizing radiations, since 

 most ionizations originate from a single mechanism, namely charged 

 particle collisions affecting the external electrons. Therefore the number 

 of ionizations produced in a gas frecjuently serves as a measure of the 

 energy dissipated within it. Table 1-4 shows the numerical values of the 



