236 RADIATION BIOLOGY 



particularly unfortunate in view of the basic role which ionized species 

 play in radiation-chemical reactions, and it is to be hoped that the 

 photochemical studies which would provide such an important background 

 for radiation chemistry and radiobiology may be undertaken in the 

 future. It may be mentioned, however, that whereas this is quite 

 feasible for gaseous reactions, condensed systems would be even more 

 difficult to study because, as a result of the very great absorption 

 coefficients for the ionization continua, the primary processes would occur 

 predominantly in surface regions. 



4-2. SURVEY OF IONIZATION PHENOMENA PRODUCED 

 BY HIGH-ENERGY RADIATIONS 



A detailed theoretical analysis of the mechanism of a reaction caused by 

 absorption of high-energy radiation would reciuire information, not only 

 on the numbers, but also on the different species of excited and ionized 

 atoms and molecules produced (cf. Sect. 1-3), and, for treatment of the 

 kinetics of ensuing elementary processes, on their spatial distributions. 

 This information is, in general, simply not available, and constitutes an 

 important goal for future work. 



For very simple atoms or molecules it may be taken as a fairly adequate 

 first approximation that most of the excitation acts bring the system to 

 its lowest electronic excited state, and most of the ionization acts similarly 

 produce ions in their ground state. Thus, Eyring, Hirschfelder, and 

 Taylor (1936) were able to formulate a satisfactory theory for certain 

 radiation-chemical reactions in hydrogen gas by considering as primary 

 products only the »S+ excited state of H2 (cf. Fig. 3-3) and the ^S, state of 

 H^, and analyzing the various secondary reactions which these entities 

 can undergo. In more complex systems such simplification is not 

 possible, and a "catalogue" of the various species most abundantly pro- 

 duced is required. Dainton (1948) has made an initial approach to this 

 problem for the important case of H2O. 



The individual acts whereby impact of a swiftly moving charged 

 particle excites or ionizes an atom or molecule are on the whole very well 

 understood. Thus, there exist abundant experimental data concerning 

 excitation and ionization produced by impact of electrons of low and 

 moderate velocities on atoms and both simple and complex molecules, 

 and there are highly developed theoretical methods for interpreting these 

 data and calculating unmeasured cases (Massey and Burhop, 1952). 

 Similar processes in which swiftly moving charged particles (such as a 

 particles or fast electrons) provide the energy have not been measured 

 directly but are amenable to theoretical treatment. In the case of high- 

 energy radiations, however, much of the effect is secondary, tertiary, etc. ; 

 thus, about one-half to two-thirds of the total ionization produced by a or 

 13 particles is caused by secondary, tertiary, etc., electrons which were 



