196 RADIATION BIOLOGY 



of first or higher order); and reactions between "free" atoms or radicals, 

 formed in earher stages, and normal molecules (usually proceeding 

 according to rate laws of second or higher order). Only the first class is 

 distinctive of photochemical and radiation-chemical reactions, and to it 

 attention in the following pages is chiefly confined. 



2-2. THE PRIMARY PROCESSES OF RADIATION CHEMISTRY 



The absorption of high-energy radiation by matter produces a great 

 number of excited and of ionized atoms or molecules in a variety of 

 different states. Some, or all — and in some instances almost none — of 

 these different states may be effective in inducing permanent chemical 

 change (if some change is possible in the system). Except for the 

 simplest media (hydrogen gas, for example), little has been accomplished 

 in analyzing the relative effectiveness of different excited or ionized 

 states. Information can often be obtained, however, on the roles of 

 "excitation" and "ionization" considered as separate classes of primary 

 processes. 



Because the individual energy of a photon or particle of high-energy 

 radiation is very great compared to binding energies of valence electrons 

 in atoms or molecules, or to binding energies of atoms in molecules or in 

 liquids or solids, other types of primary processes may occur which are 

 impossible in the absorption of light. For example, bombardment with 

 particles of atomic mass (such as protons or a particles) can eject atoms 

 from their positions in molecules or in solid structures and project them 

 forward with great kinetic energy: in other words, momentum may be 

 transferred from a bombarding particle to an atom as a whole, rather than 

 to just an electron (as in the case of ionization). Such processes (except 

 in the unique case of fast-neutron irradiation) are always much less 

 numerous than electronic excitations and ionizations. If the latter 

 should be chemically ineffective, however, ejection of atoms may be 

 responsible for the effects of the radiation. Thus in the case of heavy- 

 particle irradiation of certain solid substances (e.g., metals), ordinary 

 excitation or ionization is ineffective because any electronic displace- 

 ments are easily reversed ; the observed disordering of the solid structure 

 is believed to arise from momentum transfer to atoms (Seitz, 1949). 



It should be noted that still other primary processes, less probable than 

 ordinary excitation or ionization but involving a much more violent effect 

 on the atom, are possible with high-energy radiation. Thus, with any 

 type of high-energy radiation (but never with visible light or ultraviolet 

 radiation), a substantial fraction of the ionization acts consists of the 

 ejection, not of a valence electron, but of an inner electron instead. Such 

 an event is usually followed by an Auger process or a series of them, 

 resulting in the formation of a multiply charged ion at the original site 



