/• 



On the Primary Processes 

 in Radiation Chemistry and Biology 



ROBERT L. PLATZMAN 



Department of Physics 

 Purdue University 

 Lafayette, Indiana 



Introduction 



The first stage in the interpretation of chemical or biological changes 

 which result from the exposure of any material to radiation of high en- 

 ergy must be the elucidation of the primary physical effects of the radi- 

 ation. 



Radiation chemistry and radiobiology bear a close relationship to 

 photochemistry and "photobiology." In both, radiation from an exter- 

 nal source (less commonly, from an internal one) is allowed to impinge 

 upon an aggregate of molecules in stable, or almost stable, states, 

 thereby raising some of the molecules to excited states and permitting 

 chemical interactions which could not proceed between normal mole- 

 cules. The incident energy functions as activation energy, or as requisite 

 energy for endothermal processes, or both, and is ultimately partitioned 

 between degraded energy (heat), altered chemical (potential) energy of 

 the material, and occasionally also emitted radiation. 



The primary processes of "high-energy radiation" effects, however, dif- 

 fer from those of photoeffects in their tremendously greater complexity. 

 This distinction is incisive and affects even the simplest reactions in- 

 duced by high-energy radiations. Its origin lies in the fact that ea;Ch 

 particle of high-energy radiation has energy of from thousands to many 

 millions of electron volts (ev) * — magnitudes very much greater than the 

 energies of the more probable transitions of a molecule, which are usu- 

 ally less than 15 ev. On the other hand, the energy units absorbed in 



* This discussion is for the most part restricted to radiations of energies not 

 greatly in excess of those of common nuclear radiations, for which (with the excep- 

 tion of neutrons) specific nuclear interactions are so rare as to have negligible in- 

 fluence on chemical or biological effects. The problems are therefore ones of atomic 

 and molecular physics and of chemistry, and not at all of nuclear physics. 



97 



