100 PRIMARY PROCESSES 



be found to differ in important respects. Especially if a complex mole- 

 cule should be exceptionally resistant to the effects of a small energy 

 transfer, or should have ability to recover from such effects, must al- 

 lowance for other possibilities be made! (That such behavior may not 

 be uncommon is suggested by both empirical evidence and theory.) 

 Great energy transfer to such a molecule arises, according to the point 

 of view commonly held, only from the accumulated effect of a number 

 of small energy transfers to individual atoms of the molecule considered 

 as isolated entities. The probability of great energy transfer would thus 

 be intimately related to the specific ionization at the appropriate point 

 along the path of the incident particle. In the case of densely ionizing 

 particles, such as alpha particles and especially heavier positive ions 

 like recoil atoms or fission fragments, rather great energy transfer to a 

 single large molecule may be achieved, and under favorable circum- 

 stances much greater permanent changes than would result from equiva- 

 lent dosages of less densely ionizing radiations can be anticipated. Of 

 course, the effects of specific ionization or, expressed somewhat more 

 significantly, of the spatial distribution of excited and ionized atoms 

 and molecules at the instant after the incident radiation has penetrated 

 the medium, and of the fluctuations in this distribution, are well appreci- 

 ated and are an important factor in the interpretation of ultimate chemi- 

 cal and biological effects of radiations, although they have not yet been 

 analyzed in any quantitative detail. 



However, the fact that the individual component of incident radiation 

 has energy that is orders of magnitude greater than the average energy 

 transfer per excitation or ionization act should always be borne in mind. 

 Relatively great energy transfer from a single particle to a single mole- 

 cule is certainly infrequent, but it is not impossible. The smallness of 

 the average energy transfer results from the particular nature of the 

 chief interaction between incident particle and molecule, and other types 

 of interaction, usually much less probable, could conceivably lead to 

 energy transfers of different magnitude. [One should not regard as 

 being in this category the great primary energy transfers which certainly 

 occur when a very fast secondary electron (delta ray) is produced by a 

 charged particle, or a photon of bremsstrahlung is radiated by a fast 

 electron, for this great quantity of energy is subsequently dissipated in 

 a number of secondary events to many molecules, the average energy 

 transfer per molecule still being in the 10-ev region.] It therefore seems 

 not without importance to examine possibilities of energy transfer other 

 than simple excitation and ionization, and several such are considered 

 below. The discussion is deliberately rather general, but is supplemented 

 by calculations for an illustrative case. The processes all have rather low 



