104 



tration of slow electrons. It is still very difficult but nevertheless it has to be 

 done. 



We need an answer to the question of the effect of chemical binding, either 

 primary or secondary, on the career of slow electrons. That probably is anoth- 

 er complication which may eventually be included in the treatments which have 

 been given by Dr. Platzman and Dr. Magee. The work of Thompson (1) is ex- 

 tremely interesting in that you find small effects of chemical binding with such 

 radiations as high-energy protons. However, I fail to see how these phenomena 

 can be related at present to the questions raised concerning slow particles. 



For the most part I think that we ought simply to review the situation as it 

 stands now with regard to the essential mechanism of radiation interaction. It 

 is agreed this involves mainly glancing collisions or gradual attrition. This re- 

 sults in dissipation of energy in small driblets corresponding very closely (with- 

 in a factor of 2) to the average binding energy of the electrons in the atom. The 

 end result is that in gases you get ion pairs corresponding to the dissipation of 

 energy in very small amounts by comparison with the initial beam energy, and, 

 in fact, the ion- pair event, except for the distribution, has no memory of what 

 caused it. It is just the result of a glancing collision and it is a matter of 

 chance where it occurs. The idea is that the only difference between various 

 radiations is in the distribution of these events and the hope is you can explain 

 differences in biological effects of various radiations by differences in distribu- 

 tion. 



In the gas we usually talk about partition between ionization and excitation. 

 This is certainly an erroneous assumption with regard to liquid, but it has been 

 made in most radiobiologists' discussions. It is customary to say that water 

 vapor will require approximately 30 ev per ion pair and if we go to a liquid a 

 good enough assumption is that the situation is as though vapor were com- 

 pressed a factor of 1,000. For most of us, that was a perfectly good assump- 

 tion until yesterday. It came as a revelation to me that there is some other way 

 of thinking about it and I suppose it did to most of the biologists here. And so I 

 think it should be emphasized again in this conference that probably the most 

 important thing that has happened is a clear presentation of considerations which 

 mark a tentative beginning for a study of radiation effects in liquids. 



There appear to be two alternative mechanisms at the moment. 



One approach implies that the electron leaving its parent ion, eventually 

 produces an H atom at a very great distance from the ionization site. Corollary 

 to this is the assertion that there is very little initial recombination. The oth- 

 er approach, which appears diametrically opposed, pictures the electron as 

 penetrating a very short distance, say 10 to 50 A, and then coulomb fields take 

 over and lead to a high probability of initial (or as Dr. Onsager called it yester- 

 day, "preferential" ) . This recombination gives a radical pair with very little 

 separation in space as contrasted with the result described above. 



In pure water the production of a single ion pair by itself is not going to give 

 hydrogen or hydrogen peroxide, and in order to get these you have to have three 

 or four radical pairs in close enough proximity so there can be a reaction. 

 However, if the radical pairs formed close to each other are most likely to be 

 dissipated producing H 2 and H^CK, I would think they would be less available 

 for radiobiologic effects. It would seem that radical pairs formed at some dis- 

 tance from each other would be most effective, on this basis. All biological 

 systems would correspond not to the situation in which we have pure water bu^ 

 rather scavenger present (protein, fat, carbohydrate, salts, etc.). 



