DISCUSSION 115 



cross section (which is vahd for such coUisions) and the total stopping power of 

 water (from Fig. 1). Note, however, that this gives only the number of violent 

 primary coUisions; the total number of atoms ejected will be greater — often as 

 much as twice as great — because some of the recoils eject other atoms. The 

 figure shows clearly that at low proton energy (for water, below 1 Mev) most of 

 the ejections occur at the end of the range, whereas at high energy most of them 

 are distributed along the range, in approximately constant proportion to the 

 total (that is, electronic) energy loss. 



REFERENCES (FOR APPENDIX) 



1. Bohr, N., Kgl. Danske Videnskah. Selskah, Mat.-fys. Medd., 18: No. 8, 1948. 



2. Brown, L. M., Phys. Rev., 79: 297, 1950. 



3. Crenshaw, C. M., Phys. Rev., 62: 54, 1942. 



4. Henneberg, W., Z. Phy.uk, 86: 592, 1933. 



5. Hirschfelder, J. O., and J. L. Magee, Phys. Rev., 73: 207, 1948. 



6. Livingston, M. S., and H. A. Bethe, Revs. Modern Phys., 9: 245, 1937. 



7. Platzman, R. L., "Influences of Details of Electronic Binding on Penetration 

 Phenomena, and the Penetration of Energetic Charged Particles through Liquid 

 Water," Paper No. 9, p. 139 of this volume. 



DISCUSSION 

 Fano: 



I sympathize very much with the general idea of emphasizing the limitations 

 to the help that physics can give in these matters. At the same time I wonder 

 whether Platzman's remarks might not cause undue concern in the opposite 

 direction. On the whole, it would seem that physical theory does provide a 

 reliable guide on how to analyze most of the practical questions relating to the 

 physical action of radiation. True enough, the theory does not yield quanti- 

 tative predictions as accurate as one might wish, but this lack of accuracy does 

 not seem to me to be too critical at the present time. 



Platzman : 



I quite agree with Fano that radiation physics has contributed immensely by 

 making possible the treatment of most of what he terms the "practical questions" 

 of radiation action. There was no intention of depreciating this contribution. 

 Rather, I have sought to stress how radiation physics has been inadequately 

 developed, thus far, as an aid in understanding the fundamental chemical 

 mechanisms of radiation effects. 



Magee: 



I should like to say a word about the chemical effect following the Auger 

 process. It is my opinion that to consider the chemical effect as the result of 

 electrostatic repulsion between the two charges is viewing the situation too 

 simply. Considerable work has been done at Notre Dame on the isomeric tran- 

 sitions in hydrogen bromide and deuterium bromide. Theory indicates that the 



