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Influences of Details of Electronic Binding 



on Penetration Phenomena, and the 



Penetration of Energetic Charged Particles 



through Liquid Water 



ROBERT L. PLATZMAN 



Department of Physics 

 Purdue University 

 Lafayette, Indiana 



I. Introduction 



Liquid water is perhaps the most important inorganic chemical sub- 

 stance and is certainly the fundamental biological material. The inter- 

 action of high-energy radiations with water is thus a subject of the 

 greatest consequence for studies of radiation action. Yet almost 

 nothing is known directly about the phenomena attending passage of 

 swiftly moving charged atomic particles through liquid water. The im- 

 portance of this problem in radiobiology is emphasized by the reminder 

 that the phenomena referred to are chemically non-specific: the pene- 

 trating particle affects atoms and molecules encountered along its path 

 simply in approximate proportion to the populations of the various 

 species. Hence the 'primary effects in biological systems are intimately 

 related to the effects in pure water and, indeed, are often roughly iden- 

 tical with them. 



Although we have today a wide knowledge of, and deep insight into, 

 high-energy penetration phenomena for gases, particularly monatomic 

 gases, we know less about the corresponding phenomena for polyatomic 

 gases, very little indeed about solids, and almost nothing concerning 

 liquids. Admittedly, the quantitative differences in the phenomena for 

 the last three cases — when the material is compared with a hypothetical 

 mixture of monatomic gases of identical over-all chemical composition — 

 have up to the present time usually been comparable to or smaller than 

 the relevant experimental uncertainties or the accuracies demanded in 

 common applications. Indeed, it is to this lack of obvious practical im- 

 portance that neglect of the fundamental aspects of the question is 



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