140 PENETRATION PHENOMENA IN LIQUID WATER 



probably to be ascribed. In the case of liquid water, however, the con- 

 sequence in theoretical radiobiology and radiation chemistry of ap- 

 preciable manifestation of m^mmolecular binding and miermolecular 

 interaction is potentially so great that further disregard of the possi- 

 bilities can no longer be tolerated. 



Formidable experimental difficulties are responsible for the fact that 

 only very few experimental studies relevant to this problem have been 

 made. Thus-, neither the primary nor the total ionization produced in 

 liquid water by any type of radiation has ever been measured, no method 

 having yet been advanced by means of which the ionization can be 

 directly observed. The stopping power of liquid water for energetic 

 charged particles has been determined only once: the difficulty here lies 

 in preparing a section of water which is sufficiently thin (for the energy 

 region hitherto accessible). There exist also three measurements of the 

 range of natural alpha particles in water. The results are not in accord- 

 ance with one another, and it is sufficient at this point to note that they 

 do not much clarify the basic questions. Moreover, no theoretical treat- 

 ment of penetration phenomena for the specific case of liquid water has 

 heretofore been given. This inattention should be attributed to the 

 opinion that liquid water must behave toward high-energy radiation 

 very much like a gas, or to the barrier presented by lack of any de- 

 tailed knowledge of the electronic properties of liquid water. 



In the present paper we shall review the appropriate experimental 

 results, consider the theoretical problems involved, and also point out 

 directions which future investigation might pursue. It will prove grat- 

 ifying if the discussion should serve to emphasize our ignorance of many 

 of the fundamental aspects of the subject and help to stimulate their 

 study. 



The subject of high-energy radiation effects in water is, of course, a 

 great one, embracing a number of diverse phenomena. It is therefore 

 necessary to restrict the present discussion to a few aspects which are 

 of foremost practical importance, or which seem most likely to shed light 

 on the question of possible peculiarities of liquid water. Topics which 

 merit particular attention are the stopping power and range of swiftly 

 moving charged particles, effects of electrons and positive ions of inter- 

 mediate and low velocity, and the mean over-all efficiency of ionization. 



II. Review of Experimental Information 



A. MICHL (1914) (1) 



In this first measurement on liquid water a thin platinum wire covered 

 with a very thin layer of polonium rested on a photographic plate, the 



