172 PENETRATION PHENOMENA IN LIQUID WATER 



dependent on the molecular constitution; it exerts its influence on that 

 portion of the ionization caused by slow secondary electrons. 



The paucity of information in this energy domain is in many respects 

 a result of the greater experimental difficulties which investigations with 

 slower particles entail, and in a practical sense reflects also the lesser 

 importance of this energy region in experiments of nuclear physics. 

 However, these phenomena are of extreme importance in radiobiology, 

 for many important studies involve intimately the effects of slow par- 

 ticles (for instance, secondary electrons from any high-energy radiation, 

 and recoils from neutron irradiation), and almost all studies involve 

 them to some extent. 



X. Suggestions for Further Study 



An important purpose of this paper is the examination of lacunae in 

 the contemporary understanding of the fundamental physical processes 

 underlying the interpretation of radiation effects on chemical and 

 biological systems involving liquid water. In this section there are ac- 

 cordingly collected specific suggestions for theoretical or experimental 

 investigations bearing, directly or indirectly, on the interactions of 

 ionizing radiations with liquid water. Most of them have been analyzed 

 in more or less detail in the paragraphs above. 



1. Measurement of the stopping power of liquid water for extremely fast 

 protons or alpha particles, using one of the new high-energy accelerators. 

 This is a relatively simple experiment and should lead to an accurate 

 value for I-r^qO). 



2. Further experimental study of the range of natural alpha particles in 

 liquid water, and also of the stopping power of water. Such investigation 

 is important, not only to clarify the bases of previous experimental errors, 

 but also because theory, even after /h2o(1) is known, cannot provide 

 the needed stopping-power data in the moderate- and low-energy re- 

 gions. It is likely that Am^"^^ would provide a better source than Po, for 

 practical reasons. 



3. Study of the electronic energy levels of liquid water, using any or all 

 relevant methods; for example, the far-ultraviolet and soft-x-ray spectra, 

 photochemical and radiation-chemical effects, and theory. 



4. Refined investigation of the deviations from the Bragg rule. For the 

 simplest molecule, H2, both theoretical and experimental work are re- 

 quired. Studies on more complex molecules, and most particularly on 

 water vapor, would also be valuable. An accurate experimental value of 

 Inioig) is, of course, necessary for the theoretical interpretation of 

 /h,o(1). 



