SUMMARY 173 



5. Study of the ionization in liquid water. It would be extremely help- 

 ful to have some direct experimental information, especially on slow 

 electrons, but work with all high-energy radiations would be very im- 

 portant. A theoretical investigation is also demanded. 



6. Study of the stopping power of an ionic crystal. (Cf. Section V.) 



Acknowledgments 



The writer wishes to express his appreciation to Drs. R. K. Appleyard, 

 H. de Carvalho, and H. Yagoda for most generously furnishing de- 

 tailed information regarding their respective experimental investiga- 

 tions in advance of publication. He is also deeply grateful to Professor 

 James Franck for the privilege of many invaluable discussions. 



Summary 



The few and discordant experimental studies of the penetration of 

 natural alpha particles into liquid water are reviewed critically. As a 

 necessary background for their possible interpretation, the Bragg rule, 

 which claims additivity of atomic stopping powers for all media, is 

 analyzed in the light of available experimental data and also in the light 

 of theory. The analysis discloses a variety of unsolved problems con- 

 cerning penetration processes in complex systems. A purely theoretical 

 approach to an important and tractable instance in which to study the 

 Bragg rule — the stopping power of molecular hydrogen as compared to 

 that of atomic hydrogen — is discussed qualitatively. A new calculation 

 of stopping-power and range data for swiftly moving charged particles 

 penetrating water, presuming only a small departure from the Bragg 

 rule, is presented, and the assumptions underlying the calculation eval- 

 uated. Possible causes for failure of the Bragg rule in the case of liquid 

 water are examined, and it is concluded that theory predicts an ab- 

 normally great stopping power, in the same sense as has been several 

 times observed; however, the simple theoretical considerations here 

 given do not permit an estimate of the magnitude of this effect, so that 

 the reality of the reported results cannot yet be decided on a theoretical 

 basis. Some possible consequences of the effects of liquid state on inter- 

 pretations of radiation effects in water and aqueous systems are sug- 

 gested, and, finally, some promising lines for future investigation pro- 

 posed. 



