142 PENETRATION PHENOMENA IN LIQUID WATER 



This is the only determination yet made of the true stopping power of 

 Hqiiid water; the other measurements (1, 3, 6) cited all yield ranges. 

 Appleyard found the stopping power corresponding to an alpha-particle 

 energy of roughly 4.5 Mev to be 1.71 ± 0.05 (per molecule of H2O rel- 

 ative to one "atom" of air). (The uncertainty in energy is unimportant, 

 since the relative stopping power is known to be rather insensitive to 

 energy variations in this energy region; cf. Fig. 1, p. 165.) This value is 

 15 per cent higher than the theoretical one for water vapor (cf. Table 1). 



D. DE CARVALHO AND YAGODA (1950) (6, 7) 



In this measurement, the photographic emulsion technique was again 

 employed. The source of alpha rays was a tiny particle ("radiocolloid") 

 of polonium or radium sulfate; many such particles were sprinkled over 

 the surface of the emulsion, which was then immersed in water. The 

 procedure was in a sense a modern refinement of Michl's method. Those 

 alpha particles emitted by a radiocolloid particle almost tangentially to 

 the emulsion struck the latter when they were very close to the ends of 

 their ranges and hence delineated the range in water. 



The experiments yielded values of the ranges of Po and of RaC 

 alpha particles in vapor, liquid, and solid water which, when reduced to 

 the same density, agreed closely with each other (to within 1 per cent) 

 and with the value calculated for the vapor (cf. Table 1). The results 

 are thus in utter disagreement with those of the three other investiga- 

 tions. 



It might be noted that de Carvalho (6) states that his emulsions are 

 sensitive only to alpha-particle energies greater than 0.2 Mev. This 

 should make his ranges too short in the liquid, but this difference is not 

 great and he is able to correct for it. [Similarly, the scintillation method, 

 employed by Philipp, has a visual threshold of between 0.13 and 0.17 

 Mev (8).] 



E. DISCUSSION OF THE EXPERIMENTAL INFORMATION 



The experimental results are summarized in Table 1. The semitheo- 

 retical values for water vapor listed there are taken from a new calcula- 

 tion, the results of which are presented in Figs. 1 and 2 (pp. 165 and 

 166); the basis for this calculation will be discussed in Section VI below. 

 The quantity Sr should not be confused with s. Whereas s is the true 

 stopping power, relative to that of air at the same particle energy and 

 for the same density of atoms, 1/Sr is the average reciprocal stopping 

 power (the average being over all values of particle energy as the par- 

 ticle is progressively slowed down) relative to the corresponding average 

 in air for a particle of the same initial energy. More simply, 1/Sr is 



