STOPPING POWER AND RANGE IN WATER VAPOR 



165 



shaw are indicated in the figure. The pecuHar shape of the curve 

 at low energies is anticipated, since the "Bragg"-t3'pe curves of water 

 and of air have their maxima at different energies. As the energy 

 of the particle increases indefinitely, the value of s must approach the 

 ratio of total number of electrons in water and in air, that is, 10/7.22 

 = 1.39. The curve exhibits a slow approach to this value. 



,, Mev 



12.0 



Fig. 1. Stopping power of water for protons and alpha particles, per molecule, rela- 

 tive to one "atom" of air. (Dotted curve is Sr = R^irN aii/ Rh2oNh20-) The circles 

 are experimental points of Crenshaw (29), whose data were used to establish the 

 low-energy region of the stopping-power curve. 



Figure 1 also presents values of the relative ranges in water and air, as 

 represented by Sr = Eair-^airZ-RHjO-^HaO- These are more uncertain, at 

 proton energies less than several Mev, than values of s. The error in 

 Sr is different for the two particles; thus the Sr curve should in fact 

 separate into individual curves for protons and alpha particles, and at a 

 greater energy than that at which the individual s curves separate. 



Figure 2 gives values of the range of protons and alpha particles in 

 water vapor, calculated for a density of 1 gm per cm^. In any applica- 

 tion the appropriate range from the figure must be divided by the den- 

 sity of the vapor. The range-energy relation was obtained by nu- 

 merical integration of the stopping-power data. For proton energies 

 greater than about 2 Mev it should be a trustworthy guide, within the 

 limitations set forth above, except for an unknown additive constant 

 which arises from the erroneous stopping powers at low energies. This 

 unknown additive constant is different for protons and alpha particles. 



