REVIEW OF EXPERIMENTAL INFORMATION 



143 



the range of the alpha particle in water divided by its range in air, cor- 

 rected for the difference in atomic densities. Thus, although Sr is more 

 directly a relative range, it is often called the average or mean relative 

 stopping power and sometimes even the relative stopping power. How- 

 ever, Sr and s would be equal only if s were independent of particle 



TABLE 1 



Summary of Experimental Information on Range and Stopping Power: 

 Alpha Particles in Liquid Water 



Definitions: 



s = stopping power, per molecule of water, relative to that of one "atom" of 



air 

 R = range in water of density 1 gm per cm^, in microns 



Sr = reciprocal of range in water, relative to range in air, reduced to equal 

 numbers of molecules (H2O) and "atoms" (air) per unit volume 

 = 1.525. 10-3i2^i,/i2H2O 



Note: for explanation of uncertainties in calculated values of R and Sr indicated 

 by brackets, cf. Section VI. 



energy. This is occasionally approximately but never exactly the case 

 (cf. Fig. 1). The quantities Sr and s are often confused in the literature. 

 It is a striking fact that both the older measurements of Sr by Michl 

 and by Philipp and the later one of s by Appleyard give values approx- 

 imately 15-20 per cent higher for the liquid than those calculated theo- 

 retically for the vapor. The approximate agreement between these 



