IONISATION CAUSED BY PENETRATING y RAYS. 139 



velocity, from an average value which will be inversely 

 proportional to the average loss of energy per collision, and 

 to the density of the material. If the range (d) of the P 

 ray in a given material is defined, in terms of the mass of 

 metal traversed, as the mass of a cylinder whose axis is 

 the straightened path of the ray divided by its cross section, 

 a quantity is obtained which is independent of the density 

 (p) of the material. 



If the y rays have the same intensity everywhere in a 

 block of any material, the sum of all the tracks completed 

 in a unit volume per second will be proportional to the 

 number of /? rays formed in that volume (Ikp) and to the 

 distance the fi ray travels (d/p) and is quite defined by the 

 product Ikd. This quantity depends only on the nature of 

 the material, not on its homogeneity or density. 



Any cavity in the substance does not alter the value of 

 Ikd anywhere within the boundaries of the material includ- 

 ing the cavity itself, since every fi ray must cross a mass 

 d of the substance and crossing the cavity does not count 

 in this. If air is now introduced into the cavity the value 

 of fed in the cavity will not be altered unless the energy of 

 the P rays is appreciably absorbed in crossing the chamber, 

 and the value of kd for air is much different from that for 

 the material surrounding it. 



The ionisation produced in the cavity per unit volume 

 will be proportional to the sum of all the paths of the /? 

 rays completed within unit volume. This sum has just 

 been shown to be proportional to Ikd; if, therefore, an 

 ionisation chamber is prepared with walls thick enough to 

 prevent any /3 rays formed on the inside from penetrating 

 to the outside, and with an air chamber so small that no 

 appreciable amount of ft ray energy is absorbed in crossing 

 the chamber at the pressure of gas employed, then the 

 ionisation will give a measure of Ikd for the substance of 

 which the chamber is composed. 



