328 Messrs. Moseley and Robinson on the Number of 



The number of ions produced by the a particles from a 

 gram of radium has been accurately measured by Geiger *. 

 Eve f has devised an elegant method for finding the ioniza- 

 tion by the j3 and 7 radiations. The ionization at different 

 distances in the air surrounding a known quantity of active 

 material is sampled inside a small chamber, with walls and 

 framework so light that they do not affect the radiation. 

 One gram of radium then produces in all, 



N = I ±7rr 2 ndr (1) 



pairs of ions per second ; where n, the ionization per cc, 

 is a function of r. In the case of the /3 radiation the value 

 of this function can be found experimentally for all distances 

 for which n remains appreciable. The effect of the 7 radia- 

 tion is spread over far too large a volume for this to be 

 possible. 



Fortunately the penetrating part of the 7 radiation, which 

 carries very much more energy than do the softer types, is 

 absorbed according to an exponential law, and so in this 

 case 



« = 5'-* r (2) 



and 



N 



= 1 47rKe-^r=^=: . . . . (3) 

 Jo A* 



If the value of /x, the absorption coefficient in air, is 

 already known, N can be found by using the value of the 

 the constant K given by a single measurement of n. Eve 

 has used equation (3) to calculate the ionization produced 

 both by the /? and the 7 radiations. Since, however, the 

 absorption of the ft radiation does not even approximate to 

 an exponential law, great uncertainty attaches in this ease 

 to his result. Our chief object in repeating Eve's work was 

 to measure the /3 ray ionization directly by means of equa- 

 tion (1), and so to include the effect of the softer types of 

 radiation, which in many of Eve's experiments were stopped 

 by the walls of the tube containing the active matter. 



In the case of the 7 radiation, where these objections do 

 not apply, there is a serious discrepancy, which will be 

 discus. Hater, between our results and those of Eve. 



Our scarce of radiation was from 1 to 50 millicuries of 



* Geiger, Proc. Roy. Soc. A. lxxxii. p. 486 (1909). 



t Eve, Phil. Mag. xxii. p. 551 (1911) ; xxvii. p. 394 (1914). 



