546 W. R. Barss — Measurements of Radio-activity. 



Art. XLV. — Note on Measurements of Radio-activity by 

 means of Alpha Rays ; by W. R. Barss. 



It is a well known fact that in a gas ionized by a-particles 

 a saturation current is obtained only when a much larger poten- 

 tial gradient is applied between the plates of the ionization 

 chamber than is necessary when j3- or X-rays are the ionizing 

 agents. Bragg and Kleeman* showed that a current through a 

 gas, ionized by a-particles, was still unsaturated when calculation 

 showed that the number of ions lost by general recombination 

 was small. The effect was ascribed to "Initial Recombina- 

 tion " ; i.e.. to some of the ions being but partially separated 

 from their parent molecules by the action of the a-particles. 

 In the absence of an external electric field these ions fall back 

 on their parent molecules and are thus neutralized. An intense 

 electric field is supposed to complete the separation of the ions 

 and to produce saturation. On this hypothesis, lack of satura- 

 tion would not depend on the size or shape of the ionization 

 vessel and saturation would be more easily obtained under 

 diminished pressure. 



Kleemanf has shown that lack of saturation with weak ioni- 

 zation by a-particles is not due to diffusion of the ions, nor 

 does it depend on the recombination coefficient. He has 

 shown that " Initial Recombination " is very small in gases 

 ionized by /3-, 7- and X-rays ; in other words, these ionizing 

 agents effect a more complete separation of negative ions from 

 their parent molecules. 



Moulin^ has proposed an explanation of the mechanism of 

 ionization by a-particles as follows. The ions formed by the 

 a-particles are not distributed uniformly throughout the gas, 

 but each a-particle has, associated with it, a column of ions, the 

 axis of the column being along the path of the particle. Lack 

 of saturation is explained by recombination of ions of opposite 

 sign within each column. This recombination between ions of 

 the same column ought to exceed that which would be obtained 

 for the same number of ions distributed throughout the volume 

 of the gas. The amount of the recombination between ions of 

 the same column should be much greater when the field is 

 applied in a direction parallel to the direction of the column, 

 than when it is applied in a direction perpendicular to it ; for 

 the parallel field would leave the. columns intact, while the 

 perpendicular field would break each column into two parts by 



* Phil. Mag., xi, p. 466, 1906. 

 fPhil. Mag., xii, p. 273, 1906. 

 % Le Radium, May, 1908, p. 136. 



