8 M. Levin — Absorption of the a-Rays from Polonium. 



Art. III. — On the Absorption of the a-Rays from Polonium ; 



by M. Levin. 



[Corninunicated by Professor E. Rutherford, F.R.S.] 



In their well known investigations on the absorption of 

 a-particles emitted by radium, Bragg* and Bragg and Kleemanf 

 have shown that the a-particles move in a rectilinear course, 

 spending their energy in ionization, until their velocity becomes 

 so small that they cease ionizing. They have further shown, that 

 the a-rays possess the power of ionizing a gas only within a 

 limited distance from the source and that this distance is very 

 sharply defined. The investigation of the a-ray products of 

 radium has shown that each of the four a-ray products emits 

 a-rays of the same range in air and the same initial velocity, 

 and that these ranges and velocities are different for the dif- 

 ferent products. This was confirmed by experiments of Ruth- 

 erford,;}: who has shown that the photographic and phosphor- 

 escent action of the a-rays from radium C ends abruptly at a 

 definite distance, and that the range found by these methods 

 agrees closely with the value determined by Bragg and Klee- 

 man by the electric method. The hypothesis of Bragg and 

 Kleeman that the a-particles from each product are initially 

 projected at exactly the same velocity, and that this velocity is 

 cut down by a definite amount by passing through different 

 thicknesses of matter, has been completely confirmed by 

 direct experiment by Rutherford. 



Since the a-particles emitted by a thick layer of radio-active 

 matter of one kind come from different depths, the particles 

 which escape into the air move at different velocities. For this rea- 

 son it is advisable to work with very thin films of radio-active 

 matter. As the active" deposits from radio-active emanations are 

 deposited on bodies in infinitely thin layers, they represent ideal 

 sources of rays for this kind of experiment. Thus the range of 

 radium C was measured by McClung§ by using as a source 

 of a-rays a wire which had been exposed to the radium emana- 

 tion, and in the same way Hahnjj has recently found the ranges 

 of thorium B and thorium C. Bragg pointed out in his 

 first paper that polonium, for the same reason, should be a 

 very convenient source of a-rays, as it can be obtained in a very 

 thin layer on a bismuth plate. 



The object of the present experiments was to determine 

 accurately the range in air of the ionization of the particles 

 from polonium. As the a-rays emitted by each a-ray product 

 have eacii a definite and distinct range in air, the range of the 



* Phil. Mag., Dec. 1904. 



+ Phil. Mag., Dec. 1904, Sept. 1905, Nov. 1905, Apr. 1906. 



% Phil. Mag., July 1905. § Phil. Mag., Jau. 1906. || Phil. Mag., 1906. 



