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XXXII. The Ionization Ranges of the cc Rays of Actinium. 

 By 0. Hahn, Ph.D* 



IX previous papers f I gave an account of the methods 

 employed to determine the ranges in air of the ionization 

 due to the various a. ray products of thorium- This paper 

 contains a similar investigation for the ranges of the a rays 

 emitted by the products of actinium. Apart from the physical 

 importance of accurately determining the range in air of the 

 ol rays from each of the products, I have found these investiga- 

 tions of great value in throwing light on the successive changes 

 which occur in the new product radioactinium J. 



In this paper, therefore, an account will be given of the 

 determination of the ranges in air of the a. particles from the 

 following products : — 



(1) Active Deposit of Actinium. 



(2) Actinium X. 



(3) Radioactinum. 



(4) Actinium emanation. 



The preparations used for the experiments were : — 



(1) Actinium (Emanium), activity about 300, prepared 

 by Giesel, provided by Rutherford. 



(2) Actinium, activity about 700, prepared by Debierne, 

 kindly lent to me by Sir William Ramsay. 



The apparatus employed was the same as previously used 

 for thorium, and similar to that employed by Bragg and 

 Kleeman in their investigations of the ranges of the a rays 

 from the radium products. It is, therefore, not necessary to 

 give again the details of its construction and the method 

 of use. 



As I pointed out in my last paper, the chief difficulty in 

 the investigations on thorium arose from the strong emanating 

 power of all the preparations containing thorium X. In 

 order to avoid, as far as possible, the disturbances due to the 

 emanation, the active preparation was placed inside a funnel, 

 and a constant air-current passed through this funnel in 

 order to carry away the emanation. This arrangement had 

 worked satisfactorily at some distance of the active prepara- 

 tion from the ionization-chamber, but for small distances 

 disturbances were not to be avoided. In some cases, a thin 

 mica screen completely enclosed the radioactive preparation in 

 order to prevent the escape of the emanation. 



* Communicated by Prof. E. Rutherford, F.R.S. 

 t Phil. Mag. June & July 1906. 

 X O. Halm, < Nature/ April 1906. 



