72 Ve fines — Retardation of Alpha Particles by Metals. 



the air equivalent drops off quite rapidly near the end of the 

 ranpje. The same results were also obtained when the 23oint, 

 discharge method was used. 



In order that the range may be accurately determined by 

 the discharge point method, it is essential that the point be 

 sensitive enough to respond to the small ionization produced in 

 the chamber when the range of the alpha particle ends imme- 

 diately after entering the chamber. In determining the end 

 of the range by this method, the distance was measured from 

 the lower end of the chamber. 



The results obtained are shown by the curves plotted in 

 £gure 2, the emergent range being plotted as the abscissa, and 

 the air equivalent of the foil as the ordinate. The points 

 obtained by the scintillation method are given by circles, while 

 those obtained by the point discharge method are given by 

 crosses. As shown by the curves, the change in air equivalent 

 near the end of the range is more pronounced in the case of 

 the thicker foils and especially in the case of those of higher 

 atomic weight. 



It is quite evident that in any investigation where the range 

 of alpha particles must be determined, the point discharge 

 method is as equally w^ell adapted as the scintillation method. 

 In observing scintillations, there is always a certain amount of 

 strain on the eye, and besides a considerable length of time is 

 required for getting the eye adapted to darkness. This disad- 

 vantage is altogether eliminated in the point discharge method, 

 for the deflection of an electrometer fiber indicating a discharge 

 is very definite and can be seen without the aid of a dark room. 



In conclusion the writer wishes to state that this work has 

 been carried out under the direction of Professor Alois F. 

 Kovarik, and much credit is due him for the success which was 

 attained. The work was done at the Physical Laboratory of 

 the University of Minnesota. 



