Ven7ies — Retardation of Alpha Particles hy Metals. 69 



Art. yill. — The Retardation of Alpha Particles hy Metals; 

 by H. J. Yennes. 



The retardation of alpha particles by metals has been inves- 

 tigated by several experimenters, and among the most recent 

 is a series of experiments carried out by Marsden and Rich- 

 ardson."^ They found that the amount by which a metal foil 

 reduces the range of alpha particles depends on the part of 

 the range in which the foil is placed. The air equivalent in 

 the foil was shown to be considerably greater when placed 

 directly over the source than when placed near the end of the 

 range. 



In an earlier experiment, Taylorf showed that when a layer 

 of hydrogen was used in place of a metal foil, the ionization 

 near the end of the range of alpha particles was greater when 

 the layer of hydrogen was placed directly over the source than 

 when placed near the end of the range. When a metal foil 

 was used, ionization was greatest when the foil was placed 

 near the end of the range. In the experiments carried out by 

 Marsden and Richardson, the scintillation method was used 

 for determining the end of the range. Taylor, in his experi- 

 ments, did not actually determine the range of the alpha parti- 

 cles after passing through the foils and layers of hydrogen, but 

 made experiments on the relative amounts of ionization when 

 they were placed in different parts of the range. 



The purpose of the experiments carried out by the writer 

 was primarily to test the point discharge method in determin- 

 ing the range of alpha particles after passing through matter. 

 TJiis was done by carrying out experiments, firstly, using the 

 scintillation method, the procedure being almost identical to 

 that followed by Marsden and Richardson, and secondly, using 

 the point discharge method of counting the alpha particles in 

 ' place of the scintillation method. 



The apparatus used in connection with this experiment is 

 shown in the accompanying diagram, and consists of an ordi- 

 nary microscope from which the stage has been removed. The 

 zinc sulphide screen is held in place by means of a brass frame, 

 the upper part of which is clamped to the objective of the 

 microscope. The metal foils and source of alpha particles are 

 held in position by two small arms which can be clamped in 

 any position on the vertical rod fastened to the base of the 

 instrument. The point discharge chamber is constructed as 

 shown in the diagram at " B," and is provided w^ith an arm 



* Phil. Mag., XXV, p. 184, 1913. 

 flbid., xviii, p. 604, 1909. 



