Taylor — Retardation of Alpha Bays hy Metals. - 357 



Art. XXXYI. — On the Retardation of Alpha Rays hy Metals 

 and Gases ; by T. S. Taylor. 



[Contributions from the Sloane Physical Laboratory of Yale University.] 



Introduction. 



In a preliminary paper* " On the Retardation of Alpha 

 Rays by Metal Foils and its Variation with the Speed of the 

 Alpha Particles," the writer described some experiments 

 which showed clearly that the air-equivalents of metal foils 

 decrease with the range of the alpha particles entering 

 the foils. f By " air equivalent" is meant the amount by 

 which the range of the a-particles in air is cut down by their 

 passage through the foil. It was shown that the change in 

 the air-equivalents is small for thin foils of the lighter metals 

 when the speed of the alpha particles entering the sheets is 

 high ; but, when the speed of the particles is low for thin 

 sheets or when the sheets are thicker, the change becomes 

 quite marked. A comparison of the change for sheets of 

 different metals of nearly equal air-equivalent showed the rate 

 of change to be in the order of the atomic weights of the 

 metals. The results obtained in these experiments were not suffi- 

 cient to furnish an explanation of the phenomenon ; but the 

 continuation of the experiments during the last year under 

 somewhat different conditions has furnished results which do 

 lead to conclusions of some interest. 



Scattering of the Alpha Rays. 



In the determination of the variation in the air-equivalents 

 with the speed of the alpha particle as described in the paper 

 cited above, the source of rays (polonium), with the metal sheet 

 over it, was set at such a distance from the ionization chamber 

 that some part of the top, or nearly horizontal portion, of the 

 Bragg ionization curve fell within the ionization chamber. A 

 slight increase in the range of the particle in this portion of the 

 curve corresponds to a considerable increase in the ionization. 



* This Journal, vol. xxvi, pp. 169-179, Sept., 1908. 



f The phenomenon upon which this work was based was first observed by 

 Mme. Curie and has later been investigated by several others. Bragg & 

 Kleeman(Phil. Mag., Sept, 1905, and April, 1907) observed that the stopping 

 power of a metal was not independent of the speed. Kucera & Masek (Phys. 

 Zeitschr., xix, pp. 630-40, 1906), and Meitner (Phys. Zeitschr., viii, 489, 

 1907), ascribe the effect to a difference in the amount of scattering. McClung 

 (Phil. Mag., Jan., 1906), Eutherford (Phil. Mag., Aug., 1906), and Levin 

 (Phys. Zeitschr., xv, 519-521, 1906) obtained results which indicate that 

 each successive layer of aluminium foil diminishes the range of the a-parti- 

 cle by the same amount. 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 166.— October, 1909. 

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