[ 18* ] 



XX. The Retardation of a Particles by Metals. By 

 E. Marsden, M.Se., Lecturer in Physics, and H. 

 Eichardson, B.Sc, University of Manchester * . 



ONE o£ the most characteristic properties of the a par- 

 ticles is the existence of a definite range or distance 

 through which they can travel in air or other material. 

 This range differs for a particles from different radio- 

 active products, but is characteristic of the product. Bragg 

 and others have shown that in the case of a particles from a 

 single product the length of the range depends not only on 

 the density of the material penetrated but also on its 

 chemical nature. Thus, if different gases be taken and 

 adjusted by temperature or pressure until they are of the 

 same density, it is found that they absorb a particles 

 by different amounts, the absorption being apparently a 

 function of the atomic weights only. Further, Bragg has 

 shown that if a thin sheet of metal foil is interposed in 

 the path of a homogeneous pencil of a particles in air, 

 the range of each a. particle is approximately reduced by 

 the same amount, depending on the thickness and material 

 of the foil interposed, In other words, the individual 

 a particles are not stopped by the foil but have their 

 velocities reduced by the same amount. In this w r ay Bragg 

 has determined the amounts of various substances necessary 

 to cut down the range of a particles by one centimetre in air, 

 and has noticed that the weights per unit area necessary for 

 different materials are approximately proportional to the 

 square roots of the atomic weights. 



It was observed, however, that in the case of a particles 

 from Ra C, for example, the air-equivalent of a metal foil, 

 or the amount by which the foil cuts down the range in air 

 at atmospheric pressure, is not quite the same when the foil 

 is placed directly over the Ra C as when it is placed a few 

 centimetres from the source f. Taylor % has investigated 

 this question more fully, and has shown that for layers of 

 material of atomic weight greater than the average atomic 

 weight of air, the air-equivalent descreases with decreasing 

 velocity, or range, of the entering a particles, while in the 

 case of hydrogen, whose atomic weight is less than that of 

 air, the air-equivalent increases with decreasing velocity. 



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

 t Bragg-, Phil. Mag. vol. xiii. p. 511 (1907). 



j Taylor, Amer. Journ. Sci. vol. xxvii. p. 357 (1909) ; Phil. Mag. 

 vol. xviii. p. 604 (1909). 



