Ranges of <x particles from Radioactive Substances. 613 



types of rays. On the basis of this assumption the whole 

 number of ions produced by a /3 particle of high speed per cm. 

 of its path at atmospheric pressure is 67. 



Our thanks are due to Professor Rutherford for his help 

 and interest in these experiments. 



Physical Laboratories. 

 The Victoria University, Manchester. 



LVI I. The Ranges of the a particles from Various Radioactive 

 Substances and a Relation between Range and Period of 

 'transformation. By H. Geiger, Ph.D., and J. M. 

 Nuttall, B Sc, University of Manchester* . 



IT is well known that the a particles from different radio- 

 active substances are characterized by their ranges, 

 i. e. by the distance through which they can travel in air at 

 atmospheric pressure. This was first pointed out by Bragg, 

 and the ranges of a number of products have been deter- 

 mined by him and his co-workers. The method applied by 

 Bragg to determine the ranges is well known. By means of 

 a set of parallel tubes placed directly above the active plate 

 a rays with practically parallel paths were obtained. The 

 ionization produced by these rays was measured at different 

 distances in a shallow ionization vessel, and the distance in 

 air at which the ionization just disappeared was taken as the 

 range of the « particles. 



It was shown by Rutherford that at the same distance 

 from the source at which the a particles fail to produce ioni- 

 zation they also lose their power of producing scintillations. 

 The observation of the scintillations at different distances 

 from the source therefore presents another way of deter- 

 mining the ranges of the a particles, and this method has 

 frequently been made use of by Hahn and other observers. 

 It appears, however, that the scintillations method gives 

 somewhat smaller values for the ranges than the ionization 

 method. 



Great difficulty has been experienced in the determination 

 of the ranges of the very inactive substances uranium and 

 thorium. In these cases, the methods mentioned above are 

 not applicable. Estimates of the ranges of these products 

 were, however, made by Bragg f, but more accurate values 



* Communicated by Prof. E. Kutherford, F.B.S. 

 t W. H. Bragg, Phil. Mag. xi. p. 754 (1906). 



Phil. Mag. S. 6. Vol. 22. No. 130. Oct. 1911. 2 S 



