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LXTII. The Range and Ionization of the a Particles from 

 Radium C and Thorium C. By Gr. H. B.EXDERSON, 

 M.A., 1851 Exhibition Scholar of Dalliousie University, 

 Halifax, 'M.S.* 



[Plate XVII.] 

 § 1. Object of Experiments. 



SINCE 1905, when Bragg and Kleeman introduced the 

 idea that the a particles from a particular radioactive 

 substance possessed a definite characteristic range, many 

 determinations of the ranges of the various types of a 

 particles have been made "f. Most of these, however, were 

 made at a comparatively early date and with no attempt at 

 any great accuracy. As an example of the heed for more 

 accurate knowledge of these ranges, the Greiger-Nuttall 

 relationship between range and transformation period may 

 be cited. A knowledge of the limits of accuracy within 

 which this rule holds seems called for as the next step 

 towards the explanation of this important relationship. 



One object of the present experiments was to determine as 

 accurately as possible the ranges in air of the a particles 

 from radium C and thorium 0, the substances most easily 

 obtainable in sufficient quantity for a direct determination of 

 the ranges. Standard determinations of these ranges once 

 made, it might be possible to measure the ranges of other 

 types of rays to the same degree of accuracy by a comparison 

 method. 



Another object was a careful investigation of the ionization 

 curve % of these a. particles near the end of their range, in 

 view of certain anomalies which have been noticed by some 

 observers in the case of polonium. These anomalies will be 

 discussed in detail later. 



Finally it was hoped that a further study of the ionization 

 curve might throw light on the mechanism involved in the 

 passage of a. rays through matter, particularly near the end 

 of their range. 



* Communicated bv Professor Sir E. Rutherford, F.R.S. 



t For references to this earlier work see Rutherford, ' Radioactive 

 Substances and their Radiations.' Cambridge, 1913. 



I For brevity the term " ionization curve " is used to denote a curve 

 having as coordinate axes the distance of a thin ionization chamber 

 from the radioactive source and the corresponding ionization current 

 measured in the chamber. 



