770 Electrical Method for Measurement of Recoil Radiations. 



curve A. It will be seen that the principal difference is 

 that the ionization curve gives a greater range for the recoii 

 atoms than the scintillation curve. This may be because 

 different values have been taken for the stopping powers of 

 the absorption screens used in the two ca«es or to the fact 

 that the scintillation curve was made with mixed absorptions 

 of aluminium and mica. At the same time, in the analogous 

 case of the absorption of a homogeneous b v am of a. rays, 

 ionization is detected beyond the range of the particles as 

 determined by scintillations, and the difference here may be 

 due to the same causes. 



Curve I. is less definite than curve II. because the glass 

 of the «-ray tube used as the source was not of uniform 

 thickness. Some a particles of range just over 5 cms. were 

 present, and probably every range was represented between 

 this and zero. 



The difference between the ranges of the recoil particles 

 given by this curve and the scintillation curve E is not 

 accounted for by the small difference in the maximum ranges 

 of the a particles producing them, and must be given an 

 explanation similar to that suggested for the long range 

 particles of curve II. 



The above remarks indicate the success of the research 

 from the point of view of two of its objects. It confirms 

 Professor Rutherford's scintillation experiments, which 

 show that in close collisions between a particles and hydrogen 

 atoms, the nuclei cannot be considered as point charges, and 

 that most of them are thrown straight forward. It does not 

 show any essential difference between curves which plot 

 ionization against absorption and those which plot scintilla- 

 tions against absorption, except the small difference in range 

 above discussed. The effect of increase in ionizing power of 

 a particle as its velocity decreases is probably masked by 

 differences in the experimental arrangements used to obtain 

 the scintillation and ionization curves. 



With regard to the first object of the research, the 

 electrical method with the apparatus as finally used is 

 suitable for the investigation of recoil radiations, and should 

 be capable of modification for use on an effect only about a 

 fifth as great, with similar quantities of radioactive material. 

 The maximum effect was about two divisions on the micro- 

 scope scale per minute, for an activity corresponding to one 

 milligram of radium. 



My best thanks are due to Professor Rutherford for his 

 continued interest and advice throughout the course of this 

 work. 



