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IX. The Recoil of Radium C from Radium B. By Walter 

 Makower, M.A., D.Sc, Assistant Lecturer and Demon- 

 strator, and Sidney Russ, D.Sc. } Demonstrator in Physics 

 in the University, Manchester *. 



Introduction. 



IT has been shown in a previous paper j, that during a 

 radioactive transformation involving the expulsion of an 

 a. particle, the residue of the atom from which the « particle 

 has been expelled recoils in an opposite direction to that in 

 which the a particle is emitted and can travel a considerable 

 distance through a gas if the pressure is sufficiently low. A 

 similar effect was also demonstrable in the case of the trans- 

 formation of radium B into radium C, although this change 

 is supposed to be accompanied by the expulsion of only /3 

 rays, no a rays having ever been detected. (It has recently 

 been shown by Halm and Meitner that radium C is complex ; 

 but it is only the product whose period is 19 minutes with 

 which we are concerned in what follows J.) 



Now it was thought that this transformation was worthy 

 of further study, since it is possible to investigate it by itself 

 without any complications arising from other radioactive 

 processes taking place simultaneously. If a plate is exposed 

 as the negative electrode in an electric field to radium 

 emanation for a long time, radium A, radium B, and radium C 

 will be found on the plate after removal from the emanation; 

 but if the plate is then left for half or three-quarters of an 

 hour, the radium A on the plate will have diminished to a 

 small fraction of its initial value, and we are left with only 

 radium B and radium C, which are being transformed 

 respectively to radium C and radium D. A disk which is 

 suspended above such a plate therefore receives radium C and 

 radium D projected on it as a result of the recoil during the 

 transformation of radium B and radium 0. Now, since the 

 time period of radium D is exceedingly long, the activity of 

 the disk due to the presence of this substance after exposure 

 to the active plate is inappreciable, and we should therefore 

 be in a position to study the disintegration of radium B by 

 itself. 



The phenomena were, however, soon found to be more 

 complicated than had been anticipated, and it was found 

 possible only under certain conditions to study the recoil of 



* Communicated by the Physical Society : read November 26, 1909. 



t Kuss & Makower, Proc. Roy. Soc. A. vol. lxxxii. 1909. 



J Hahn & Meitner, Phxjs. Zeiischr. x. no. 20, pp. 697-703 (1909). 



