( The Excitation of 7 Rays by a Rays. 193 



We are indebted to Prof. Rutherford for his kind en- 

 couraging interest in these experiments, and also to the 

 Royal Society Government Grant Committee for a grant 

 to one of us, out of which part of the expenses have been 

 paid. 



Additional Note added Dec, 23, 1912. 



It was noticed in the above experiments that when the 

 zinc sulphide screen was moved so as to gradually approach 

 the source of a, particles, a considerable illumination of the 

 screen appeared just prior to the point where the scintillations 

 began to appear in large numbers on the screen. Near the 

 end of the range the scintillations themselves consist of very 

 faint spots of light, more localized than those due to a particles 

 of high velocity. The illumination mentioned above appears 

 uniform, and was specially noticeable in the experiments 

 where adjustments of the end of the range were made with a 

 gold or other metal foil placed within 1 mm. of the ZnS screen. 

 The illumination appears to be connected with the a particles 

 and not to be due to a (3 or 7 radiation emitted from the 

 source. The radiation producing it can penetrate about 

 1 mm. of air, which seems to exclude the idea of S rays 

 being the cause. A transverse magnetic field of 2500 gausses 

 had no appreciable influence on the luminosity, and this, 

 in consideration of the recent experiments by Chad wick, 

 suggests that it may be due to 7 rays excited by the cc rays. 

 Such 7 rays would be of course much softer than any yet 

 investigated by the ionization method, but might conceivably 

 belong to a series similar to those investigated by Barkla. 



Experiments with willemite and BaPtCy, which might be 

 expected to give a larger relative illumination of 7 to a rays, 

 were somewhat unsatisfactory owing to the large illumination 

 effect of the /3 and 7 rays given off from the source. 



XXI. The Excitation of 7 Rays by a Rays. By J. Chad- 

 wick, M.Sc.j Beyer Fellow of the University of Manchester*. 



AT the end of a paper f published recently the writer 

 stated that some evidence had been obtained of the 

 excitation of y rays when a rays impinge on matter. This 

 has now been definitely established, and the preliminary part 

 of the experiments will be described. 



The source of a rays used in these experiments was radium 

 emanation with its active deposit contained in a very thin 



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

 f Chadwick, Phil. Mag. xxiv. p. 594 (1912). 



Phil. Mag. S. 6. Vol. 25. No. 115. Jan. 1913. 



