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LXIX. The Wave-Length of Hard Gamma Rays. By 

 Arthuk H. Compton, Ph.D., Physics Laboratory, Wash- 

 ington University *. 



THE only recorded attempt to measure directly the wave- 

 length o£ hard gamma' rays is apparently that of 

 Rutherford and Andradef, using the method of reflexion 

 from a crystal of rock-salt. In these experiments spectrum 

 lines were observed at angles as small as about 44 minutes, 

 corresponding to a wave-length of about 007 A.U. It was 

 thought that this line, as well as one of wave-length 0*10 A.U., 

 could be detected through a 6-millimetre screen of lead, 

 which would make it appear that these lines represent the 

 hard gamma rays from radium C. Professor .Rutherford 

 informs me, however, that the appearance of these lines 

 through the lead screen was doubtful. His more recent 

 measurements of the absorption of X-rays of very high 

 frequency J have indicated rather that radiation, whose wave- 

 length is about 0*08 A.U., has an absorption coefficient in 

 lead that is very much greater than that of the hard gamma 

 rays from radium. Thus, while the crystal reflexion measure- 

 ments show that radium gives off gamma rays of wave- 

 lengths 0'07 A.U. and longer, the very penetrating radiation 

 which it emits probably has a much shorter wave-length. 



Various lines of theoretical reasoning suggest that there 

 are in hard gamma rays components ranging in wave-length 

 from 0'01 to 0'04 A.U. Rutherford has pointed out § that 

 radium C gives off beta rays with an energy corresponding 

 to a fall through from 5 to 20 x 10 5 volts. According to 

 the quantum relation, hv = eY, the limiting wave-length 

 produced by the slower of these electrons would be about 

 0*03 A.U., while that due to the fastest ones would be as 



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short as 0'007 A.U. In the second place, using an absorption 

 formula which is satisfactory for hard X-rays of known 

 wave-length, it is found || by extrapolation that the absorption 

 coefficient of hard gamma rays corresponds to a wave-length 

 of about 0*04 A.U. And finally, knowing approximately the 



* Communicated by Prof. Sir E. Rutherford, 

 t Rutherford and Andrade, Phil. Mag. xxviii. p. 263 Q914). 

 t E. Rutherford, Phil. Mag. xxxiv. p. 153 (1917). 

 § Ibid. 



|| A. H. Compton, Washington University Studies,' Scientific Series, 

 Jan. 1921. 



