Wave-Length of Hard Gamma Rays. Ill 



and Dunlop's experimental values for the scattering of 

 X-rays at 90°. Since their measurements included wave- 

 lengths as short as 3 A.U., we may conclude from equa- 

 tion (2) that the effective wave-length of the gamma rays 

 here employed is less than 



5 ^ X 0-3 = 0-037 A.U. 



sin 45 



If Barkla and Dunlop's results are extrapolated according to 

 the theoretical curve for lead shown in fig. 1, it will be seen 

 that the ratio Ipb/Icu a ^ 90° begins to increase appreciably 

 for wave-lengths in the neighbourhood of '2 to '25 A.U. If 

 from Table I. we take 10° as the angle at which the ratio 

 begins to increase for gamma rays, the effective wave-length 

 of these waves is by equation (2) between 0*025 and 0'030A.U. 

 According to the present experiment this may therefore be 

 taken as the approximate wave-length of hard gamma rays 

 which have traversed about 8 mm. of lead. 



The three principal elements of uncertainty which enter 

 into this calculation are : (1) the wave-length 02 A.U. at 90° 

 is an extrapolated value, (2) a possible error in the experi- 

 ments, and (3) the lack of positive evidence that the radiation 

 measured in these experiments contains an appreciable fraction 

 of truly scattered rays. None of these difficulties seem 

 sufficiently serious to render improbable the correctness of 

 the result as to order of magnitude. Indeed this method 

 of estimating the wave-length of hard gamma rays is perhaps 

 the most direct one that has been employed, and in as far as 

 its results are in agreement with the predicted values con- 

 sidered at the beginning of the paper, they may be taken as 

 a support of the theoretical bases of these predictions. 



The writer performed this experiment at Cavendish 

 Laboratory as National Research Fellow. He desires to 

 thank Professor Sir E. Rutherford for the free use of the 

 laboratory facilities and for valuable suggestions with regard 

 to the experimental procedure. 



Washington University, 

 Saint Louis, U.S.A. 

 December 1, 1920. 



