774 Prof. A. H. Conipton on the 



is clear that sin(0'/2)/A/=c=sin (0/2)/\, whence 



V=^X (2) 



sin 0/2 



Thus, i£ it is possible to find an angle at which gamma rays 

 scattered from lead and copper are in the same ratio as the 

 X-rays scattered at 90° in Barkla and Dunlop's experiments, 

 we have the data necessary to calculate the effective wave- 

 length of the gamma rays. 



Scattering of Hard Gamma Rays at Small Angles. — An 

 element of uncertainty is introduced into the application of 

 this method of determining the wave-length of gamma rays 

 by the fact that recent measurements have shown* that only 

 a very small part, if any, of the secondary gamma' rays 

 observed at large angles with the primary beam is truly 

 scattered radiation. At an angle of 45° these measurements 

 indicated that perhaps 5 or 10 per cent, of the secondary 

 radiation consisted of scattered primary rays., though the 

 absorption coefficient of the fluorescent secondary radiation 

 was so nearly the same as that of the primary rays that it 

 was not possible to establish with certainty the existence of 

 any scattered rays. With the hope of placing the present 

 wave-length experiments on a more certain footing, careful 

 examination of the character of the secondary radiation at 

 22°*5 was made, using, with some refinements, the same 

 general method as that employed in the earlier experiments. 

 It was found that, though for small thicknesses of the ab- 

 sorption screen the absorption coefficient of the secondary 

 radiation differed by only about 8 per cent, from that of the 

 primary beam, even after traversing 5*6 cm. of lead the two 

 absorption coefficients were still measurably different. Thus 

 at least 50 per cent, of the radiation at this angle is certainly 

 fluorescent in nature, but the fluorescent radiation is absorbed 

 at so nearly the same rate as the primary rays that it was 

 impossible to decide what part of the remaining 50 per cent, 

 was fluorescent and what part might be scattered radiation. 

 The experiments are, however, consistent with the view that 

 for angles smaller than 30° with the primary beam a con- 

 siderable portion of the secondary gamma radiation consists 

 of truly scattered radiation. 



Several investigators have found that at angles greater 

 than 30° the ratio of the secondary radiation from one element 



* A. H. Compton, supra, p. 749. 



