Energy of Scattered JL-radiation. 649 



estimates the energy of radiation from a given mass to be six 

 times that given by the writer, and concludes that the number of 

 scattering electrons per atom is three times the atomic weight. 



Now it was early shown by the writer* that elements of 

 low atomic weight, up to and including sulphur, scatter to 

 the same extent mass for mass, and that the scattering is 

 independent of the penetrating power of the Rontgen radia- 

 tion used. Mr. Crowther later verified both these results f. 

 We should therefore expeot to find the same amount of 

 energy scattered in the two cases from equal masses of air 

 and aluminium. The experimental values are evidently in 

 conflict, and it becomes a matter of interest to examine them, 

 not merely for the sake of any evidence the result may afford 

 as to atomic constitution, but in order to explain certain 

 phenomena of absorption. 



From the results of experiments by the writer it was con- 

 cluded that a la}'er of atmospheric air of 1 centimetre thick- 

 ness scatters about '00024 of the energy of Rontgen radiation 

 passing through it. Thus if — d\ s represents the diminution 

 due to scattering in the intensity of a beam during 

 transmission through a layer of air of thickness dx, then 

 dl s -— -000211^1'/ Calling the quantity '00024 the co- 



efficient of scattering s, we get - = 2 approximately, where 



p is the density of air or any substance of low atomic weight. 

 Mr. Crowther's value in the case of aluminium is 1*18. 



We will briefly consider these results in the light of other 

 experiments. 



(1) As the radiation considered is scattered radiation, it 

 involves a corresponding diminution in the intensity of the 

 primary beam. That is if the intensity of a beam proceeding 

 in a given direction be expressed by the equation I = I £ _U , 

 A cannot be less than s, or the total diminution of intensity of 

 the beam proceeding in the original direction of propagation can- 

 not be less than the loss due to scattering alone %. Yet the total 



* Phil. Mag. June 1903, pp. 685-698: May 1004, pp. 543-560; June 

 1906, pp. $12-828. 



t Phil. Mag. Nov. 1907, pp. 653-675. (Hydrogen is a possible though 

 not certain exception to the first law.) 



\ For want of a better term X will throughout be termed ike absorp- 

 tion coefficient, though some of the energy is merely scattered and some 

 is re-emitted in a different form. Correctly it is the rate of diminution 

 with distance of the primary beam, as a primary beam, or it is the rate 

 of diminution of intensity of an infinitely narrow pencil of radiation 

 during transmission through matter. In experiments on absorption cave 

 has to be taken to get the primary beam after transmission practically 

 free from the scattered and re-scattered radiations, as well as from the 

 fluorescent X-radiation and the corpuscular radiation, or at any rate to 

 arrange for the effects to be small and to correct for them. 



Phil May. S. 6. Vol. 21. No. 125. May 1911. 2 V 



