4S C. D. Cooksey — Secondary Cathode Rays. 



Art. V. — On the Asymmetry in the Distribution of Sec- 

 ondary Cathode Rays produced by X-rays ; and its Depend- 

 ence on the Penetrating Power of the Exciting Rays; by 

 C. D. Cooksey. 



It has been sbown by Bragg and Madsen* that the amount 

 of secondary /3-radiation excited in various solids by 7-rays is 

 not symmetrically distributed about a plane perpendicular to 

 the direction of propagation of the 7-rays, but that the /3-radia- 

 tiou which comes from the side from which the 7-rays emerge 

 is greater than that coming from the side on which the 7-rays 

 are incident. The same effect is observed, though to a less 

 degree, in the case of secondary X-rays, f and the secondary 

 cathode rays produced by X-rays,:}: and those produced by ultra- 

 violet light. § This lack of symmetry is less for soft 7-rays 

 than for hard and still less for X-rays, which are usually con- 

 sidered as very soft 7-rays. The order of magnitude of the 

 ratio of emergence to incidence radiation ranges all the way 

 from 20:1 down to unity, depending on the nature of the 

 radiations used and the substance in which the secondary rays 

 are excited. 



Since I first showed that this effect was true for the cathode 

 rays produced by X-rays, I have been experimenting with a 

 view to find how the ratio of emergence to incidence radia- 

 tion depends on the hardness of the exciting rays. But with 

 these rays the effect is so small at best, that the variations 

 which might be produced by the widest possible variation in 

 hardness of the primary are not likely to be much greater than 

 the experimental errors always inherent in X-ray measure- 

 ments. Some results which I at first obtained! seemed to 

 indicate that there was a slight increase in the ratio with an 

 increase of hardness of the primary. But owing to the hetero- 

 geneity of the rays from an ordinary tube, and the difficulty, 

 at that time, of sorting out and using rays of a single penetra- 

 ting power, these results were not very convincing. 



Since these results were obtained, however, the work of 

 Prof. Barkla and others on "Fluorescent" X-radiations, a sum- 

 mary of which is to be found in the Phil. Mag. for Sept., 1911, 

 page 396, has afforded a convenient means of obtaining homo- 

 geneous beams of X-rays of known penetrating powers over a 

 wide range. It was by this means that the experiments 

 described in the present paper were performed. 



* Trans. Eoy. Soc. South Australia, vol. xxxii, May, 1908. 



t Bragg and Classon, loc. cit., Oct., 1908. 



i Cooksey, Nature, vol. lxxvii, p. 509, 1908. 



S Stuhlmaun, Pliil. Mag., vol. xxii, p. 854, 1911. 



(Nature, vol. Ixxxii, p. 128, 1909. 



