163- 



Secondary 7 Radiation. 



By J. P. V. Madsen, D.Sc. (Adel.), B.E. (Syd.), Lecturer 

 in Electrical Engineering, University of Adelaide. 



[Read July 17, 1908.] 



Introduction. 



As a result of the passage of y rays through matter, sec- 

 ondary rays of two types make their appearance. As it will 

 be necessary to distinguish between the secondary rays which 

 proceed from the sides at which the original y rays enter and 

 emerge from the plate which they penetrate, we shall refer 

 to these as the "incidence" and 'emergence" rays respectively. 



The secondary radiation consists of )8 and of -y rays. 



The former appear on both sides of the plate; the "in- 

 cidence" )8 rays have been recently investigated in some de- 

 tail by Kleeman (Phil. Mag., Nov., 1907) and by Eve (Phil. 

 Mag., June, 1908). 



In papers by Professor Bragg and myself (Phil. Mag., 

 May, 1908; Trans. Roy. Soc, S. Aus., v. xxxii., 1908) it is 

 shown that most of the experimental results so far obtained 

 with these rays can be very simply explained on the ''mate- 

 rial" theory, if we suppose that the /3 radiation is produced 

 directly from the y particle and at the outset moves in the 

 direction of the original y radiation, subsequently under- 

 going scattering in the ordinary manner of )3 rays. 



The second type of secondary radiation resulting from 

 the primary y rays, viz., the secondary y rays, have been 

 investigated on the incidence side of plates of different mate- 

 rial by Kleeman (Phil. Mag., May, 1908), and later by Eve 

 (Phil. Mag., Aug., 1908). 



It was a deduction made by Professor Bragg (Trans. Roy. 

 Soc, S. Aus., Jan., 1908) from the theory of the material 

 nature of X- and of y rays previously propounded by him, 

 that ''the existence of modified or softened y rays might be 

 suspected, since there is an analogous effect in the case of 

 X-rays ; and probably they would be found more at the back 

 of the penetrated plate than in front of it." The back and 

 front sides spoken of here refer of course to the sides of 

 emergence and incidence respectively. 



It will be shown in the present paper that this predic- 

 tion is fulfilled very exactly : that the want of symmetry in 

 the amount of radiation from the two sides of the plate is 

 very marked, that a softening of the original rays is effected, 

 f2 



