produced by Partly-absorbed Gamma Rays. 433 



and '12 mm. thick. Absorbing screens A/3 could be placed 

 over the side o£ the electroscope, and other screens, Ay,. 

 directly over the source. 



Method. 



Series of observations were taken o£ the ionization in the 

 electroscope both with the radiator in place and with the 

 radiator removed. The ionization due to the secondary 

 radiation from the radiator is then obtained by subtraction 

 of the smoothed curves. 



Two methods of arranging the observations were used. 

 In the first, a series of absorption curves for the secondary 

 /3 rays was taken, the different curves corresponding to 

 different thicknesses of matter traversed by the primary 

 7 rays. Each curve shows the ionization plotted against 

 the thickness of aluminium in the screen A/3 which absorbs 

 the secondary radiation. Any marked change in the energy- 

 distribution of the secondary radiation should then be made 

 apparent in a change of shape of the curve as we pass down 

 the series. 



Fig. 2. 



30 



25 



20 



8 '5 



10 



f 





























\ 















\ 



) 













\ x 















V, 



X— ^ 









1 R 















O Af3 -25 -5 -75 1-0 1-25 1-5 mm. AL 



Thickness of screen over electroscope. 



Fig. 2 shows a pair of experimental curves typical of 

 these observations, that is, ionization (in arbitrary units) 

 Phil. Mag. S. 6. Vol. 31. No. 185. May 1916. 2 G 



