Primary and Secondary 7 Rays, 923 



II. Secondary 7 radiation. 



(1) The distribution of secondary 7 radiation : 



(a) produced by different thicknesses of the 



same material ; 



(b) produced by different materials. 



(2) The quality o£ the secondary 7 radiation. 



The variation of quality with 



(a) Position of electroscope. 



(b) Material of radiator. 



(c) Thickness of radiator. 



(d) Area of radiator. 



(e) Screening of radium. 



(3) A discussion of the question whether the secondary 



7-rays are true secondary rays or scattered 

 primary rays. 



I. Initial Absorption of 7 Rays. 



When y-rays are absorbed by a substance such as lead it 

 has been observed by most experimenters that the coefficient 

 of absorption decreases with an increase in thickness of the 

 absorbing material. This has been generally explained by 

 assuming the original radiation to be heterogeneous, and 

 consequently the softer radiation to be cut out more in pro- 

 portion than the harder radiation. That the 7-rays from 

 radium are heterogeneous is no doubt true, but the above 

 assumption requires modification to explain all the cases that 

 are likely to arise. Observers do not agree in the exact 

 values to be assigned for the initial absorption of 7-rays. A 

 few experiments were performed to see what was the reason 

 of this divergence. 



Apparatus. 



The electroscope was of lead, 3 mm. thick and 7 cm. cube. 

 The top, bottom, and the two sides of the electroscope con- 

 taining the windows were surrounded in lead about 1 cm. 

 thick. The face through which the 7-rays penetrated was 

 3 mm. thick. A screen of lead *7 cm. could be used to test 

 the quality of the radiation. The quality is determined by 

 calculating \ the absorption coefficient of this *7 cm. of lead 

 from the relation I f = Io e~ Kt , where I is the leak, in the 

 electroscope before the lead screen is placed in position. 

 The electroscope was supported on an iron pipe about 1^ 

 inches in diameter. The radium was placed near the edge 

 of a table, and in some cases the lead screen was supported 



3P2 



