Primary and Secondary <y Rays. 



925 



The absorption coefficient is now determined for a screen of 

 lead (11 x 11 x *7) cm., when it is placed against the lead 

 screen in front of the radium ; and secondly, when it is 

 placed against the electroscope. 



•7 cm. against the lead screen \='50 

 *7 „ „ ,, electroscope X = *55, 



This effect seemed to be caused by the secondary radiation 

 emitted by the screen. 



A narrow screen 3'4 cm. wide and 1*3 cm. thick is placed 

 in front of the radium. A lead screen (13 x 13 X l'l)cm. 

 is divided into three strips. One of these narrow strips is 

 placed against the lead screen in front of the radium, and the 

 coefficient of absorption is thus determined. 



The apparent absorption coefficient A, — *51. The two other 

 strips are now added to continue the plate, and for this large 

 plate A, = *46. The side portions of the screen which are added 

 in this latter case do not cut off any of the direct primary radia- 

 tion from the electroscope. They add, however, the secondary 

 radiation due to the passage of the primary radiation. Hence 

 the area of the absorbing screen will modify the value of the 

 coefficient of absorption. Experiments also show that bodies 

 in the neighbourhood, from which a secondary radiation can 

 be produced, will cause a variation in the value of results. 

 The secondary radiation from the air due to the passage of 

 the primary radiation would most probably be very small. 



Importance of secondary <y-rays in tlie measurement of the 

 absorption coefficient of primary <y-rays. 



The radium is kept in a constant position 80 cm. from the 

 electroscope. A large screen (20 X 20 X l'01)cm. can be 

 placed in any position between the radium and electroscope. 

 A lead screen \7 cm. thick is placed against the electroscope 

 to test the quality of the radiation. 



Position of large screen ... 



Position I. 



Position II. 



Position III. 



Distance from electroscope... 



77 '5 cm. 



66 cm. 



1 cm. 



No screen against electroscope 



5*05 div./min. 



4*65 div./min. 



4-29 div./min. 



Screen against electroscope . . . 



3*26 div./min. 



317 div./min. 



2-82 div./min. 



Absorption coefficient 



A = '62 



\ = -54 



\ = -60 





It will be seen that the magnitude of the leak and also the 

 absorption coefficient of the radiation change considerably 



