the Absorption of the Gamma Rays. 



135 



§ 3. Experimental Details. 



The general arrangement o£ the apparatus is shown 

 in fig. 2. Radium-emanation in a closed glass tube was 

 used as the source of gamma rays. The source R was 

 placed near the end A of a cylindrical hole AA', 1*4 cm. 

 in diameter, passing through the centre of a lead cube LL, 

 of 25 cm. side, so that a narrow beam of rays was obtained, 

 of angle of about 2° in most cases. 



Fig. 2. 



The electroscope E was a cube of 10 cm. side, made of 

 aluminium plate 3'2 mm. thick, and was covered with 

 lead plate 2*5 mm. thick except the two sides through 

 which the beam passed. Glass windows for observing the 

 leaf were protected by lead, in order to exclude secondary 

 /3 rays. The gold leaf was charged to 400 volts, and the 

 natural leak was *20 to "18 div./min. throughout the experi- 

 ments. The distance between the electroscope E and the 

 front face N of the lead block LL was 36 cm. To make 

 sure that any stray rays, which might pass outside the 

 hole AA X , could not enter the electroscope, and also to 

 cut off the scattered rays from plates at R x at wide angles, 

 a thick lead block CC, 7*5 cm. thick, with a hole 5 cm. in 

 diameter, was placed close to the receiving surface of the 

 electroscope. The semi-angle of the cone described by A' 

 as vertex and the hole in CC as the base was not greater 

 than 5°. 



From 100 to 150 millicuries of radium emanation were 

 used as a source of gamma rays. The rays were screened 



