Radioactivity of Lead and other Metals. 



7G5 



measured, and the difference between its value and that due 

 to the natural conductivity of the air was noted, and recorded 

 as being due to the gamma rays from the radium bromide 

 together with the secondary rays produced by this radiation. 

 Each of the first eight cylinders referred to in Table I. 

 was then used in turn as a screen between the lead block 

 containing the radium and the testing cylinder, and the cor- 

 responding saturation current measured. The differences 

 between these readings and that taken with the radium 

 before the screens were inserted, were taken as a measure 

 of the absorption of the gamma rays by the respective 

 cylinders. From these differences, combined with the 

 ionization produced by the gamma rays impinging directly 

 on the testing cylinder, the absorptive power of each of the 

 cylinders was calculated as a percentage of the intensity of 

 the penetrating rays issuing from the lead block, and these 

 are given in Table III. The absorption of the gamma rays 

 by two sheets of aluminium 073 mm. and 1 '46 mm. in thick- 

 ness was also determined in the same manner, and these 

 are recorded, together with the others^ as Nos. 9 and 10 in 

 Table III. 



Table III. 



Cylinder 

 No. 



Material. 



Percentage 

 in mm. 



Percentage 



absorption by 



gamma rays. 



No. of ions pro- 

 duced by natural 

 ionization. 

 (See Table I.) 



1 



Lead 



1-85 



15-36 



23 



2 



,, 



2-25 



16-29 



160 



3 



}) 



1-45 



9-36 



37 



4 



,, 



1-85 



11-2 



78 



5 







1-80 



12 92 



34 



6 



J} 



1-80 



1012 



55 



7 



,, 



1-80 



13-23 



61 



8 



Zinc 



1-62 



462 



15 



9 



Aluminium 



1-43 



•92 





10 



" 



•73 



•46 





A set of measurements was next made on the saturation 

 currents in each of the first three cylinders given in Tables I. 

 and III. These were taken (a) with the air under natural 

 ionization; (b) with the cylinder lined with aluminium, but 

 otherwise the same as in (a) ; (c) with the radium bromide 

 in the lead block mentioned above at a distance of 1 metre 

 from the unlined cylinders; and (d) with all the conditions 

 the same as in (b) excepting that the cylinders were lined 

 with the sheet aluminium. 



