PENETRATION OF GAMMA RAYS. 



8 9 



TABLE 56. Penetration of radium radiation. Cylindrical glass fog chamber without 

 casket. 8j> = 22 cm. Lead tube, 30 cm, or 60 cm. long. Walls, 0.5 cm. thick. 



' Air effect frequently tested. 



Flat against chamber. 



Ill the first part of table 56 the radiation passes through thick lead 

 tubes, P, placed parallel to and contiguous with the end of fog cham- 

 ber A, as shown in fig. 88. There is no observable effect due to the cap, 

 nor to the length of the lead tubes, whence it follows that the rays pro- 

 ducing the nuclei actually pass through the heavy walls of lead. If 

 mean values be taken for the periodic data, the results are (curve 89) : 



Radium in thin sealed aluminum tube, N ' = = 27,000 



Radium in lead tube, walls 0.5 cm., N '-= 23,000 



walls i.o cm., ^ = =18,000 



or 85 per cent and 67 per cent pass, respectively, through the walls of 

 lead 0.5 cm. and i cm. thick. The important result follows here, as 

 above, that the extremely penetrating rays are responsible for the 

 observed nucleation. 



<T1. Radiation from a distance. It is surprising to compare with the 

 penetrating effect of the radiation the relatively marked diminution of 

 its intensity with distance. Indeed, if distances be reckoned from 

 the line of sight (supposing this to be justified) about 10 cm. from the 

 end of the chamber, the mean data are (curve 90) : 



z>= 10 



JV X io- 3 = 30 

 ND X io~* = 30 



20 



13 

 26 



40 



8 



32 



60 



5 

 30 



no 



3 



33 



210 cm. 



2 



42 



