XVII. RADIOACTIVE TRACERS 



619 



silver mirror deposit on the inside of the cyHndrical glass jacket and 

 the anode is a tungsten wire stretched down the center. P>om the 

 name of an early manufacturer it is known as an Eck and Krebs tube 

 (see Fig. 8). The minimum wall thickness that can be used in this 

 type of construction is about 0.15 mm., which will completely 

 stop /3 radiation of energies less than 0.17 m.e.v. and will absorb 

 a large fraction of the /3 rays unless their energy is at least 0.4 m.e.v. 



Silvered glass cathode 



^ ^ 



/ 

 Glass shell Fine tungsten wire 



Fig. 8. Cylindrical glass GM tube. 



Glass cap 



Metal she! 



Fine tungsten 

 wire 



Mica window^ Protecting brass ring 

 Fig. 9. Mica end window counter of bell type GM tube. 



There are a number of commerical sources for this type of tube 

 {86-87) and, since there is much less that can go wrong with this kind 

 of construction, the tubes are recommended for work with energetic 

 ^ emitters. Holders with adjustable shelves can be obtained {85) 

 and thick-walled tubes of the same sort that will allow only 7 rays 

 and the most energetic /3 rays to pass are also available. 



For work with emitters of low to average energy /3 rays, the mica 

 end window counter is. generally used (see Fig. 9). In this type the 

 cathode is a metal cylinder that may be the shell of the tube or may be 



