306 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 10 



cathode surface they are neutralized by ejecting an electron from the wall. 

 This is immediately followed by the emission of one or more photons from 

 each atom as it returns to the ground state. For some gases such as argon, 

 helium, and hydrogen, the emitted radiation lies in the ultraviolet and 



BF 



mnn//l/niiitiiiuuiiiiii>ii /iiiii 



^VW-^ZZ3- 



s 



}JJ/Jj>j>j>rtjsji, /,, i /,//,,, , j / r , / f f r f , f -? 



Fig. 74. Easily constructed metal counting-tube designs. 



A. Cylindrical brass or copper tube counter using Kovar seals (cosmic rays, gamma rays, 

 and neutrons). 



B. Copper "bell" counter with Kovar seal pump out and anode lead (gamma rays and 

 neutrons). 



C. Arrangement of radiator in fast neutron counters showing hydrogenous layer coated on a 

 thin platinum plate. 



D. Bell counter with mica window and with mounting for standard sample position (beta 

 particles). 



possesses sufficient energy to eject photoelectrons from the counter walls 

 where it is absorbed. Although the photoelectric efficiency is of the order 

 of one ejected electron per 10 4 photons, if 10 10 ions are formed in the discharge 

 process, the probability of electron ejection is high and consequently the 

 discharge is continued. In addition, positive ion bombardment of the 

 cathode at potentionals normally used in operating counter tubes frequently 



