ASSAY OF RADIOACTIVITY 



of the cathode should exceed unity. For a Geiger tube filled with a pure 

 gas it is possible to prevent a continuous discharge from occurring by connec- 

 ting the" tube to the type of pulse generator circuit called a 'quench probe' 

 which, immediately it is triggered, feeds back to the anode a negative square 

 wave of several hundred V, thus reducing the anode potential well below the 

 normal operating level for a pre-determined period. A suitable circuit for 

 achieving this is shown in Figure 31.5. 



Another way of obviating a continuous discharge is to add to the gas in 

 the tube some molecules capable of absorbing the photons before they reach 



+235 +285 



Input 

 from °^ 

 Geiger 



8 2pF Cc ^56k <82k 



r-»— 1? 



If 



1^3 = EAC 91 

 H =EB 91 



-95 -95 -95 



Figure 31.5 Quench probe, redrawn A.E.R.E. type 1014 



the cathode. Alcohol vapour is often used for this purpose, and recently 

 halogen-quenched tubes have become available. The role of the quenching 

 vapour is to absorb the photons generated by the electron avalanche at the 

 anode and to prevent the occurrence of the spurious discharges which can 

 arise during the neutralization of the positive ions at the cathode. When the 

 positive ions reach the cathode they lose their charge by drawing an electron 

 from the cathode surface, but are then in an excited state, having an excess 

 energy /—</>, where /is the ionization energy and </> the work function of the 

 cathode. If they rid themselves of this energy by emitting a photon, the 

 consequence may be the production of a photoelectron from the cathode, 

 and the re-initiation of the discharge. In tubes filled only with argon or 

 neon this effect may lead to frequent spurious counts, but it can be largely 

 overcome by the presence of a small proportion of organic or diatomic 

 halogen molecules. These substances have lower ionization potentials 

 than the rare gases, so that the charge on the positive ions tends to be trans- 

 ferred to them through inter-molecular collisions in the gas, and the ions 

 which finally arrive at the cathode are mostly those of the quenching agent. 

 Being either poly- or diatomic, the excited quenching molecules have the 

 virtue of dissipating excess energy acquired on neutralization by splitting 



428 



