FORMATION OF THE AVALANCHE IN GEIGER-MOLLER COUNTERS 



their motion is rapid compared with that of the much heavier positive ions 

 liberated at the same time. As the electrons approach the thin wire anode 

 they are increasingly accelerated, since the voltage gradient is greatest near 

 the wire. Soon they are travelling fast enough to cause further ionizations 

 by collisions with the gas molecules, giving rise to secondary electrons, which 

 in their turn are attracted to the anode. The secondary electrons may then 

 breed tertiary electrons, and so on. The consequence of this cumulative 

 process is that a shower of over 10^ electrons reaches the anode. However, 

 the discharge of electrons at the anode ceases after a period of the order of 



Filling 

 tube 



Filling 

 tube 



r^H 



Glass-metal 

 seal 



Anode wire 



Metal 

 cathode 



n 



Anode wire 



Cathode 

 connection 



Graphite 

 layer on 

 inside of 

 glass wall 



Thin-walled immersion 

 End-window tube ''-'be 



Figure 31.4 Basic features of two types of Geiger tube 



OT /^sec, because the slow-moving positive ions build up a virtually stationary 

 space charge which neutralizes the effective voltage gradient in the vicinity 

 of the anode. Before the tube can discharge again the positive ions have to 

 be sufficiently dispersed towards the cathode to restore the voltage gradient at 

 the anode; this occurs relatively slowly, and may take 100 /^sec or more. 

 The period during which the tube is thus rendered inoperative is called its 

 'dead time'. It is followed by a rather longer interval (while the positive 

 ions are still being collected at the cathode) during which the tube gives an 

 output pulse of reduced size if triggered again. 



Quenching mechanisms 



When the avalanche of electrons reaches the anode, photons of electro- 

 magnetic radiation are produced. Unless precautionary steps are taken, 

 these are now capable of initiating a fresh discharge by ejecting electrons 

 from the material of the cathode, thus setting up a state of continuous dis- 

 charge. The limiting condition for a continuous discharge is that each 

 avalanche should give rise to just one photoelectron at the cathode, i.e. that 

 the product of the gas multiplication factor and the photoelectric efficiency 



427 



