OUTPUT CHARACTERISTICS OF GEIGER TUBES 



into fragments, rather than by photon emission. The chances of initiating 

 a spurious discharge are therefore substantially reduced. 



Alcohol as a quenching agent has the disadvantage that its dissociation 

 products cannot reconstitute the original molecules, so that a small quantity 

 of alcohol is used up during each discharge. This limits the useful life of the 

 tube, and means that it may be irretrievably damaged if it is allowed to go 

 into a continuous discharge (which may occur despite the presence of alcohol 

 if too high an anode voltage is applied) for any length of time. In halogen- 

 quenched tubes the halogen atoms formed on dissociation can subsequently 

 re-combine, so that such tubes have a very long life and are less likely to be 

 harmed if they accidentally pass into a continuous discharge. Halogens 

 have the additional advantage that they can be used to quench low-voltage 

 Geiger tubes filled with argon-neon mixtures; such tubes have operating 

 potentials around 500 V whereas conventional argon-alcohol tubes need 

 about 1,200 V. The main disadvantages of halogen-quenched tubes are that 

 they are at present more expensive, and that they have somewhat less than 

 100 per cent efficiency for recording /9-particles within their sensitive volume. 

 However, their greater cost is offset by their longer life, and except for making 

 absolute measurements or coincidence experiments it does not greatly matter 

 if a small (but, of course, fixed) proportion of counts is missed. 



OUTPUT CHARACTERISTICS OF GEIGER TUBES 



The discharge of electrons at the anode and positive ions at the cathode 

 causes a transient flow of current through the Geiger tube, and the potential 

 of the centre wire drops temporarily. This fall in potential constitutes the 

 output pulse, which is then fed via a probe unit to a device for measuring 

 the rate of arrival of pulses, i.e. to a scaling circuit or ratemeter. Much the 

 greater part of the output pulse is contributed by the positive ions, since they 

 have to move through most of the applied potential gradient before reaching 

 the cathode, whereas the electrons only move a short distance to the anode. 

 The size of the output pulse depends on the total amount of charge on all 

 the positive ions (which increases with the length of the tube and with the 

 applied potential), on the total capacity (C) between anode and cathode, to 

 which it is inversely proportional, and on the recovery time constant RC, 

 where R is the parallel resistance of the EHT feed resistor and the input 

 resistance of the probe unit (see Figures 31.5 and 31.6). 



For a given value of C, maximum pulse amplitude requires a value of RC 

 large compared with the total period (200-500 ^asec) needed for complete 

 collection of the positive ions; but if R is made large enough to achieve this, 

 the potential on the centre wire returns excessively slowly to its resting level 

 after each pulse, which may be disadvantageous if fast counting rates are to 

 be dealt with. In practice it is usual to compromise by making RC about 

 50 /isec. This gives pulses of about half the maximum size, generally of the 

 order of a few volts. In order to avoid loading the tube with a long length 

 of cable, thus increasing C and reducing pulse size, the probe unit is mounted 

 close to the tube. There is something to be said (see below) in favour of 

 using the type of quench probe shown in Figure 31.5, even with a self- 

 quenching Geiger tube ; but with such tubes it is not essential to provide a 



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