Figure 10.8
Characteristic curve for a gas ionization detector.An important feature of all gas ionization detectors is their dead or paralysis time, which has a direct
bearing on their utility. Once initiated a voltage pulse takes several hundred microseconds to die away,
and may be prolonged by the discharge of secondary electrons from the cathode as the argon ions are
reduced. Until this first pulse is terminated the tube is 'dead' to further radiations and the 'recorded
count' CM will be less than the 'true count' CT. The relation between the two values will depend upon the
length of the dead time t
The dead time can be shortened by reducing the anode potential immediately the pulse is initiated, and
by the incorporation of electron attracting materials which absorb secondary electrons. Typically ethyl
alcohol, bromine or carbon dioxide are used for this purpose. Quenching a pulse by a combination of
the two methods can reduce dead times of Geiger tubes (which generate large pulses) to the range 200–
400 μs and for proportional counters with their smaller pulses to a few microseconds only. In the latter
case corrections may generally be ignored, but for a typical Geiger tube dead time losses may amount to
20% at a count rate of 100 counts s–^1.
Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple
and robust in construction and may be employed as 'static' or 'flow detectors'. Flow studies have
received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas
chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas
chromatography column. To achieve