Radiation Detectors
Three main types of detector are in widespread use, namely, gas ionization, scintillation and
semiconductor. Although there are a number of common features they are best discussed separately for
the sake of clarity.
Gas Ionization Detectors
These are based on the interaction between the ionizing radiations and a suitable ionizable gas (e.g.
argon), within a closed tube. The electrons produced in the ionization generate voltage pulses when they
are collected by a thin wire anode (300–3000 V) whilst the argon ions are reduced at an envelope
cathode (connected to earth) (Figure 10.7). Two important types of gas ionization detector
(proportional and Geiger–Müller) have this simple construction. Their characteristics may be
appreciated by considering the change in the number of counts recorded as the voltage supplied to the
anode is varied, whilst presenting a source of constant activity to the detector. The curve obtained
(Figure 10.8) is divided into four parts. At low potentials (< 150 V) the potential gradient between the
electrodes is shallow. Thus the primary electrons produced in the ionizing event move slowly towards
the anode and they mostly recombine with argon ions before being discharged. As the potential
increases so does the efficiency of collection of electrons until all are collected and over a short
potential range a plateau is produced. A steadily increasing potential will however cause the electrons to
be accelerated towards the anode and produce secondary ionization or gas phase multiplication in
collisions with argon atoms. As a result, the characteristic curve will rise steeply with increase in
potential. The size of the pulse produced will be proportional to the energy carried by the incident
particle or photon, whence a measure of pulse height analysis may be applied to the signals. Eventually
gas ionization reaches its ultimate when a single ionizing event leads to complete ionization of the
filling gas, saturating the detector. A further plateau, known as the Geiger region, results, within which
the number of electrons produced is largely independent of the applied potential.
Figure 10.7
Radiation induced ionization in an argon-filled detector.