Physics and Engineering of Radiation Detection

3.6. Geiger-Mueller Counters 193

γuv

γuv γuv

γuv

γuv

γuv

γuv

e

e

e

Incident Localized Avalanche

Radiation

e

HV

Signal

Anode

Cathode

Cathode

Figure 3.6.1: Spread of Geiger avalanche due to the ultraviolet photons in a GM

counter.

amplitude irrespective of its energy. Hence these tubes are absolutely useless for
spectroscopic purposes or for making any measurement to reveal properties of the
incident radiation. This implies that the GM detectors can be used for particle
counting purposes only.

3.6.A Current-VoltageCharacteristics.................

Let us go back to Fig.3.3.1 and have a closer look at the Geiger Mueller region. A
closer view of this region is shown in Fig.3.6.2. It is evident that the pulse height at a
certain bias voltage is independent of the energy delivered by the incident radiation.
However the pulse height is not really independent of the applied voltage and a small
positive slope is clearly visible. This occurs since at higher voltages it takes more
time for the space charge to build up and decrease the effective electric field below
the threshold for avalanche.
This slope, however, is of no significance as far as the operation of GM tubes
is concerned. The reason is that these detectors are generally connected with a
discriminator circuitry that increments an internal counter as soon as the pulse
crosses its preset threshold that is chosen corresponding to the operational voltage.

E 1

E 2

E 1 >E 2

Limited Proportionality GM Region

Breakdown

Bias Voltage

Pulse Height

Figure 3.6.2: Pulse height vari-

ation with respect to applied

voltage in and around the

Geiger Mueller region of a gas

filled detector.