A diagram of a Geiger-Mueller tube is shown in Figure 1.
A typical Geiger tube can detect separate particles as long as they arrive more than 200 microseconds apartand therefore it has a maximum count rate of 5000 counts per second.
This device is basically a gas-filled cold-cathode diode, in which the anode is a metal rod fixed along the axis of a cylindricalcathode.
The anode should be thin, so that an intense electric field is produced near it when a potential is connected between the anode and cathode. The end of a tube is closed by a ‘window’, thethickness of which varies from tube to tube depending on the type of radiation it is designed to detect.
The thickness of the end window is quoted in mg cm-2 for alpha-particles it is about 2, forbeta-particles about 25 and for gamma-rays many hundred. The tube contains neon at about 10 cm of mercury pressure, and a potential of about 450 V is applied between anode and cathode.
When aparticle enters through the end window ions are produced in the gas.
The positive ions travel towards the cathode while the electrons move towards the anode (Figure 2). As they move they produce furtherions by collisions, a process known as secondary ionisation, and an avalanche of ions reaches the detecting electrodes. For an electron about 108 ions are produced in a few microseconds. This pulseis amplified in an external circuit and detected as either a meter reading or a sound. To prevent continuous secondary ionisation a little bromine gas is added to the tube, acting as a ‘quenchingagent’ and absorbing the kinetic energy of the positive ions.
If the characteristics of the Geiger tube (the anode voltage related to the count rate) are recorded as shown in Figure 3, it can be seen thatthe tube should be operated in the so-called plateau region. In this area a small change of anode potential will have little effect on the count rate.
The Geiger tube may be fitted to...