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The Geiger-Mueller tube

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 apart and 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 cylindrical cathode.

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', the thickness 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, for beta-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 a particle 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 further ions 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 pulse is 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 'quenching agent' 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 that the 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 a variety of detectors for investigating the activity of a radioactive source:
(a) a scaler - this device simply records the total number of pulses;
(b) a speaker and an amplifier - this will give an audible signal that becomes a continuous crackle when the activity is high;
(c) a ratemeter - this actually records the count rate (dN/dt) and the output may be fed to a meter or to a storage facility.
If a Geiger tube with a thin end window is used in a darkened room, flashes of light may be observed in the tube when it is used to detect particles from an alpha source.

The energy of an alpha-particle emitted by polonium-210 is 3.9 MeV, and its range in air at standard temperature and pressure (s.t.p.) is 35 mm. As it passes through the air the alpha particle produces ion pairs, the energy required to produce each ion pair being about 30 eV.
(a) Estimate the number of ion pairs formed per mm of path
(b) The ionisation per mm increases towards the end of the path. Suggest a reason for this.

If the pressure was reduced to one-hundredth of the original value, how would this affect the number of ions per mm? Explain your answer.
© Keith Gibbs 2011