Ionising radiation detectors

Picture 13. Plot of variation of ionisation current against applied voltage for a co-axial wire cylinder gaseous radiation detector.

Townsend avalanche discharges are fundamental to the operation of gaseous ionization detectors such as the Geiger–Müller tube and the Proportional counter in either detecting ionizing radiation or measuring its energy. The incident radiation will ionise atoms or molecules in the gaseous medium to produce ion pairs, but different use is made by each detector type of the resultant avalanche effects.

In the case of a GM tube the high electric field strength is sufficient to cause complete ionisation of the fill gas surrounding the anode from the initial creation of just one ion pair. The GM tube output carries information that the event has occurred, but no information about the energy of the incident radiation.

In the case of proportional counters, multiple creation of ion pairs occurs in the "ion drift" region near the cathode. The electric field and chamber geometries are selected so that an "avalanche region" is created in the immediate proximity of the anode. A negative ion drifting towards the anode enters this region and creates a localised avalanche that is independent of those from other ion pairs, but which can still provide a multiplication effect. In this way spectroscopic information on the energy of the incident radiation is available by the magnitude of the output pulse from each initiating event.

The accompanying plot shows the variation of ionisation current for a co-axial cylinder system. In the ion chamber region, there are no avalanches and the applied voltage only serves to move the ions towards the electrodes to prevent re-combination. In the proportional region, localised avalanches occur in the gas space immediately round the anode. Increasing the voltage increases the number of avalanches and thereby current, until the Geiger region is reached where the full volume of the fill gas around the anodes ionised, and all energy information is lost. Beyond the Geiger region the gas is in continuous discharge owing to the high electric field strength.

Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons.

Materials conduct electricity if they contain mobile charge carriers. There are two types of charge carrier in a semiconductor: free electrons and electron holes. A fixed electron in a reverse-biased diode may break free due to its thermal energy, creating an electron-hole pair. If there is a voltage gradient in the semiconductor, the electron will move towards the positive voltage while the hole will "move" towards the negative voltage. Most of the time, the electron and hole will just move to opposite ends of the crystal and stop. Under the right circumstances, however, (i.e. when the voltage is high enough) the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (i.e. more charge carriers), increasing the current. Fast-"moving" holes may also result in more electron-hole pairs being formed. In a fraction of a nanosecond, the whole crystal begins to conduct.

The large voltage drop and possibly large current during breakdown necessarily leads to the generation of heat. Therefore, a diode placed into a reverse blocking power application will usually be destroyed by breakdown, as the external circuit will be able to sustain a large current and dump excessive amounts of heat. In principle, however, avalanche breakdown only involves the passage of electrons, and intrinsically need not cause damage to the crystal. Avalanche diodes (commonly encountered as high voltage Zener diodes) are constructed to have a uniform junction that breaks down at a uniform voltage, to avoid current crowding during breakdown. These diodes can indefinitely sustain a moderate level of current while on the edge of breakdown.

The voltage at which the breakdown occurs is called the breakdown voltage. There is a hysteresis effect; once avalanche breakdown has occurred, the material will continue to conduct even if the voltage across it drops below the breakdown voltage. This is different from a Zener diode, which will stop conducting once the reverse voltage drops below the breakdown voltage [2,4,5].