Sandtray for GM tube changes

Quenching and dead time

Follwing the primary Geiger discharge there is considerable potential for the creation of spurious pulses not due to radiation ionisation. This can happen when the positive ions reach the cathode, where they are neutralised by each gaining an electron. However, the considerable ionisation potential of the main fill gas can result in the release of another electron which makes its way to the anode, gains energy, and triggers another Geiger discharge. This process can be repeated with the result of multiple spurious discharges, and even a continuous discharge. The is highly undesirable as no new radiation ionisation can be counted during this time and damage to the tube may well result. The solution is "quenching" of the main Geiger discharge in a controlled fashion, which puts the tube into a passive state awaiting the next radiation electron.

Quenching can be internal, due to processes within the tube, and external, where the applied tube voltage is controlled by external circuitry.

Internal quenching

The Geiger-Muller tube has a mix of two gases, which have different functions. The primary fill gas, which is the main gas for producing avalances, and a quench gas, which ensures the proper termination of each main Geiger discharge, and has a typical concenration of 5-10%. The quench gas works by absorbing the energy from the excited primary fill gas molecules by transfer of the positive charge from the positive ions of the primary gas to those of the quench gas. This readily occurs because of the lower ionisation potential of the quench gas. With the correct concentration of quench gas this will ensure all the positive ions arriving at the cathode are those of the quench gas. When they are neutralised at the cathode, they dissociate into neutral quencher molecules in preference to releasing an electron from the cathode surface. Spurious pulses are thereby avoided, and there is no tendency to sustain a continuous discharge.

Early tubes used a small amount of organic vapour such as butane or ethanol as the quench gas, but these permanently dissociated and depleted their quench effect, with a finite life of about 10⁹. In modern tubes (since the 1950s) a halogen such as bromine or chlorine is mainly used. The halogen gases have much longer lives as they re-combine, but the number of discharges is then limited by other effects in the tube, such as the deposition of polymerised oproducts. ^[1]

External quenching

External quenching, (sometimes called active quenching or electronic quenching) takes two forms. In it simplest form it uses the current pulse through the anode resistor to generate a voltage across the resistor and reduce the tube's electric field, which prevents further avalanche production, and limits the duration of the pulse. However due to uncertainty at high count rates introduced by the dead time, the count rate can become unreliable due to missing rapidly incoming radiation counts. This varies according to the dead time of the tube and associated electronic circuit, but is nominally 10³ counts per second.

The more complex forms uses high speed electronics to overcome dead time effects. One technique is the "time-to-first-count method" using statistical signal processing and high speed switching, which rapidly removes and re-applies the high voltage to the tube for a fixed time after each discharge. With this count rates of 10⁵ counts per second are achievable, two orders of magnitude larger than the normal effective limit. The time-to-first-count method is more complex to implement than the simple anode resistor method.^[1].

Dead time

As previously mentioned, the time for the geiger discharge to be quenched creates a dead tie, when the tube is insensitive to further ionsation events due to radiation.

When ionizing radiation strikes the tube, positively charged ions and free electrons known as ion pairs are generated in the gas. The strong electric field created by the voltage across the tube's electrodes accelerates the positive ions towards the cathode and the electrons towards the anode. Close to the anode in the "avalanche region" where the electric field strength rises exponentially as the anode is approached, free electrons gain sufficient energy to ionize additional gas molecules by collision and create a large number of electron avalanches. These spread along the anode and effectively throughout the avalanche region. This is the "gas multiplication" effect which gives the tube its key characteristic of being able to produce a significant output pulse from a single original ionising event. For alpha and beta detection the fill gas is ionized directly by the incident radiation, but for photon detection the dominant effect is more indirect and if the tube cathode is made of such as stainless steel, secondary electrons are liberated in it which migrate into the gas to cause ionisation.

^[1]

^ ^a ^b ^c ^d Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons, ISBN 0-471-07338-5

[knoll-1] Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons, ISBN 0-471-07338-5

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