Gaseous Detectors, Operational Modes

  The charge collected by the anode of a chamber depends on the intensity of the electric field applied to the chamber. At some low voltage, the recombination of electrons and ions is overcome, but no gas multiplication occurs; a detector in this mode is insensitive to the voltage, and is called an ionization chamber ; the output signal is weak, and corresponds to the number of primary electrons.

As the voltage is increased, which happens in the fields of wire chambers, the primary ionization electrons cause electron avalanches to form: the accelerating electric field is high enough to impart to the electrons, generated by the primary ionization in the gas, an energy higher than the first ionization potential of the gas. These electrons then produce ion-electron pairs while continuing along their path; the secondary electrons may, in turn, form further pairs, and the phenomenon is called gas multiplication . Eventually, the freed electrons drift towards the anode and produce an analogue signal that can be used for position and energy loss measurement. Most wire chambers work in this proportional mode, viz. the signals recorded by the detector are much higher and still proportional to the energy loss of the traversing particle. In most practical chambers, the electric field close to the thin ( ) anode wire has a high gradient, so that a multiplication factor of 10⁵ to 10⁶ is reached, with multiplication occuring mostly very close to the wire, where the field is strongest.

Strict proportionality assumes that space charge (due to the longer-lived positive ions) and induced effects remain negligible, compared to the external field. At higher electric fields, or in a high flux of charged particles, the space charge effects alter the effective electric field, the chamber works in the mode of limited proportionality: the signal is no longer strictly proportional to the energy loss of the particle; the relation between collected charge and can still be put to use, though.

Further increase of the electric field eventually leads to electric breakdown of the gas. This takes place when the space charge inside the avalanche is strong enough to shield the external field. A recombination of ions then occurs, resulting in photon emission and in secondary ionization with new avalanches beyond the initial one. If the process propagates (backwards, from the avalanche tail) until an ion column links anode and cathode, a spark discharge will eventually occur, and a chamber or counter is said to operate in the Geiger-Müller mode.

In the limited Geiger mode , this discharge is not allowed to happen, which can be achieved by adding quenching agents to the gas ( Gas Mixtures in Gaseous Detectorss); output pulses at the anode are much higher in this mode than in the proportional mode. The process of spark discharge can also be stopped by manipulating the electric field: if only short (a few ns) pulses of high voltage are applied, short discharges develop from the ion trail of a crossing particle (streamers ), and a track image can be obtained by photography (streamer chamber ).

A similar effect as for the limited Geiger mode can be obtained using thick (50-100  , as opposed to the usual 20-30  ) anode wires [Brehin75] without using quenchers. This mode of operation, attractive because of its high mechanical reliability due to the thick wires, is called the limited streamer mode .

For more details, [Sauli91], [Blum93].