Gas Mixtures in Gaseous Detectorss

  Avalanche multiplication is essential in all gaseous detectors, in order to produce an electrical signal of sufficient amplitude. In principle, all gases can be used for generating electron avalanches, if the electric field near the (sense) wire is strong enough. However, depending on the mode of operation ( Gaseous Detectors, Operational Modes) and the intended use of the chambers, specific requirements towards, e.g. signal proportionality, high gain, good drift properties, or short recovery times, limit the choice of gases or gas mixtures.

Multiplication occurs in noble gases at lower fields than in gases with complex molecules; the addition of other components increases the threshold voltage. This suggests a noble gas as the main component of a chamber gas. Noble gases do not, however, allow operation at high enough gas gain without entering into a permanent discharge operation; the atoms excited during the avalanche process return to the ground state emitting photons at high enough energies to initiate a new avalanche in the gas or around the cathode. The latter may also be induced by the neutralization of ions that travel to the cathode. This problem is solved by the addition of a quenching gas which absorbs energetic photons; usually this is an organic gas like isobutane (CH₃)₂CHCH₃. Most organic compounds in the hydrocarbon and alcohol families are efficient in absorbing photons in the relevant energy ranges. The molecules dissipate the excess energy either by elastic collisions, or by dissociation into simpler radicals. Even a small amount of a polyatomic quencher added to a noble gas changes completely the operational characteristics of a chamber, and may allow gains in excess of 10⁶ to be obtained before discharge.

Classical gas mixtures for proportional counters are P10 (90% Ar+10% CH₄) and for (proportional) multiwire chambers, MWPCs for short, the ``magic gas'' mixture: 75% Ar + 24.5% isobutane + 0.5% freon.

Different requirements apply to chambers with long drift time; they include (besides the properties of gases of MWPCs) particularly good drift properties: gas purity is important, and special attention must be given to the drift velocity. If the chamber is to operate at high counting rates, the drift velocity should be high, to avoid losses due to dead time. For better spatial resolution, drift velocities should be lower, to minimize the influence of timing errors on position resolution. Characteristic for this category are gases like dimethylether (DME) or CO₂.

In microstrip gas chambers, MSGCs for short, the gas mixtures should have the following characteristics (see [Schmitz94]):

• - high primary ionization density of the gas mixture, to reach full efficiency in a thin layer of gas;
• - high electron drift velocity, to achieve a large signal and keep the detector occupancy low in a high flux environment;
• - high maximum gas amplification factor, to match the noise level of the electronics;
• - the gas mixture should not cause fast detector aging;
• - for use in magnetic fields, the gas should have a low Lorentz angle, which is achieved by a high electric field in the drift region.

Typical mixtures proposed for MSGCs are of the type Xe + CO₂ + DME. Detailed studies can be found in [Geijsberts92], [Beckers94].

For introductory reading, see [Blum93]. More about gases in wire chambers can be found in [Sauli91] or [Peisert84]. [Va'vra92] discusses in detail the simulation of the behaviour of gas mixtures on computers. On aging of wire chambers, Radiation Damage in Gaseous Detectors.