2 2 transport parameters of operational gas mixtures
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2.2 Transport Parameters of Operational Gas Mixtures. Introduction. Particle physics experiments rely on the detection of charged and neutral particles by gaseous electronics. A suitable gas mixture within an electric field between electrodes detects charged particles.

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Particle physics experiments rely on the detection of charged and neutral particles by gaseous electronics

A suitable gas mixture within an electric field between electrodes detects charged particles

Ionizing radiation passing through liberates free charge as electrons and ions moving due to the electric field to the electrodes.

The study of the drift and amplification of electrons in a uniform (or non-uniform field) has been an intensive area of research over the past century.

Requirements for gas mixtures
Requirements for Gas Mixtures

  • Fast: an event must be unambiguously identified with its bunch crossing

    • Leads to compromise between high drift velocity and large primary ionization statistics

  • Drift velocity saturated or have small variations with electric and magnetic fields

  • Well quenched with no secondary effects like photon feedback and field emission: stable gain well separated form electronics noise

  • Fast ion mobility to inhibit space charge effects

Electron ion pair production in a gas
Electron-Ion Pair Production in a Gas

An ionizing particles passing through a gas produces free electrons and ions in amounts that depend on the atomic number, density and ionization potential of the gas and energy and charge of the incident particle

Np: number of primary electron pair per cm.

Nt: total number of electron ion pairs (from further ionization)

Electron transport properties
Electron Transport Properties

With no electric field, free electrons in a gas move randomly, colliding with gas molecules with a Maxwell energy distribution (average thermal energy 3/2 kT), with velocity v


When an electric field is applied, they drift in the field direction with a mean velocity vd

Energy distribution is Maxwellian with no field, but becomes complicated with an electric field

Noble gases
Noble Gases

Electrons moving in an electric field may still attain a steady distribution if the energy gain per mean free path << electron energy

Cross-section for electron collisions in Argon

Momentum transfer per collision is not constant.

Electrons near Ramsauer minimum have long mean free paths and therefore gain more energy before experiencing a collision.

Drift velocity depends on pressure, temperature and the presence of pollutants (e.g. water or oxygen)

Poly atomic gases
Poly-atomic gases

Electron collision cross-sections forCO2

Poly-atomic molecular and organic gases have other modes of dissipating energy: molecular vibrations and rotations

In CO2vibrational collisions are produced at smaller energies (0.1 to 1 eV) than excitation or ionization

Vibrational and rotational cross-sections results in large mean fractional energy loss and low mean electron energy

Mean or ‘characteristic electron energy’ represents the average ‘temperature’ of drifting electrons

Electron diffusion
Electron Diffusion

Electrons also disperse symmetrically while drifting in the electric field: volume diffusion transverse and longitudinally to the direction of motion

In cold gases, e.g. CO2, diffusion is small and the drift velocity low and unsaturated: non-linear space-time relation


Warm gases, e.gAr, have higher diffusion. Mixing with polyatomic/organic gases with vibrational thresholds between 0.1 and 0.5 eV reduces diffusion

Lorentz angle
Lorentz Angle


Due to the deflection effect due to a B field perpendicular to the E field, the electron moves in a helical trajectory with lowered drift velocity and transverse dispersion


The Lorentz angle is the angle the drifting electrons make with the electric field

Large at small electric field but smaller for large electric fields


Linear with increasing magnetic field

Gases with low electron energies have small Lorentz angle

Properties of helium
Properties of Helium


Lorentz Angle for Helium-Isobutane




Longitudinal Diffusion Constant for Ne-CO2 mixtures

Diffusion in argon
Diffusion in Argon

Transverse Diffusion in Ar-DME mixture

Transverse Diffusion in Ar-CH4

No B field

With B field


Lorentz Angle in Ar/CO2

Drift Velocity for Pure Argon

Possible gas for single photon detectors



In medical imaging, the gas choice is determined by spatial resolution: CO2 added to improve diffusion

Pure Xenon


Transport Parameters for Pure DME

Low diffusion characteristics and small Lorentz angles  used to obtain high accuracy

Lorentz angle in dme based mixtures
Lorentz angle in DME-based mixtures

  • Introduced as a better photon quencher than isobutane.

  • Absoption edge of 195nm: stable operation with convenient gas multiplication factors

  • High gains and rates without sparking.

Townsend coefficient
Townsend Coefficient

Mean number of ionizing collisions per unit drift length



Ion transport properties
Ion Transport Properties

Ion drift velocity

Electric field


Constant up to rather high fields


Pollutants modify the transport parameters and electron loss occurs (capture by electro-negative pollutants)

The static electric dipole moment of water increases inelastic cross-section for low energy electrons thus dramatically reducing the drift velocity

Mean electron capture length

Electron capture phenomenon has a non negligible electron detachment probability