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.
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.
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)
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
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)
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
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
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
Lorentz Angle for Helium-Isobutane
Longitudinal Diffusion Constant for Ne-CO2 mixtures
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
Transport Parameters for Pure DME
Low diffusion characteristics and small Lorentz angles used to obtain high accuracy
Mean number of ionizing collisions per unit drift length
Ion drift velocity
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