2.2 Transport Parameters of Operational Gas Mixtures

1. 2.2 Transport Parameters of Operational Gas Mixtures

2. 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 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.

3. 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

4. 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)

5. 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 vd 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

6. 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)

7. 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

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

9. Lorentz Angle B 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 F 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 θ Linear with increasing magnetic field Gases with low electron energies have small Lorentz angle

10. Properties of Helium Helium-Ethane Lorentz Angle for Helium-Isobutane Drift Diffusion

11. Neon Longitudinal Diffusion Constant for Ne-CO2 mixtures

12. Diffusion in Argon Transverse Diffusion in Ar-DME mixture Transverse Diffusion in Ar-CH4 No B field With B field

13. Argon Lorentz Angle in Ar/CO2 Drift Velocity for Pure Argon Possible gas for single photon detectors

14. Xenon Xenon-CO2 In medical imaging, the gas choice is determined by spatial resolution: CO2 added to improve diffusion Pure Xenon

15. DME Transport Parameters for Pure DME Low diffusion characteristics and small Lorentz angles  used to obtain high accuracy

16. 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.

17. Townsend Coefficient Mean number of ionizing collisions per unit drift length Helium-Ethane DME/CO2

18. Ion Transport Properties Ion drift velocity Electric field pressure Constant up to rather high fields

19. Pollutants 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