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Drift velocity

Drift velocity. Adding polyatomic molecules (e.g. CH 4 or CO 2 ) to noble gases reduces electron instantaneous velocity; this cools electrons to a region where scattering cross-sections are lower, so τ and v d increase

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Drift velocity

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  1. Drift velocity • Adding polyatomic molecules (e.g. CH4 or CO2) to noble gases reduces electron instantaneous velocity; this cools electrons to a region where scattering cross-sections are lower, so τ and vdincrease • Drift velocity is sensitiveto contaminants, gasdensity (P, T), appliedfield • Often need to calibratevd to 1% or better Phys 521A

  2. Diffusion • Electron cloud diffuses as it drifts; diffusion ∝√distance • Dominates position resolution over long drift paths • Transverse (σT) and longitudinal (σL) diffusion differ • σT suppressed in parallel E, B fields as electrons spiral around lines of B:σT(B) ~ σT(0) / √(1+ω2τ2) [ω=eB/m] • Gas properties can be calculated with MAGBOLTZ code Phys 521A

  3. Multiplication • Mean free path for ionization, λi, decreases exponentially with increasing E • Inverse, α = 1/λi, is 1st Townsend coefficient • Multiplication N = N0 eαx in uniform E • Gas amplification large forE >~ 20kV/cm (106 V/m) • Note large range for eachgas and large difference in threshold field for enteringthe amplification region Phys 521A

  4. Attachment, recombination • Electronegative molecules (e.g., O2, H2O, CF4) capture electrons to form negative ions; reduces signal, impacts measurement of ionization energy loss (dE/dx) • Effect highly sensitive to contaminant concentration, differs in different primary gas mixtures • Recombination most likely in regions of low E field Phys 521A

  5. Streamers, quenching • UV photons are emitted by excited noble gas atoms • Photon propagation is independent of E field direction (unlike electrons) • UV photon absorption lengths are about 10-4 gm/cm2, or ~0.06 cm in Argon gas • Compare mean-free-path in Argon for electron ionization (inverse 1st Townsend coefficient), ~0.01 cm in reasonable gas amplification fields • Polyatomic molecules absorb UV photons; prevent (quench) spread of ionization in space (streamers) Phys 521A

  6. Ar e- ~400 Positive ions • Ion drift velocities << electron vd • Mobility μ (velocity / applied field) independent of field, inversely proportional to density • Cloud of accumulated ions can change electrostatics, affect gain, distort field • Issue mostly for high-ratedetectors (large ionizationdensity) Phys 521A

  7. Radiation damage, aging • Presence of hydrocarbons and high integrated ionization rate can lead to growth of polymers (sometimes spanning conductors); degrades performance • Local hots spots (electron leakage) or dead spots can form • Recipes exist for adding trace gases (often H2O) to address problems with aging; underlying physics (chemistry) not well understood • In well-made chamber can collect few C/cm (for gain of 104 this corresponds to ~1014 mips) Phys 521A

  8. Operating gas detectors: voltage • Tunable parameter: field strength (fn of operating voltage in a given detector) • Qualitatively differentbehavior vs. field • In proportional region,signal ∝ nT, but gaindepends strongly on field • Geiger region yieldslarge signals Phys 521A

  9. Multiwire Proportional Chambers • Georges Charpak, 1960s (1992 Nobel Prize in Physics) • Many anode wires in a plane collect drifting ionization • Allow position and energy measurement over large collection areas • E field in drift region is constant; increases as 1/r near anode wires • Mechanical stability places limit on wire length/spacing: • Anode wire diameters ~10-20 μm; Tungsten allow often used for strength (maximum tension 0.16-0.65 Newton) Phys 521A

  10. MWPC electric field configuration • MWPC has long region of parallel, constant E field • Field is radial and grows as 1/r near anode wire • Segmented cathod pads allow use of induced signal to further localize ionization; charge sharing improves resolution relative to pad size Phys 521A

  11. MWPC resolution, efficiency • Efficiency approaches 100% for MIPs traversing enough gas to liberate nT>10 primary electrons • Resolution along wire based on charge division (due to differential resistance between location of charge deposit and each end; requires electronics at each end of wire) • Resolution transverse to wire depends on wire spacing (rms = 1/√12 of spacing) • Addition of segmented cathode pads allows charge-sharing to be used, provides sub-mm accuracy Phys 521A

  12. Drift chambers • Careful shaping of field in MWPC allows substantial improvement • Uniform E field allows arrival time to determine coordinate; fewer anode wires/cm required • Goal is uniform time-to-distance relation across entire cell Phys 521A

  13. Drift chamber operation • Need careful control of drift velocity, since d ~ vd (t-t0) • Calibration based on dedicated laser or on collected particles • In practice need full time-to-distance function; complications near anode wire and near cell edges • Need electronics with good time resolution on rising edge of ionization pulse • Resolution sensitive to fluctuations in ionization density • Discrete ambiguities must be resolved with external information • Chamber design can help with ambiguity resolution (e.g. staggering wires along anode plane in jet chamber) • Like MWPC, resolution along wire is poor Phys 521A

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