1 / 13

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

kyle
Download Presentation

Drift velocity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related