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Drift velocity Adding polyatomic molecules (e.g. CH4 or CO2) to noble gases reduces electron instantaneous velocity; this cools electrons to a region where.

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Presentation on theme: "Drift velocity Adding polyatomic molecules (e.g. CH4 or CO2) to noble gases reduces electron instantaneous velocity; this cools electrons to a region where."— 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 vd increase Drift velocity is sensitive to contaminants, gas density (P, T), applied field Often need to calibrate vd to 1% or better Spring, 2009 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 Spring, 2009 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 for E >~ 20kV/cm (106 V/m) Note large range for each gas and large difference in threshold field for entering the amplification region Spring, 2009 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 Spring, 2009 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) Spring, 2009 Phys 521A

6 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-rate detectors (large ionization density) Ar e- ~400 Spring, 2009 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) Spring, 2009 Phys 521A

8 Operating gas detectors: voltage
Tunable parameter: field strength (fn of operating voltage in a given detector) Qualitatively different behavior vs. field In proportional region, signal ∝ nT, but gain depends strongly on field Geiger region yields large signals Spring, 2009 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 Newton) Spring, 2009 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 Spring, 2009 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 Spring, 2009 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 Spring, 2009 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 Spring, 2009 Phys 521A


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