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Spring, 2009Phys 521A1 Drift velocity Adding polyatomic molecules (e.g. CH 4 or CO 2 ) to noble gases reduces electron instantaneous velocity; this cools.

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Presentation on theme: "Spring, 2009Phys 521A1 Drift velocity Adding polyatomic molecules (e.g. CH 4 or CO 2 ) to noble gases reduces electron instantaneous velocity; this cools."— Presentation transcript:

1 Spring, 2009Phys 521A1 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 Drift velocity is sensitive to contaminants, gas density (P, T), applied field Often need to calibrate v d to 1% or better

2 Spring, 2009Phys 521A2 Diffusion Electron cloud diffuses as it drifts; diffusion ∝ √distance Dominates position resolution over long drift paths Transverse ( σ T ) and longitudinal ( σ L ) diffusion differ σ T suppressed in parallel E, B fields as electrons spiral around lines of B: σ T (B) ~ σ T (0) / √ (1+ω 2 τ 2 ) [ω=eB/m] Gas properties can be calculated with MAGBOLTZ code

3 Spring, 2009Phys 521A3 Multiplication Mean free path for ionization, λ i, decreases exponentially with increasing E Inverse, α = 1/λ i, is 1 st Townsend coefficient Multiplication N = N 0 e αx in uniform E Gas amplification large for E >~ 20kV/cm (10 6 V/m) Note large range for each gas and large difference in threshold field for entering the amplification region

4 Spring, 2009Phys 521A4 Attachment, recombination Electronegative molecules (e.g., O 2, H 2 O, CF 4 ) 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

5 Spring, 2009Phys 521A5 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/cm 2, or ~0.06 cm in Argon gas Compare mean-free-path in Argon for electron ionization (inverse 1 st Townsend coefficient), ~0.01 cm in reasonable gas amplification fields Polyatomic molecules absorb UV photons; prevent (quench) spread of ionization in space (streamers)

6 Spring, 2009Phys 521A6 Positive ions Ion drift velocities << electron v d 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

7 Spring, 2009Phys 521A7 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 H 2 O) to address problems with aging; underlying physics (chemistry) not well understood In well-made chamber can collect few C/cm (for gain of 10 4 this corresponds to ~10 14 mips)

8 Spring, 2009Phys 521A8 Operating gas detectors: voltage Tunable parameter: field strength (f n of operating voltage in a given detector) Qualitatively different behavior vs. field In proportional region, signal ∝ n T, but gain depends strongly on field Geiger region yields large signals

9 Spring, 2009Phys 521A9 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)

10 Spring, 2009Phys 521A10 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

11 Spring, 2009Phys 521A11 MWPC resolution, efficiency Efficiency approaches 100% for MIPs traversing enough gas to liberate n T >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

12 Spring, 2009Phys 521A12 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

13 Spring, 2009Phys 521A13 Drift chamber operation Need careful control of drift velocity, since d ~ v d (t-t 0 ) –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

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