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Drift Chambers

Drift Chambers. AMY experiment at e + e - TRISTAN collider CDF experiment. B = 3 T B = 1.4 T. E-field, Drift-Time Relation. A charged particle enters gas in E-field, Ionizes gas, produces e - -ion pairs ~300 m m / pair

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Drift Chambers

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  1. Drift Chambers AMY experiment at e+e- TRISTAN collider CDF experiment B = 3 T B = 1.4 T Nigel Lockyer / Young-Kee Kim

  2. E-field, Drift-Time Relation A charged particle enters gas in E-field, Ionizes gas, produces e--ion pairs ~300 mm / pair Ionization E ~30 eV / pair Primary ionization electrons drift toward anode (sense) wire (low E field region) Avalanche multiplication of charges by electron-atom collision in high E field region - within a few radii of the wire. Signal induced via motion of charges. Measure “drift time”, Dtdrift, (first arrival time) of electrons at sense wire relative to a time t0 (e.g. collision time) Locate the position of “first” electrons D = ∫t0t0 + Dt v(t) dt D = v Dtdrift if the drift velocity is independent of E field. drift time e-- ion pairs Q t E p+ r cathode gas anode (sense) avalanche Nigel Lockyer / Young-Kee Kim

  3. Left-Right Ambiguity, B field Left-right ambiguity solved by staggering Apply Magnetic field to measure momentum Measure curvature, C = 1 / R P = 0.3 RB B out of page Nigel Lockyer / Young-Kee Kim

  4. B-fields, Lorentz Angle Magnetic fields complicate the drift-time relation Introduce the drift angle (Lorentz angle). COT Argon: Ethane = 50:50 Lorentz angle of ~300 Tilt cells e-- ion pairs p+ Nigel Lockyer / Young-Kee Kim

  5. Choice of Gas (desired property) Drift velocity independent of E field Good linear distance-time correlations. v ~ 50 mm / ns Low working voltage High gain operation Small diffusion coefficient for electrons traveling though the gas. Amount of diffusion that occurs limits the spatial resolution obtainable Argon is widely used as a drift chamber gas. The argon may be mixed with gases consisting of heavy organic molecules (“quenchers”). Quenchers - heavy organic molecules that have many degrees of freedom and can efficiently absorb energy from the gas. The effective temperature of the electron is reduced. The drift velocity is increased. The diffusion is decreased. Carbon dioxide, isobutane, … Avoid elements with strong affinity of electrons. Otherwise the detection efficiency will depend on the drift distance. Avoid O2, H2O, F, Cl Nigel Lockyer / Young-Kee Kim

  6. Signal formation, dE/dx • What happens • Charged track traverses the cell and deposits a number of clusters-Poisson distributed • Clusters are distributed along the track trajectory uniformly • The number of electrons per cluster follows a particular distribution, peaked at about one, mean about 3 in Argon, but with along tail-Landau like tail • Drift electrons in along field lines-drift time depends on gas, field, trajectory to the wire • Avalanche takes place near wire-Polya distribution • Signal forms from electron pulse (10%) and ion drift to cathode-preamplifier cuts this off-pulse shape formed Nigel Lockyer / Young-Kee Kim

  7. Drift time contours (isochrones) Nigel Lockyer / Young-Kee Kim

  8. Electrons per Cluster Nigel Lockyer / Young-Kee Kim

  9. Polya Distribution Nigel Lockyer / Young-Kee Kim

  10. Signal Formation Nigel Lockyer / Young-Kee Kim

  11. Cluster Time Distribution Nigel Lockyer / Young-Kee Kim

  12. Charge versus Tstart Nigel Lockyer / Young-Kee Kim

  13. Charge versus Width Nigel Lockyer / Young-Kee Kim

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