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Drift Chambers: The Science behind the Art

Drift Chambers: The Science behind the Art. What are they? how do they work? ionization of the gas by particles “drifting” of the electrons the “avalanche” at the wire how tracking works frequently asked questions ( your part!). the motivation (you get to go to Vienna !).

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Drift Chambers: The Science behind the Art

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  1. Drift Chambers:The Science behind the Art • What are they? how do they work? • ionization of the gas by particles • “drifting” of the electrons • the “avalanche” at the wire • how tracking works • frequently asked questions (your part!) the motivation (you get to go to Vienna !) What’s a Drift Chamber? Mac Mestayer

  2. there are alternatives! • Micro-pattern gas detectors • No wires to break, accurate patterns, fast ion clearing, anode at ground • Ideal for TPC’s; not as uniform as wires • Less multiple scattering than Silicon • Multi-GEM’s -> less ion feedback • more stable at same gain • shape of dielectric important • Micro-megas w/ resistive anodes -> competitive with GEM’s • Flexible readout schemes ! What’s a Drift Chamber? Mac Mestayer

  3. Monolithic pixel detectors from Bellazini’s talk What’s a Drift Chamber? Mac Mestayer

  4. New Types of Silicon Trackers What’s a Drift Chamber? Mac Mestayer

  5. so why drift chambers ? $ - economical way to cover a large volume with tracking chambers What’s a Drift Chamber? Mac Mestayer

  6. gas ionization by particles cosmic ray tube gas wire (at positive high-voltage) ~1 ionization/ 300 mm 1 - 10 electrons / ionization ~ 100 electrons/cm What’s a Drift Chamber? Mac Mestayer

  7. “drifting” of the electrons wire at positive voltage • electrons drift to the wire • strike a molecule every 2 mm • velocity ~ 50 mm/ns (max) What’s a Drift Chamber? Mac Mestayer

  8. wire radius at which E = i / mfp i ~ 20 eV mfp ~ 2 mm Ecrit ~ 200 kV/cm every mfp the number of electrons doubles here comes the electron! the “avalanche” close-up of wire What’s a Drift Chamber? Mac Mestayer

  9. wire radius at which E = i / mfp i ~ 20 eV mfp ~ 2 mm Ecrit ~ 200 kV/cm every mfp the number of electrons doubles here comes the electron! the “avalanche” (cont.) gain ~ 2 ndbl ndbl = (rcrit-rwire)/mfp Ecrit = k * V/ rcrit gain ~ 2 k*V rule of thumb: gain doubles every 75 or 100 Volts What’s a Drift Chamber? Mac Mestayer

  10. “all-wire” drift chamber wires in layers “brick-wall” fashion F What’s a Drift Chamber? Mac Mestayer

  11. how tracking works hit wires shown in yellow minimize rms between track and calculated distance What’s a Drift Chamber? Mac Mestayer

  12. Drift Velocity Calculation 20 mm wire 2325 V 88:12 AR:CO2 30 mm wire 2475 V 92:08 AR:CO2 same gain 58% faster - and more linear ! What’s a Drift Chamber? Mac Mestayer

  13. drift velocity calibration distance (cm) time (ns) What’s a Drift Chamber? Mac Mestayer

  14. What’s a Drift Chamber? Mac Mestayer

  15. Installing Pre-tensioning Wires Pre-tensioning - before we start stringing - use springs on guard wires - gradual release of tension What’s a Drift Chamber? Mac Mestayer

  16. Operating successfully for ~10 years …. What’s a Drift Chamber? Mac Mestayer

  17. FAQ’s (your turn!) efficiency vs. noise trade-off drift time calibration Malter effect magnetic field effects fast gas cathode emission material choices: wires, endplates quenching gas reference books HV plateau What’s a Drift Chamber? Mac Mestayer

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