Drift Chambers: The Science behind the Art

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Drift ChambersThe 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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there are alternatives!

• Micro-pattern gas detectors
• No wires to break, accurate patterns, fast ion
  clearing, anode at ground
• Ideal for TPCs not as uniform as wires
• Less multiple scattering than Silicon
• Multi-GEMs -gt less ion feedback
• more stable at same gain
• shape of dielectric important
• Micro-megas w/ resistive anodes
• -gt competitive with GEMs
• Flexible readout schemes !

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• Monolithic pixel detectors

from Bellazinis talk
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New Types of Silicon Trackers
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so why drift chambers ?

- economical way to cover a large volume with
tracking chambers
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gas ionization by particles
cosmic ray
tube gas wire (at positive high-voltage)
1 ionization/ 300 mm 1 - 10 electrons /
ionization
100 electrons/cm
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drifting of the electrons
wire at positive voltage

• electrons drift to the wire
• strike a molecule every 2 mm
• velocity 50 mm/ns (max)

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the avalanche
close-up of wire
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the avalanche (cont.)
gain 2 ndbl ndbl (rcrit-rwire)/mfp Ecrit k
V/ rcrit gain 2 kV rule of thumb gain
doubles every 75 or 100 Volts
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all-wire drift chamber
wires in layers brick-wall fashion
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how tracking works
hit wires shown in yellow
minimize rms between track and calculated distance
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drift velocity calibration
distance (cm)
time (ns)
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Installing Pre-tensioning Wires
Pre-tensioning - before we start stringing - use
springs on guard wires - gradual release of
tension
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• Operating successfully for 10 years .

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FAQs (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