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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
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

there are alternatives
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

slide3
Monolithic pixel detectors

from

Bellazini’s

talk

What’s a Drift Chamber? Mac Mestayer

new types of silicon trackers
New Types of Silicon Trackers

What’s a Drift Chamber? Mac Mestayer

so why drift chambers
so why drift chambers ?

$

- economical way to cover a

large volume with tracking chambers

What’s a Drift Chamber? Mac Mestayer

gas ionization by particles
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

drifting of the electrons
“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

the avalanche

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

the avalanche cont

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

all wire drift chamber
“all-wire” drift chamber

wires in layers

“brick-wall” fashion

F

What’s a Drift Chamber? Mac Mestayer

how tracking works
how tracking works

hit wires

shown in

yellow

minimize rms between

track and calculated distance

What’s a Drift Chamber? Mac Mestayer

slide12

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

drift velocity calibration
drift velocity calibration

distance

(cm)

time (ns)

What’s a Drift Chamber? Mac Mestayer

installing pre tensioning wires
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

slide16
Operating successfully for ~10 years ….

What’s a Drift Chamber? Mac Mestayer

faq s your turn
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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