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High Precision Wire Chambers. at LHC. Historic remarks Wire chambers at LHC Precision tracking: ATLAS Muon Drift Tubes Precision timing: LHCb Muon Trigger Chambers. Werner Riegler, CERN. The Basic Objects. Tube Geiger- Müller, 1928. Multi Wire Geometry, in H. Friedmann 1949 .

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High Precision Wire Chambers

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High precision wire chambers l.jpg

High Precision Wire Chambers

at LHC

  • Historic remarks

  • Wire chambers at LHC

  • Precision tracking: ATLAS Muon Drift Tubes

  • Precision timing: LHCb Muon Trigger Chambers

Werner Riegler, CERN

W. Riegler/CERN

The basic objects l.jpg

The Basic Objects

Tube Geiger- Müller, 1928

Multi Wire Geometry, in H. Friedmann 1949

G. Charpak 1968

  • These geometries are widely used at LHC

  • Basic elements are unchanged since many years

  • Electronics has changed considerably

W. Riegler/CERN

historic remarks

Detector electronics 1925 l.jpg

Detector + Electronics 1925

were quite different from today

‘Über das Wesen des Compton Effekts’

W. Bothe, H. Geiger, April 1925

  • Bohr, Kramers, Slater Theorie:

  • energy is only conserved statistically

  • testing Compton effect

‘ Spitzenzähler ’

W. Riegler/CERN

historic remarks

Detector electronics 19254 l.jpg

Detector + Electronics 1925

‘Über das Wesen des Compton Effekts’, W. Bothe, H. Geiger, April 1925

  • ‘’Electronics’’:

    • Cylinders ‘P’ are on HV.

    • The needles of the counters are insulated and connected to electrometers.

  • Coincidence Photographs:

    • A light source is projecting both electrometers on a moving film role.

    • Discharges in the counters move the electrometers , which are recorded on the film.

    • The coincidences are observed by looking through many meters of film.

W. Riegler/CERN

historic remarks

Detector electronics 1929 l.jpg

Detector + Electronics 1929

‘were not very different from today’

‘Zur Vereinfachung von Koinzidenzzählungen’

W. Bothe, November 1929

Coincidence circuit for 2 tubes

Geiger Müller Tubes, 1928

W. Riegler/CERN

historic remarks

1930 1934 l.jpg

1930 - 1934

Cosmic ray telescope 1934

Rossi 1930: Coincidence circuit for n tubes

W. Riegler/CERN

historic remarks

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ATLAS Muon Chamber 2000

looks fairly similar to 1934

W. Riegler/CERN

historic remarks

Wire chambers at lhc l.jpg

Wire Chambers at LHC

  • Cloud Chambers, Bubble Chambers, Spark Chambers … have disappeared but wire chambers are still popular.

  • While the principle detecting element has changed very little, the readout electronics integration has changed dramatically.

  • Situation can be compared to astronomy. Telescope mirrors haven’t changed much but detecting elements (CCDs etc.) improved a lot.

W. Riegler/CERN

wire chamber at LHC

Atlas l.jpg


  • Cathode Strip Chambers:

    • h=2.54mm, s=2.54mm

    • 67k cathode channels

    • Ar/CO2/CF4

    •   60m

  • Thin Gap Chambers

    • h=1.4mm, s=1.8mm

    • 440k cathode and anode channels

    • n-Pentane /CO2 45/55

    • : 99% in 25ns with single plane

  • Monitored Drift Tubes

    • R=15mm

    • 370k anode channels

    • Ar/CO2 93/7

    •   80m

  • Transition Radiation Tracker

    • R=2mm

    • 372k anode channels

    • Xe/CO2/CF4 70/10/20

    •   150m

Other than that - Silicon and RPCs

W. Riegler/CERN

wire chamber at LHC

Slide10 l.jpg


  • Cathode Strip Chambers:

    • 2h=9.5mm, s=3.12mm

    • 211k anode channels for timing

    • 273k cathode channels for position

    • Ar/CO2/CF4 30/50/20

    •   75-150m

  • Rectangular ‘Drift Tubes’

    • w=42mm, h=10.5mm

    • 195k anode channels

    • Ar/CO2 85/15

    •   250m

Other than that - Silicon and RPCs

W. Riegler/CERN

wire chamber at LHC

Slide11 l.jpg


  • Cathode Strip Chambers:

    • h=2.5mm, s=1.5mm

    • 80k cathode and anode channels

    • Ar/CO2/CF4 40/50/10

    • t 3ns for two layers

  • Straw Tracker

    • R=2.5mm

    • 110k (51k) anode channels

    • Ar/CO2/CF4 75/10/15

    •   200m

Other than that - Silicon and RPCs

W. Riegler/CERN

wire chamber at LHC

Alice l.jpg


  • TPC with wire chamber

    • 1.25-2.5mm wire pitch

    • 2 - 3 mm plane separation

    • 570k Readout Pads

    • Ne/CO2 90/10

    •   1mm

W. Riegler/CERN

wire chambers at LHC

Slide13 l.jpg

Precision tracking: ATLAS muon system

Precision timing: LHCb muon system

W. Riegler/CERN

Precision tracking atlas muon drift tubes l.jpg

Precision Tracking: ATLAS Muon Drift Tubes

  • Magnetic field

    • Toroidal magnetic field (0.5T) provided by 8 superconducting coils.

  • Three muon stations

    • ~1200 chambers,

    • Outer diameter ~20m,

    • Sagitta measurement  muons with pT=1 TeV/c will show a sagitta of ~500 µm

    •  for 10% momentum resolution we need a sagitta measurement accuracy of ~50 µm

  • Monitored Drift Tubes (MDT)

    • each chamber consists of 23 (24) layers of drift tubes ( 3cm)

    • chamber deformations monitored with in-plane alignment system

precision tracking

Principle of operation l.jpg

Principle of Operation

  • Ionization

    • muon produces 100 clusters/cm with 2-3 e- (3 bars Ar/CO2 93/7)

  • Electron Drift

    • maximum drift-time ~800ns for baseline gas

  • Space-drift-time-relation  radius r

    • obtained by auto-calibration

W. Riegler/CERN

precision tracking

Drift chambers l.jpg

Drift Chambers

  • Auto-calibration of the rt-relation

    • start with good estimate for rt-relation

    • track fit  residual distribution

    • rt-relation corrected with the mean of the residual distribution

    • convergence after a few iterations

    • muon tracks with an angular spread (~10°) are used to avoid systematic errors

    •  rt-relation with a typical accuracy of 10µm

precision tracking

Requirements l.jpg


  • Parameters

    • resolution < 80µm for a single wire

    •  3cm, 370 000 channels

    • rates up to 500 Hz/cm2 (400 kHz/tube)

    • total charge 1C/cm in 10 years

  • Choice of gas

    • low diffusion, fast, linear, stand 1C/cm

    • Ar/N2/CH4 91/4/5: fast, linear, cannot stand 1C/cm

    • Ar/CO2 93/7: slow, nonlinear, however the only gas known to survive 1C/cm

      Therefore Ar/CO2 the ATLAS baseline gas !

W. Riegler/CERN

precision tracking

Single tube performance l.jpg

Single Tube Performance

for Ar/N2/CH4

NIMA 443(2000) 156-63

  • ‘typical’ signal shape

    • for tp=5 and tp=15ns

    • many ‘spikes’ due to clustering

    • tp=15ns makes the signals more ‘smooth’

  • Simulated and measured resolution

    • simulation and measurement match very well

    • close to the wire: primary ionization effects

    • far from the wire: diffusion

W. Riegler/CERN

precision tracking

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Single Tube Performance

for Ar/N2/CH4

NIMA 443(2000) 156-63

  • Effect of diffusion

    • increases with distance for the wire

  • Effect of charge deposit fluctuation

    • decreases the primary cluster and time slewing effects

W. Riegler/CERN

precision tracking

Single tube performance20 l.jpg

Single Tube Performance

for Ar/N2/CH4

NIMA 443(2000) 156-63

  • Higher gas gain

    • improves resolution (less slewing effects) but no possible due to:

    • aging

    • space charge effects (gain drop)

  • Amplifier bandwidth

    • tp=5ns to tp=15ns:

    • only 10um difference

    • tp=15ns is nicer to handle

W. Riegler/CERN

precision tracking

Rate capability gain drop l.jpg

Rate Capability, Gain Drop

NIMA 446(2000) 435-43

W. Riegler/CERN

precision tracking

Space charge effects l.jpg

Space Charge Effects

for Ar/CO2

  • In addition to gain drop: space charge changes the electric field

    • shift of the rt-relation

  • Variations of the drift field

    • constant space charge wouldn’t give a problem

    • the drift field for the electrons of one event are influenced only by neighboring ion clouds (slice of 1cm)

    • only a few background events influence the drift field (~6 at 1500Hz/cm)

    • this number of preceding events of importance is Poisson distributed

    • each event has a different drift field and hence a different rt-relation

    •  resolution deterioration

  • Strong gas dependence

    • linear gases: small effect

    • non-linear gases: dominating effect

W. Riegler/CERN

precision tracking

Single tube performance23 l.jpg

Single Tube Performance

for Ar/CO2

NIMA 446(2000) 435-43

NIMA 446(2000) 435-43

  • Resolution for low rate and 1.4kHz/cm

    • space charge effect deteriorates the resolution

    • the fluctuation of the space charge is responsible

    • calculation matches data very well

  • Effect of gas gain

    • at low rate higher gas gain improves the reoslution

    • at high rate the resolution decreases

W. Riegler/CERN

precision tracking

Atlas mdt front end electronics l.jpg

ATLAS MDT Front-End Electronics

3.18 x 3.72 mm

Single Channel Block Diagram

  • 0.5m CMOS technology

    • 8 channel ASD + Wilkinson ADC

    • fully differential

    • 15ns peaking time

    • 32mW/channel

    • JATAG programmable

Harvard University, Boston University

W. Riegler/CERN

precision tracking

Precision timing lhcb muon system l.jpg

Precision Timing: LHCb Muon System

  • A muon trigger is given by a coincidence of all 5 muon stations within 25ns

  • >99% efficiency/station in 20ns time window

  • Time resolution <3ns

  • Up to 1MHz/cm2

  • 50% Wire Chambers(MWPCs)

  • 50% RPCs (<1kHz/cm2)

W. Riegler/CERN

precision timing

Segmentation l.jpg


quadrant of a single station

  • Segmentation in station 2

    • R4: 5 x 25 cm2

    • R3: 2.5 x 12.5 cm2

    • R2: 1.25 x 3.15 cm2

    • R1: 0.63 x 3.1 cm2

  • Segmentation achieved by

    • connecting wires  Wire Pad

    • segmenting cathode  Cathode Pad

    • limit to segmentation of cathode pads comes from crosstalk due to direct induction (1-2cm in this case)

s=1.5mm, 2h=5mm, developed by PNPI

W. Riegler/CERN

precision timing

Single station l.jpg

Single Station

  • One station consists of 4 gaps forming a single chamber element

  • Two independent front end channels per station


W. Riegler/CERN

precision timing

Cathode pad readout structure l.jpg

Cathode Pad Readout Structure

  • Cathode signals are guided to the chamber side with traces on the bottom of the PCBs.

  • Danger of capacitive crosstalk due to high amplifier bandwidth (tp=10ns).

  • Input resistance must be lower than 50

  • Traces were carefully designed in order to minimize crosstalk (MAXWELL).


W. Riegler/CERN

precision timing

Slide29 l.jpg

Full size prototype of inner region (close to beam-pipe)


W. Riegler/CERN

Detector parameters l.jpg

Detector Parameters

  • Parameters

    • 5mm gas gap

    • 30 m wire

    • 1.5mm wire pitch

    • Readout pads on 1.6mm G10

  • Operating point

    • Ar/CO2/CF4 40/50/10

    • 3150V on wire

    • Gain 105

    • 8kV/cm on cathode, 260kV/cm on wire

  • Gas parameters

    • 21.4 clusters in 5mm for 10 GeV muon (Heed)

    • v  90m/ns (8kV/cm, Magboltz)

    • Proportional mode

    • Average total charge induced on cathode = 0.37pC (gain=105)

    • total avalanche charge=0.74pC

W. Riegler/CERN

precision timing

Performance of a single gap l.jpg

Performance of a Single Gap


  • Intrinsic time resolution 3ns

  • optimum amplifier peaking time 10ns

W. Riegler/CERN

precision timing

Efficiency and time resolution l.jpg

Efficiency and Time Resolution

Double Gap

Efficiency and time resolution vs. HV

Efficiency and time resolution vs. threshold


W. Riegler/CERN

precision timing

Comparison with garfield l.jpg

Comparison with GARFIELD

  • Full Simulation

    • primary ionization (HEED)

    • drift, diffusion (MAGBOLTZ)

    • induced signals (GARFIELD)

  • no parameters to tune

  • we understand our detector

W. Riegler/CERN

precision timing

Measurement of edge effects l.jpg

Measurement of Edge Effects


‘chamber ends’ 1 gap size from first obstacle

‘chamber ends’ last wire

W. Riegler/CERN

precision timing

Front end electronics carioca l.jpg

Front End Electronics ‘CARIOCA’

3x4mm prototype, (2.5x3mm final)

  • 0.25m CMOS technology

    • chip is under development

    • 8 channel ASD

    • fully differential

    • 10ns peaking time

    • 30mW/channel


W. Riegler/CERN

precision timing

Pulse shaping l.jpg

Pulse Shaping

results from 3 prototype chips on a chamber


  • detector signal has 1/(t+1.5ns) tail

  • dead time leads to inefficiency

  • shaping circuit for tail cancellation

  • prototype shows <50ns average dead time at the working point

W. Riegler/CERN

precision timing

Conclusions l.jpg


  • Wire chambers will be widely used at LHC experiments for tracking and triggering.

  • Ar/CO2/CF4 gas mixtures are used because of their good aging properties.

  • Position resolutions of 80 m per single tube and 5ns per single MWPC layer are expected even for large systems.

  • The basic detector elements haven’t changed much, but the front-end electronics integration is progressing fast.

  • The long experience with wire chambers and the fact that one can calculate and predict their behavior very accurately makes this detector a competitive candidate also for future experiments.

W. Riegler/CERN

Detector simulation l.jpg

Detector Simulation

  • Garfield (Rob Veenhof)

    • electric fields, particle drift, induced signals, electronics ….

  • Magboltz (Steve Biagi)

    • transport properties of gas mixtures

  • Heed (Igor Smirnov)

    • charge deposit of fast particles in gas mixtures

  • Very reliable simulation of all the chamber and signal processes

W. Riegler/CERN

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