1. High Precision Wire Chambers
Feb. 2002
High Precision Wire Chambers
at LHC
Werner Riegler, CERN
Historic remarks
Wire chambers at LHC
Precision tracking: ATLAS Muon Drift Tubes
Precision timing: LHCb Muon Trigger Chambers
W. Riegler/CERN

2. The Basic Objects Tube Geiger- Müller, 1928
Feb. 2002
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

3. Detector + Electronics 1925
Feb. 2002
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

4. Detector + Electronics 1925
Feb. 2002
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

5. Detector + Electronics 1929
Feb. 2002
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

6. Feb. 2002
Cosmic ray telescope 1934
Rossi 1930: Coincidence circuit for n tubes
W. Riegler/CERN
historic remarks

7. ATLAS Muon Chamber 2000 looks fairly similar to 1934 Feb. 2002
W. Riegler/CERN
historic remarks

8. Feb. 2002
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

9. ATLAS Other than that - Silicon and RPCs Feb. 2002
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

10. CMS Other than that - Silicon and RPCs Feb. 2002
Rectangular ‘Drift Tubes’
w=42mm, h=10.5mm
195k anode channels
Ar/CO2 85/15
  250m
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
  m
Other than that - Silicon and RPCs
W. Riegler/CERN
wire chamber at LHC

11. LHCb Other than that - Silicon and RPCs Feb. 2002
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

12. ALICE Feb. 2002 TPC with wire chamber W. Riegler/CERN
mm wire pitch
2 - 3 mm plane separation
570k Readout Pads
Ne/CO2 90/10
  1mm
W. Riegler/CERN
wire chambers at LHC

13. Precision tracking: ATLAS muon system
Feb. 2002
Precision tracking: ATLAS muon system
Precision timing: LHCb muon system
W. Riegler/CERN

14. 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

15. Principle of Operation
Feb. 2002
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

16. 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

17. Requirements Parameters Choice of gas
Feb. 2002
Requirements
Parameters
resolution < 80µm for a single wire
 3cm, 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

18. Single Tube Performance
Feb. 2002
Single Tube Performance
for Ar/N2/CH4
NIMA 443(2000)
‘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

19. Single Tube Performance
Feb. 2002
Single Tube Performance
for Ar/N2/CH4
NIMA 443(2000)
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

20. Single Tube Performance
Feb. 2002
Single Tube Performance
for Ar/N2/CH4
NIMA 443(2000)
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

21. Rate Capability, Gain Drop
Feb. 2002
Rate Capability, Gain Drop
NIMA 446(2000)
W. Riegler/CERN
precision tracking

22. Feb. 2002
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

23. Single Tube Performance
Feb. 2002
Single Tube Performance
for Ar/CO2
NIMA 446(2000)
NIMA 446(2000)
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

24. ATLAS MDT Front-End Electronics
Feb. 2002
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

25. Precision Timing: LHCb Muon System
Feb. 2002
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

26. Segmentation quadrant of a single station Segmentation in station 2
Feb. 2002
Segmentation
quadrant of a single station
Segmentation in station 2
R4: x 25 cm2
R3: 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

27. Feb. 2002
Single Station
One station consists of 4 gaps forming a single chamber element
Two independent front end channels per station
LHCb, CERN
W. Riegler/CERN
precision timing

28. Cathode Pad Readout Structure
Feb. 2002
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).
LHCb, CERN
W. Riegler/CERN
precision timing

29. Full size prototype of inner region (close to beam-pipe)
Feb. 2002
Full size prototype of inner region (close to beam-pipe)
LHCb, CERN
W. Riegler/CERN

30. Detector Parameters Parameters Gas parameters Operating point
Feb. 2002
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 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

31. Performance of a Single Gap
Feb. 2002
Performance of a Single Gap
LHCb, CERN
Intrinsic time resolution 3ns
optimum amplifier peaking time 10ns
W. Riegler/CERN
precision timing

32. Efficiency and Time Resolution
Feb. 2002
Efficiency and Time Resolution
Double Gap
Efficiency and time resolution vs. HV
Efficiency and time resolution vs. threshold
LHCb, CERN
W. Riegler/CERN
precision timing

33. Comparison with GARFIELD
Feb. 2002
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

34. Measurement of Edge Effects
Feb. 2002
Measurement of Edge Effects
LHCb, CERN
‘chamber ends’  1 gap size from first obstacle
‘chamber ends’  last wire
W. Riegler/CERN
precision timing

35. Front End Electronics ‘CARIOCA’
Feb. 2002
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
UFRJ Rio, CERN
W. Riegler/CERN
precision timing

36. Pulse Shaping results from 3 prototype chips on a chamber Feb. 2002
LHCb, CERN
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

37. Feb. 2002
Conclusions
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

38. Detector Simulation Garfield (Rob Veenhof) Magboltz (Steve Biagi)
Feb. 2002
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