1. Pre-installation Tests of the LHCb Muon Chambers
Outline:
Overview of the LHCb Muon System
Chamber Design and Performance
Optimization of threshold finding and setting
Tests with cosmics and under large particle flux
Burkhard Schmidt, CERN On behalf of the LHCb Muon Group
22/10/2008
IEEE NSS 2008, Dresden

2. The LHCb Muon Detector Purpose: Main Requirement:
Identification of muons
Triggering on muons produced in the decay of b-hadrons by measuring PT
M1 M2 M3 M4 M5
5 Muon stations, one in front
and 4 behind the calorimeters
435m2 of detector area
with 1380 chambers
Hadron Absorber of 20 
thickness in total
Main Requirement:
-> A muon trigger requires the coincidence of hits in all 5 stations within a bunch crossing (25ns) in a region of interest that selects the muon PT
22/10/2008
IEEE NSS 2008, Dresden

3. Muon Detector Layout Chamber
Space point measurement
4 Regions with granularity
variable with distance from
beam axis
- 0.6cm x 3.1cm in Region 1,
5cm x 25cm in Region4
Particle flux varies between
- 0.5 MHz/cm2 (M1 Region 1)
- 0.2 kHz/cm2 (M5 Region 4)
Chamber
Detector Technology : - MWPC will be used in most regions
- triple GEMs are used in station M1 R1
Electronics: - System has 126k FE-channels,
reduced to 26k readout channels
22/10/2008
IEEE NSS 2008, Dresden

4. MWPC Readout Types Region 4: Region 3: Region 1+2:
Anode wire readout
Region 3:
Cathode pad readout
Region 1+2:
(in stations M2+M3)
Combined anode and cathode readout
-> Different readouts and variations
in input capacitance pose special requirements on FE-chip
22/10/2008
IEEE NSS 2008, Dresden

5. MWPC Requirements and Layout
High rate capability of chambers -> up to 0.2 MHz/cm2
Good ageing properties of detector components:
-> up to 1.6 C/cm2 on cathodes and 0.3 C/cm on wires
Detector instrumentation with good redundancy and time resolution
-> each chamber has (generally) 4 layers
-> 99% station efficiency in 20ns time window
2 independent ASD
ORed on FE Board
(similar ORing scheme
also applies to cathode
pads) HV HV HV HV
Layout: - Gas mixture: Ar/CO2/CF4 40/55/5
- Pitch: 2 mm - HV: V
- Gap: 5 mm - Gain: 7x104
- Wire: 30 µm - Total avalanche charge: 0.4pC
22/10/2008
IEEE NSS 2008, Dresden

6. Chamber Performance Double gap chamber:
95% efficiency if the threshold
is set to ~30% of the average signal.
99% efficiency if the threshold is
set to ~20% of the average signal.
In order to have a double gap
chamber well within the plateau we
want to be able to use a threshold of
~15% of the average signal.
Full simulation:
- Primary ionization (HEED)
- Drift, Diffusion (MAGBOLTZ)
- Induced Signals (GARFIELD)
-> Good agreement, we understand our detector
22/10/2008
IEEE NSS 2008, Dresden

7. Front-End Electronics
‘CARIOCA’ FE-chip:
8 channel ASD in O.25um CMOS technology
Peaking time: ~ 10ns
Rin: < 50 
Cdet : pF
Noise: ~ 2000fC + 42e/pF
Rate: up to 1MHz
Pulse width: < 50ns
UFRJ Rio, CERN
22/10/2008
IEEE NSS 2008, Dresden

8. Initial chamber tests Measurement of Detector Capacitance: pad number
Cdet for this chamber
type: 50 ± 10 pF
Variations in Cdet are
due to differences
in trace length to
bring out the signals.
22/10/2008

9. Threshold finding and setting
Challenges:
Large variation of Cdet due to different chamber types
Variations in FE-channel offsets
Bias due to discriminator
circuitry.
Extraction of ENC:
Possible from the centered and
linearized noise distribution
obtained from threshold scans.
Use the fundamental frequency
of 25 MHz as reference point,
which is related to the bandwidth
of the CARIOCA chip.
22/10/2008
IEEE NSS 2008, Dresden

10. Threshold finding and setting
Extraction of Equivalent Noise Charge ENC:
f – rate at threshold Qth
f0 =25MHz - rate at zero threshold
y0=7.4= log10f0
22/10/2008
IEEE NSS 2008, Dresden

11. Threshold finding and setting
Example:
FE-channel of a
M5R1 chamber
with Cdet of 120pF
Log (rate[Hz])
Th [r.u.]2
Slope
ENC [r.u.]
ENC [fC]
elect.
e/pF
0.0134
4.03
0.81
5054
42.3
Thr (n=6)
24.15
4.8
30321
At a gas gain of 7x104
this corresponds to 4.3 p.e.
optimal value for good time resolution
Capacitive noise term as expected
from CARIOCA design parameters
ENC = 2000e + 42e/pF
22/10/2008
IEEE NSS 2008, Dresden

12. Tests with High Voltage
Low noise count rate with both double gaps in OR
and a threshold of 5fC and HV 2550V (nominal HV)
Cathodes
Cdet ~115pF
Anodes
Cdet ~90pF
3Hz
22/10/2008
IEEE NSS 2008, Dresden

13.  Rate per channel reflects profile of wire strip width
Tests with Cosmics
Integrated count rate for Cosmics with both double gaps in AND (self-triggering mode)
Normally 7 wires per strip, but sometimes there are 6 and 9
M3R2 26A HV=2550V TH=5fC
 Rate per channel reflects profile of wire strip width
22/10/2008
IEEE NSS 2008, Dresden

14. ~300 V operational HV-plateau
Tests with Cosmics
HV scan in self-triggering mode (both double gaps in AND)
Threshold on FE: 5fC (cathode readout)
~300 V operational HV-plateau
~5% cross-talks at 2650V HV
Scan of HV on 1st double gap with fixed HV on the
2nd double gap (and vice versa)
22/10/2008
IEEE NSS 2008, Dresden

15. Tests with Cosmics HV scan in self-triggering mode.
Threshold on FE: 10fC (wire readout)
 Increase in trigger rate due to cross-talk according to expectations (about 5% 50V above knee)
slope ~ cross-talk
~20%
22/10/2008
IEEE NSS 2008, Dresden

16. Tests with large Particle Flux
Gamma irradiation Facility at CERN:
137 Cs source Eγ =662 keV Strength = 600 GBq
Procedure:
Measure current and count rate for various attenuations
22/10/2008
IEEE NSS 2008, Dresden

17. Tests with large particle flux
The good linearity confirms that neither saturation
nor space charge effects are present with particle
fluxes up to 160kHz/pad.
22/10/2008
IEEE NSS 2008, Dresden

18. Conclusions
The LHCb Muon chambers can be operated at a threshold of about 5fC (cathode readout) and
10fC (anode readout).
Important ingredient to operate the chambers also at lowest possible High Voltage to minimize detector ageing and have good cross-talk performance
Good chamber performance at low threshold has been confirmed in cosmic test and under high particle flux
Details about the LHCb Muon System commissioning will be presented in one of the following talks.
22/10/2008
IEEE NSS 2008, Dresden