Conditioning of MWPCs for the LHCb Muon System Katharina Mair for J.-S. Graulich, H.-J. Hilke, A. Kachtchouk, K. Mair, B. Schmidt, T. Schneider LHCb Muon CERN, Switzerland Contents: MWPCs in the LHCb Muon System MWPC Conditioning Conclusion
Dec 14, 2015
Conditioning of MWPCs for the LHCb Muon System
Katharina Mair for
J.-S. Graulich, H.-J. Hilke, A. Kachtchouk, K. Mair, B. Schmidt, T. Schneider
LHCb Muon CERN, Switzerland
Contents: MWPCs in the LHCb Muon System MWPC Conditioning Conclusion
LHCb Muon CERN 2/13 IEEE, October 2005
5 Muon Stations
Calorimeters
Tracker RICH-2
Vertex Locator
Magnet
RICH-1
Muon Detectors
Iron Filters Chamber Dimension
MWPCs in the LHCb Muon System
Multi Wire Proportional Chambers (MWPCs): Fast muon triggering Muon identification
5 Muon Stations, 4 Regions / Station 20 different chamber sizes 1368 chambers
LHCb Muon CERN 3/13 IEEE, October 2005
MWPC Design 4-gap MWPC gap size: 5 mm (wire plane centered) gas mixture: Ar/CO2/CF4 (40:55:5) wire: Gold-plated Tungsten, 30 μm Ø, 250 to
310 mm wire length wire spacing: 2 mm, mechanical tension: 65 gr HV = 2.650 kV field on wires: 262 kV/cm field on cathodes 6.2 kV/cm gas gain: G ≈ 50 000 gain uniformity: ≤ 30%
Not wired panel
Wired panel
Side bar
Wire fixation bar
Spacer
MWPC Sandwich
panel production: PCB coated by 35 μm copper, 5
μm nickel, 0.2 μm gold foam injected between 2 PCBs in
mould
LHCb Muon CERN 4/13 IEEE, October 2005
time resolution
MWPC Requirements fully efficient and robust level-0 high pt muon
trigger: 5-fold coincidence of at least 1 hit within 20ns in
each station high efficiency and time resolution:
high efficiency (>99%) per station requires a time resolution of ~4ns
achieved by use of 2-fold OR (double-gaps)
high rate capability: up to 0.2 MHz/cm2 for inner chambers at a
luminosity of L = 5 x 1032 cm-1 s
ageing resistance over 10 years: the accumulated charge < 1C/cm
spatial resolution: we require a pt resolution < 20%,
therefore we need a spatial resolution in the order of 1 cm in bending plane (6-30 mm)
LHCb Muon CERN 5/13 IEEE, October 2005
Chamber Conditioning
General rule: In order not to damage any surfaces (wire and cathode)
high currents must be avoided ! (nominal dark current: ~ 3 nA)
impurities on wire
Cleaning Effects due to: positive ion bombardment on cathode negative ion and electron bombardment on anode
Results: surfaces smoothened current reduction
Motivation for Conditioning: by stepwise applying positive HV on anode wire for the first time, we have difficulties
to reach operating point within safe current region < 50 nA we assume impurities on the anode wire and on the flat cathode surface responsible
for self-sustaining discharge under positive HV Conditioning Procedure:
Step 1: Inversed HV-Conditioning: negative HV up to -2300 V at anode wire Step 2: Normal HV-Conditioning: positive HV up to +2900 V at anode wire
LHCb Muon CERN 6/13 IEEE, October 2005
Step 1:Negative HV Conditioning
Procedure: applying negative HV to wire stepwise increasing HV up to -2300 V
Effect: possibly electron emission from metallic tips on wire surface positive ion bombardment on wire surface
→ E-field reduction due to wire surface smoothening Advantage:
safe procedure: weak avalanche effect at this HV range ( at -2300 V: gas gain ~100) Results:
at each HV step: fast current reduction (10 μA->100 nA) within 10 minutes mean conditioning time at -2300 V: 45 hours after this: positive HV = 2900 V is quickly reached (within 15 minutes) with a dark
current level of 3 nA
time result:
current reduction
10 min
LHCb Muon CERN 7/13 IEEE, October 2005
Step2: Positive HV Conditioning Motivation: High Rate Capability of chambers is tested at GIF
all chambers of inner regions (highest particle flux up to 0.2 MHz/cm2 ) are exposed to high gamma radiation at the Gamma Irradiation Facility (GIF) at CERN for testing.
137Cs source (photons 660 keV) similar to LHC background radiation Outcome:
in ~ 20% of all tested gaps Malter-like emission detected
Malter effect: Thin Film Field Emission (L. Malter, Phys. Rev. 50, 1936)
thin insulator film on cathode surfaces charged up by GIF irradiation
high electric field starts e- -emission
(from: J. Va’vra, NIM A367, 1995)
8 gaps
time
LHCb Muon CERN 8/13 IEEE, October 2005
Chemistry: cathode panel production: mould release agent ACMOIL36-4600 contains long C-chains
(Isoalcine C9-C12) with silicone (5-10%) → indication for a remaining insulating film on surface
panels are cleaned by hand (Isopropyl alcohol, 4-Methyl-Pentanol, n-Hexane, demineralized water), problematic region for dirt: between cathode pads
Treatment at GIF: positive HV applied in steps of 50-100 V for the range of 2.2 kV – 2.75 kV high gamma ray irradiation leads to charge up of insulator spots
→ e--emission → positive ions F-radicals are created due to 5% CF4 in gas mixture remove Si by creating SiF4 molecules, that are volatile and will be removed by gas flux
measured current reduction in gap2/M3R1#03, HV=2.75kV
y = 1.1e-0.21x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40
Time (hours)
I/I_
initia
l
Time const = 5 hours
Results: exponential current reduction conditioning time: 0.5 – 70 hours
Draw Back: molecular bonds may recover
when irradiation or HV removed
→ currents return, but decay is faster
LHCb Muon CERN 9/13 IEEE, October 2005
Test without CF4: gas mixture: Ar/CO2(40:60);
HV=2.75 kV current reduction slow remaining current level still too high [μA] HV trips observed
Test with 5% CF4: gas mixture: Ar/CO2/CF4(40:55:5);
HV=2.75 kV current reduction faster remaining current level excellent [~2 nA] no HV trips
I/I_
initi
al
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60 70 80 90 100time [h]
I/I_
initi
al
0.0000.1000.2000.3000.4000.5000.6000.7000.8000.9001.000
0 20 40 60 80time [h]
0
500
1000
1500
2000
2500
3 1 9 2 0 8 3 4 0 0 3 1 9 2 0 8 3 4 5 0 3 1 9 2 0 8 3 5 0 0 3 1 9 2 0 8 3 5 5 0 3 1 9 2 0 8 3 6 0 0 3 1 9 2 0 8 3 6 5 0
Time (10 minutes)
Cu
rren
t (n
A)
Gap3/M3R2#26curr
en
t [n
A]
time10 min
0
1
2
3
4
5
3192343100 3192343200 3192343300 3192343400 3192343500
Gap1/M3R2#26 2 3 4
curr
en
t [n
A]
time10 min
same chamber
LHCb Muon CERN 10/13 IEEE, October 2005
Emission Physics: first attempts to combine Malter-effect with field emission (Fowler-
Nordheim) can be found in literature (A. Boyarski, NIM A535, 2004)
For our MWPCs: high currents most probably due to 2 effects: electron emission from metallic tips on surfaces thin film field emission → most likely from area between cathode pads
For our measurements: measure I, V include gas gain G(V)
depending on V
approximation:
Anode (Gold-plated Tungsten wires)
Cathode pad(Gold-coated Copper foil)
Positive ions
+HVElectrons
R
I1(t)
Thin film between cathode padsElectron emission
E0
EActual=E0+EFilm
]/[1043.5
exp)(104.5 210
25 mAE
EJ
)(VGJI
V
bVG
a
V
I 1)(
1.1loglog
*
2/32*
2
y= =x
ß… field enhancement factorE… electric field in [V/m] for a gold-coated wire
→ field factor ß* estimated from the slope
LHCb Muon CERN 11/13 IEEE, October 2005
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
2500 2550 2600 2650 2700 2750 2800
HV (V)
curr
en
t [n
A]
The original I(V) relation
Measurments:
repeated measurements on the same chamber show rotation of straight lines with increasing slopes
Result: E-field decreasing
→ surface cleaned conditioning time: up to
500 hours (20 days)
log(I/V^2) vs 1/V
-7-6.5
-6-5.5
-5-4.5
-4-3.5
-3
0.0
003
0.0
00325
0.0
0035
0.0
00375
0.0
004
0.0
00425
0.0
0045
0.0
00475
0.0
005
1/V (V^-1)
log
(nA
/V^
2) Measured at t=0
50 hours at GIF=ON
70 hours
M3R2#04 (Gap2)
log
(I /
V2 )
[n
A /
V]
1/V [V-1]
1E-13
1E-12
1E-11
0.00036 0.000365 0.00037 0.000375 0.00038 0.000385 0.00039 0.000395 0.0004
1/V
log
( I /
V2
)I
/ V
2
logarithmic scale:
LHCb Muon CERN 12/13 IEEE, October 2005
Summary
MWPC Conditioning in 2 steps:
Step1: negative HV on wire apply negative HV up to 2300 V check current at positive HV=2900 V:
If I < 3 nA → Step 2 If I > 3 nA → repeat Step1
Step 2: positive HV on wire under High Gamma Ray Irradiation stepwise increase positive HV on wire for the range of 2.2 kV – 2.75 kV at each step of 50-100 V switch source off to check current:
If I < 10 μA → increase HV If I > 10 μA → repeat irradiation at same HV
LHCb Muon CERN 13/13 IEEE, October 2005
Conclusion
Achieved excellent conditioning result by applying 2 steps: Inversed HV Conditioning Normal HV Conditioning under High Gamma Ray Irradiation
Effects are: successful wire cleaning successful cathode cleaning
→→ We can assume that tWe can assume that the observed anomalous currents are self-he observed anomalous currents are self-suppressed during MWPC operation (high background radiation)suppressed during MWPC operation (high background radiation)
→ Therefore we are optimistic, that these currents will not be a problem for Therefore we are optimistic, that these currents will not be a problem for the long term operation of the LHCb Muon Systemthe long term operation of the LHCb Muon System