Transcript Conditioning of MWPCs for the LHCb Muon System

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
MWPCs in the
LHCb Muon System
Calorimeters
Tracker
RICH-2
Muon
Detectors
Chamber Dimension
Iron Filters
Magnet
RICH-1
Vertex
Locator

Multi Wire Proportional Chambers
(MWPCs):

Fast muon triggering
 Muon identification

5 Muon Stations
5 Muon Stations,
4 Regions / Station

20 different chamber sizes
 1368 chambers
IEEE, October 2005
LHCb Muon CERN
2/13
Wire fixation bar
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%

panel production:
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
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Side bar
MWPC
Sandwich
Spacer
Wired panel
Not wired panel
PCB coated by 35 μm copper,
5 μm nickel, 0.2 μm gold
foam injected between 2 PCBs
in mould
IEEE, October 2005
LHCb Muon CERN
3/13
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:


ageing resistance over 10 years:


up to 0.2 MHz/cm2 for inner chambers at a
luminosity of L = 5 x 1032 cm-1 s
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)
IEEE, October 2005
LHCb Muon CERN
4/13
Chamber Conditioning

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:
 Step 2: Normal HV-Conditioning:
General rule:

negative HV up to -2300 V at anode wire
positive HV up to +2900 V at anode wire
In order not to damage any surfaces (wire and cathode)
high currents must be avoided ! (nominal dark current: ~ 3 nA)
Cleaning Effects due to:

positive ion bombardment on cathode
 negative ion and electron bombardment on anode

Results:

surfaces smoothened
 current reduction
IEEE, October 2005
impurities on wire
LHCb Muon CERN
5/13
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

10 min
result:
current
reduction
Advantage:


tim
e
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

IEEE, October 2005
LHCb Muon CERN
6/13
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
8 gaps
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
time
(from: J. Va’vra,
NIM A367, 1995)
IEEE, October 2005
LHCb Muon CERN
7/13
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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


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measured current reduction in gap2/M3R1#03, HV=2.75kV
Results:
1

exponential current reduction
 conditioning time: 0.5 – 70 hours
0.9
y = 1.1e-0.21x
0.8
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Draw Back:

molecular bonds may recover
when irradiation or HV removed
→ currents return, but decay is
faster
I/I_initial
0.7
Time const = 5 hours
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
Time (hours)
IEEE, October 2005
LHCb Muon CERN
8/13

Test without CF4:
 gas mixture: Ar/CO2(40:60);
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_initial
HV=2.75 kV
 current reduction slow
 remaining current level still too high [μA]
 HV trips observed
I/I_initial
1
0.8
0.6
0.4
0.2
0
10
20
30
40
50
60
70
80
90
100
current [nA]
2000
1500
1000
Gap3/M3R2#26
20
40
60
time [h]
5
4
Gap1/M3R2#26
2
3
4
3
2
1
500
0
10 min
time
Tim
e (10 m inutes)
IEEE, October 2005
1.000
0.900
0.800
0.700
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0
time [h]
2500
Current (nA)
0
current [nA]
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same
chamber
LHCb Muon CERN
0
10 min
3192343100 3192343200 3192343300
3192343400 3192343500
time
9/13
80
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Emission Physics:

first attempts to combine Malter-effect with field emission (FowlerNordheim) can be found in literature (A. Boyarski, NIM A535, 2004)
 5.431010 
 [ A / m 2 ]
J  5.4 10 ( E ) exp 
E 

5

2
ß… field enhancement factor
E… electric field in [V/m]
for a gold-coated wire
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
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For our measurements:

measure I, V
 include gas gain G(V)
depending on V
I    J  G(V )
 approximation:
R
+HV
Anode (Gold-plated Tungsten wires)
Electrons
I1(t)
E0
Electron emission
EActual=E0+EFilm
Cathode pad
(Gold-coated Copper foil)
 a *2
 b 3 / 2 1
 I 
y= log V 2   log 1.1  G(V )   *  V =x


IEEE, October 2005
Positive ions
Thin film between cathode pads
LHCb Muon CERN
→ field factor ß* estimated from the slope
10/13
Measurments:
logarithmic scale:
/ V2
1E-11
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
2500
log( I / V2 ) I
current [nA]
The original I(V) relation
1E-12
1E-13
2550
2600
2650
2700
2750
0.00036 0.000365 0.00037 0.000375 0.00038 0.000385 0.00039 0.000395
2800
1/V
HV (V)
IEEE, October 2005
50 hours at GIF=ON
-1]
1/V[V
(V^-1)
1/V
LHCb Muon CERN
0.000475
0.00045
0.000425
0.0004
70 hours
0.000375
E-field decreasing
→ surface cleaned
 conditioning time: up to
500 hours (20 days)
Measured at t=0
0.00035

-3
-3.5
-4
-4.5
-5
-5.5
-6
-6.5
-7
0.000325
Result:
log(I/V^2) vs 1/V
0.0003

repeated measurements
on the same chamber
show rotation of straight
lines with increasing
slopes
log(nA/V^2)
/ V2) [nA / V]
log(I

0.0004
0.0005

M3R2#04 (Gap2)
11/13
Summary

MWPC Conditioning in 2 steps:
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Step1: negative HV on wire

apply negative HV up to 2300 V
 check current at positive HV=2900 V:
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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
IEEE, October 2005
LHCb Muon CERN
12/13
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 the observed anomalous currents are selfsuppressed during MWPC operation (high background radiation)
→ Therefore we are optimistic, that these currents will not be a problem for
the long term operation of the LHCb Muon System
IEEE, October 2005
LHCb Muon CERN
13/13