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