Malter effect study in MUON MWPCs (2014)

Dmitrii Mausuzenko, Oleg Maev (PNPI)1

Status of Malter effect study. Malter effect(ME) was manifested in many of the muon

chambers from the beginning of operation with beam.

Most of affected chambers are cured by HV training with working gas mixture at the nominal polarity with beam and at inversed polarity during Technical stops and Winter Shutdowns. But some chambers – not cured.

Suppression of the Malter current in 20 CERN production chambers on GIF (Conditioning of MWPCs for the LHCb Muon System 2005)

In November 2013 we started the study of HV training of the non cured MUON chambers with adding a small amount of Oxygen in the nominal gas mixture.

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Pollution of the MWPCs cathodes during production:• Outgasing construction materials

(fiberglass in the presence of CF4 + radiation).

• Bad cleaning of surface

(dirt between cathode pads)

Studied chambers & pollution chemistry

Silicon or/and Carbon

based insulating film

on surface

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Oxidation of the cathode covering (some forms of Cu2O have a highly electrically resistance)

Malter effect are manifested in chambers from different production => they have similar reasons to form insulating layers.

Cleaning of cathode surface

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HV+ → high current = more etching particles Etching of pollutant by active

form of Fluorine and Oxygen:

e-+ O2→ 2e- +O2+

e-+ O2→ e- + 2O*

e-+ O2→ O2-

e-+ O2→ e- + O+ + O-

e- + CF4 → e- + *CF3 + F*

e- + CF4 → *CF3 + F−

e- + CF4 → 2e- + CF3+ + F*

SiF6

CO, CO2, H2OC/CH

SiGas flux

Ionization reaction

Sources of ionization by HV+ :• External irradiation (90Sr)• Malter current.

Last results.

Strong Malter effect was detected in GAP B , after 6h of training with nominal

mixture the chamber failed to cure. Curing with O2 remove the ME after 4h! 5

In summary chambers.

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M2R4 #412 zones

M5R3#10C 1 Malter zone

M4R4 #941 zones

M5R4 #FIR05400021

M5R3#24B 3 Malter zones

M2R4 #46

M5R3 #22A

M.E. cured, but large dark currents remain.

M.E. wasn’t detected

Cured with Oxygen

M.E. wasn’t detected

Cured with Oxygen

Cured with Nominal mxture

M5R4 #FIR05400037

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Conclusion

1. Eight chambers from different productions have been studied up to now. Ten Malter zones was detected in six chambers. All of them cured successful by HV training. Four zones succumbed to cure only with Oxygen addition.

2. Insulating layers characteristics:• the place of occurrence – unpredictable;• not the same insulation properties (different Malter zones needs

different current density to ignition;• most of pollution is not resistant to etching by CF4, but they

removed faster with Oxygen admixing.

3. High current during the training = faster curing (the current even 30uA is not dangerous for life of the chamber )

4. All cured chambers are installed back in the detector. Future operating with beam will show the effect of curing.

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• Public note on this studies.

• Proposal for GIF++ in 2015 in collaboration with CMS (Malter study will be included to this program).

Future plans and perspectives.

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BACK SLIDES

Previous results.

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Malter effect was cured successful after ~1h of HV+

& ~2h HV- training with 2% of Oxygen.

Malter insulation layer in chamber M4R4#94 was removed after ~ 10 hours* of HV+ training with 2% of Oxygen.

*The optimal training mode was not yet known.

Last results.Chambers M5R3 from the low irradiation region.

Were cured successful in nominal mixture by short time

Very difficult to ignite the Malter current during the Sr90 (39MBq) scanning.

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Treatment procedure in brief

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1. Setting the chamber taken from the detector with nominal gas mixture at MUON/OT gas quality check room in the PIT.

2. Check that the chamber is properly conditioned with HV. It means that CMB has almost zero dark current at HV= +2850V and -2300V.

3. Scanning the chamber sensitive area with radioactive source at HV=2800V to provoke and localize “Malter currents zone(s)”

4. Try to remove the insulating film in Malter zone:• Setup the radioactive source over the “Malter zone”• Training the chamber at different polarities HV with

admixing a small amount of Oxygen (~2%) to the nominal gas mixture.

• Controlling the Malter currents in different places of Malter zone until the moment when it disappeared completely.

5. Check on nominal gas, that the current in “Malter zone” does not reappear again.

Addition of OXYGEN

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With ~2% of Oxygen the currents in reference point

decreased by ~20% , that corresponds to

the Gain calculations in GARFIELD

HV =2850V

Gain calculations in GARFIELD

Oxygen O%

Oxygen 2%HV=2600V HV=2600V