1. Dmitrii Mausuzenko, Oleg Maev (PNPI)
Malter effect study
in MUON MWPCs (2014)
Dmitrii Mausuzenko, Oleg Maev (PNPI)
Good day all. Now i tell you about MC investigations which we carry out with LHCB . Firstly let me remind you what we was doing in last time.

2. 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. 2
From the beginning of operation many of MUON chambers demonstrated multiple HV-trips due the Malter effect manifestation.
A probable reason of the effect can be a pollution of MWPCs cathodes during production.
Most of affected chambers are cured by with working gas mixture at the nominal polarity with beam and at inversed polarity during Technical stops and Winter Shutdowns. But not all of them. it
Due this we can conclude that the insulating film which is a consequence of Malter effect not resistant to etching by fluoro, but for some reason not in all chambers fluorine may solve this problem. Based on Plasma chemistry experience and assuming that the cathode pollution have hydrocarbon nature - In November 2013 we started the study of curing MUON chambers from Malter effect with using as an option the adding small amount of Oxygen in the nominal gas mixture.

3. Studied chambers & pollution chemistry Carbon based insulating film
Malter effect are manifested in chambers from different production => they have similar reasons to form insulating layers.
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)
Silicon
or/and
Carbon based insulating film
on surface
Oxidation of the cathode covering
(some forms of Cu2O have a highly electrically resistance)

4. Cleaning of cathode surface
HV+ → high current = more etching particles
Etching of pollutant by active form of Fluorine and Oxygen:
Sources of ionization by HV+ :
External irradiation (90Sr)
Malter current.
Ionization reaction
SiF6
e- + CF4 → e- + *CF3 + F*
e- + CF4 → *CF3 + F−
e- + CF4 → 2e- + CF3+ + F* Si
In this slide I trying to assume why the training in both polarities can works.
Firstly , the base etching particles which can remove the pollutants from cathode it is a radicals and ions of fluorine which is a part of LHCb working gas mixture. it proved successful in many chambers. But its can not be sufficiently to curing the chambers with high amount deposits or may needed very much time to do that. In this case Addition of oxygen can help. when we training the chamber in positive polarity we get many etching particles from Hihg value of Malter currents with positive ions bombing . In case of low Negative HV polarity We have only the external radiation and may be mini avalanches on pollutants film tips but its all birth near the deposits..
Gas flux
e-+ O2→ 2e- +O2+
e-+ O2→ e- + 2O*
e-+ O2→ O2-
e-+ O2→ e- + O+ + O-
CO, CO2, H2O
C/CH

5. Last results.
5 .
The Malter effect at these chambers was manifested in the different stage of it operating. But all of them are trained long time in different HV polarities without improvement.
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

6. In summary chambers. M4R4 #94 1 zones Cured with Oxygen M2R4 #41
M.E. wasn’t detected
M5R3 #22A
M5R3#10C
1 Malter zone
Cured with Nominal mxture
M5R3#24B
3 Malter zones
M5R4 #FIR
M.E. cured, but large dark currents remain.
Cured with Oxygen
M5R4 #FIR
M2R4 #46
M.E. wasn’t detected

7. Conclusion
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.
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.
High current during the training = faster curing (the current even 30uA is not dangerous for life of the chamber )
All cured chambers are installed back in the detector. Future operating with beam will show the effect of curing. 7

8. Future plans and perspectives.
Public note on this studies.
Proposal for GIF++ in 2015 in collaboration with CMS (Malter study will be included to this program).

10. BACK SLIDES

11. Previous results.
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.
3. For the first treatment we was chosen the M2R4 CMB#41. This chamber have been shown Malter effect manifestation from start of it operating and bsen long time training in different HV polarities unsuccessfully. Due scanning with Sr90 source we detected 2 zones with Malter currents on GAP A . First zone was removed during the scanning, as apparently it was very small. The second zone was removed after 2h of training in negative polarity with Sr radiation and 2% Oxygen addition. Red line shows the ignition of Malter current? Green line it is current in this tone after training
In next chamer Due the scanning all of GAPs The Malter effect manifested in one place on GAP D , this GAP repeatedly showed HV trips at the beam. At this plots The currents in CMB M4R4 was increased more faster then in M2R4 chamber and ignited at lowest current density .
5 hours HV traning with working mixture was cured
*The optimal training mode was not yet known.

12. Last results. Chambers M5R3 from the low irradiation region.
Very difficult to ignite the Malter current during the Sr90 (39MBq) scanning.
Were cured successful in nominal mixture by short time 4 4

13. Treatment procedure in brief
Setting the chamber taken from the detector with nominal gas mixture at MUON/OT gas quality check room in the PIT.
Check that the chamber is properly conditioned with HV. It means that CMB has almost zero dark current at HV= +2850V and -2300V.
Scanning the chamber sensitive area with radioactive source at HV=2800V to provoke and localize “Malter currents zone(s)”
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.
Check on nominal gas, that the current in “Malter zone” does not reappear again.

14. Gain calculations in GARFIELD
Addition of OXYGEN
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%
HV=2600V
Oxygen 2%
HV=2600V