Malter currents study in MUON MWPC (2014)

Dmitrii Mausuzenko, Maev Oleg (PNPI) 1

Malter currents in MUON MWPCsFrom the very beginning of RUN1, some of

MUON chambers demonstrated multiple HV-trips .

The self-sustained current behavior looks as a Malter effect manifestation.

A probable reason of the effect can be a pollution of MWPCs cathodes during production.

HV-training with working gas mixture at the nominal and inversed (negative) polarity during Technical stops and Winter Shutdowns usually helps for most of affected chambers but not for all of them.

2

Experimental setup in the PIT.

BUBLER

OXYGEN

OXYGEN flow meter

RESIVER

CHAMBER

WORKING MIXTURE flow meter

3

Treatment procedure in brief

4

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.

HV+ or HV-

5

HV+ → high current = more etching particles

HV- → small current = less etching particles , but it’s all near the insulating film

Production of etching particles :

e- + CF4 → *CF3 + F* + e-

e- + CF4 → *CF3 + F−

e- + CF4 → CF3+ + F* + 2e-

e-+ O2→ O2+ +2e-

e-+ O2→ 2O* + e-

e-+ O2→ O2-

e-+ O2→ e- + O+ + O-

Sources of ionization:HV+ :• External irradiation (Sr90)• Malter currentHV-:• External irradiation (Sr90)• Possible mini avalanches on tips

Scanning and treatment the CMB M2R4 #41 with radioactive sources

Scanning with Sr90 - J~70 nA/cm2 ignited the Malter current in two places only in GAP A

Scanning with source Am241

(J~0.3 nA/cm2) not give ignition!

6

Beam in 2012:J~0.1 nA/cm2

CMB M4R4#94 scanning with Sr90 - 39MBq

7

CMB M4R4#94The Malter effect manifested on GAP D at low current density J< 0.2 nA/cm2

Different chambers – different Malter currents behavior

*The currents in CMB M4R4 was increased more faster than in CMB M2R4 and ignited at lowest current density.

Curing time and results

8

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

1. Malter-currents are very good reproducible in MUON MWPC under irradiation with radioactive source, in some chambers to ignite it needs not much current density.

2. Adding a small amount of oxygen could be useful in case chambers with large value of pollutant when the only CF4 insufficiently.

3. HV+ training seems to be more effective due more etching particle production.

4. The first cured chamber M2R4 will be install on C side.

9

Conclusion

10

• After successfull treatment of two chambers we

started curing the chamber M5R4 , which has a problem not only with Malter effect but with high dark current in GAP A . As a first result , we see that after couple of days of training with Oxygen dark current has been significantly reduced but not completely yet. Will continue the work this summer.

• As a next object for study we suggest to cure a few chambers from M5R3

• Proposal for GIF++ in 2015

Future plans and perspectives

11

Thanks for attention !

BACK SLIDES

Malter effect

SOURCES: Avalanche producing polymers

deposits Some oxides are highly resistive. Constructions material and gas

pollutants. Insulating deposits left from sparks. Corona on sharp point on the cathode. Fingerprints Etc.

Ignition mechanisms:

a) Highly ionizing heavy ions.b) X-rays.c) Sparks.d) Sharp points on electrodes causing corona.

Necessary condition for electron emission:a) Localized primary ionization deposit.b) An insulator on the cathode.c) A rate of the charge build up is higher than its removal rate.d) Excessive field cathode gradients help to trigger it.e) To start the effect, it needs an ignition.

13

Addition of OXYGEN

14

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