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Malter currents study in MUON MWPC (2014) Dmitrii Mausuzenko , Maev Oleg (PNPI)

Malter currents study in MUON MWPC (2014) Dmitrii Mausuzenko , Maev Oleg (PNPI). Malter currents in MUON MWPCs. F rom the very beginning of RUN1, some of MUON chambers demonstrated multiple HV-trips . The self-sustained current behavior looks as a Malter effect manifestation.

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Malter currents study in MUON MWPC (2014) Dmitrii Mausuzenko , Maev Oleg (PNPI)

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  1. Malter currents study in MUON MWPC (2014) DmitriiMausuzenko, Maev Oleg (PNPI)

  2. Malter currents in MUON MWPCs • From the very beginning of RUN1, some of MUON chambers demonstrated multiple HV-trips . • The self-sustained current behavior looks as a Maltereffect 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.

  3. OXYGEN flow meter BUBLER WORKING MIXTURE flow meter RESIVER CHAMBER OXYGEN Experimental setup in the PIT.

  4. 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.

  5. HV+ or HV- • Sources of ionization: • HV+ : • External irradiation (Sr90) • Malter current • HV-: • External irradiation (Sr90) • Possible mini avalanches on tips HV-→ small current = less etching particles , but it’s all near the insulating film HV+→ high current = more etching particles

  6. Scanning and treatment the CMB M2R4 #41 with radioactive sources Beam in 2012: J~0.1 nA/cm2 Scanning with source Am241 (J~0.3 nA/cm2)not give ignition! Scanning with Sr90- J~70 nA/cm2 ignited the Malter current in two places only in GAP A

  7. CMB M4R4#94 scanning with Sr90 - 39MBq CMB M4R4#94 The Maltereffect manifested on GAP D at low current density J< 0.2 nA/cm2 *The currents in CMB M4R4 was increased more faster than in CMB M2R4 and ignited at lowest current density. Different chambers – different Malter currents behavior

  8. Curing time and results Malter insulation layer in CMB M4R4#94 was removed after ~ 10 hours of HV+ training with 2% of Oxygen !

  9. 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. Conclusion 9

  10. Future plans and perspectives • After successfulltreatmentof 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

  11. Thanks for attention !

  12. BACK SLIDES

  13. Malter effect 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. 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.

  14. 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 inGARFIELD Oxygen O% Oxygen 2% HV=2600V HV=2600V

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