MWPCs Status report. Paolo Pedroni. Collaboration Meeting Dubrovnik, oct. 10 th -13 th 2010

Transcription

1 MWPCs Status report Paolo Pedroni Collaboration Meeting Dubrovnik, oct. 10 th -13 th 2010

2 MWPCs (recent) history april mwpc-2 repaired (broken wire; probably a defective one) may-july august HV beam test (no Strip ADC readout) (I MAMI =6 na Moeller; target) mwpc-2 again repaired (another broken wire defective soldering) First full readout of the mwpcs signals (Wire TDC +Strip ADC) Electronic noise hunt restarted (at the end low noise!) MWPCS used during beam time mwpc-1: problems after 1 day (one wire started to loose tension ) mwpc-2: OK After the end of the beam time, mwpc-1: OK (??)

3 MWPCs (recent) history september MWPCs used during beam time (I MAMI = na; Cu radiator) (I MWPCS = 1-2 ma) mwpc-1: problems after 2 days one wire continued to loose tension and finally broke (first broken wire since 3 years ) mwpc-2 OK at 2450V problems after a few days ( Malter effect ) at 2400V could stand for the rest of the beam time (point reconstruction efficiency for single protons % ) using one point from mwpc-2 and one point given by CB cluster position charged particle trajectories can still be reconstructed. (Sasha Mushkarenkov is updating/improving the old Sven MWPCs analysis software)

4 Vertex Histos from MWPCs Analysis XY Z 2-trajectories events (mm) X Y Preliminary evaluation of the mechanical mwpcs offsets (mm)

5 (mm) Vertex Histos from MWPCs Analysis 5 cm XY LOG scale (mm)

6 Vertex Histos from MWPCs Analysis Z LOG scale (mm)

7 The Malter Effect At high rates, positive ions (from gas dissociation) form an insulating layer close to the cathode. The positive electric field created by the layer ejects electrons from the cathode which increase the mwpc current

8 MWPCs gas composition The Malter Effect Argon 69.5 % Ethane 30 % Ar + and e - induce the charge avalanche Ar + ions neutralize at the catode; their kinetic energy is enough to extract X-rays or electrons very good X-ray absorber Freon (CF 4 ) 0.5 % very good electron absorber Positive ions from Ethane dissociation combine together forming more complex and slow polymers. They enhance the Malter effect. Moreover, they keep attached to the cathode after recombination thus forming a permanent insulating layer (long term aging)

9 The Malter Effect This effect is partially compensated by the gas flow and the Freon (flourine from freon dissociation interacts with polymers preventing their deposition to the cathode) A little ageing was shown after more than 10 years of mwpcs operations with DAPHNE, but now this effect goes much faster... The ageing businnes is not a clear science since it depends on many parameters. Some recipes which helps here may fail there. M.Hildebrandt, PSI Villigen (CH)

10 Use DME (ether) gas instead of Ethane DME needs to be quite pure (bad chlorinated freon impurities) How to avoid/prevent/reduce the Malter Effect Use CO 2 instead of Ethane (and Freon) Some tests done in Pavia with 80% Argon and 20% CO 2 MWPC reponse is as the one with standard gas mixture A short HV test with Ar+CO 2 on the Bonn S-beamline was OK test are needed with the full read-out and in (Mainz) beam instabilities are likey at high rates; a small ethane % (+Freon?) will be probably still needed. Add some % of O 2 to our mixture (O 2 interacts with polymers) Add some % of alcohol (methylal) vapour to our mixture methylal (interacts with polymers) needs to be kept at zero degr. Additional complication in the gas mixing circuit

11 Future planning beginning of november: MWPCs to be taken out fom CB for MWPC-1 reparation This (not optimal) time window is due to the availability contraints of our technician We need to have A2 accessible for a part of the detector test beam of nov.4-8 ( I had contacts with K.Livingston and there is a way to have acceptable interfences) After reinstallation, MWPCs under cosmic ray tests with Argon + CO 2 (+Ethane + Freon) to find the new HV working conditions end of november: test of the new gas mixture under beam

12 MWPC-1 reparation Take MWPCs out of CB (done within 4/11 morning; just before the start of the beam time) MWPC-1 reparation and mwpcs maintenance (4 afternoon +5/11) This can be done outside A2 hall; we need a (not too dirty) room with a NIM crate (to connect HV power supply) MWPCs reinstallation inside CB in A2 -) Connect mwpcs to their mechanical support and insert flat cables into mwpcs output mwpcs signal connectors -) Transport the full frame inside CB; verification/reparation of missing connections 6/11 (8 hours) 7/11 (8 hours) -) Final verifications / Cabling completion (8/11 A2 hall accessible)

13 MWPCs interference for the charged particle detection During the last CB meetings there were again claims (without numbers) that MWPCS induce an heavy interfence to the charged particle detection in the forward polar (TAPS) region To get a quantitative evaluation of these effects I have performed a simulation of the MWPCs effects on the recoil 0 deuterons coming from the d d reaction, (one of) the worst case of interference (pure phase space reaction mechanisms ; pointlike target)

14 (only the upper half is shown ) MWPC layout (side view) cathode 2 mm rohacell 50 mm kapton 10 mm copper equivalent scintillator thickness 0.8 mm (all cathodes) flanges [13 o,17 o ] beam multiple scattering MAX o 0.8 rms (500 MeV/c polyethylene Maximum thickness seen by particles is around 3 cm protons) 0.06 r.l. TAPS MAX ( ) rms rms e e for a few % of the events

15 850MeV E 1GeV 10% of the deuterons stopped by MWPCs Complete shadowing due to CB frame emitted d d stopped by MWPCs 1GeV E 1. 5GeV 2% of the deuterons stopped by MWPCs

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