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Aging test of MWPC at Casaccia

Aging test of MWPC at Casaccia. The Calliope gamma facility @ ENEA Casaccia Setup of the test: Description of chamber prototypes Gas system Temperature and atmospheric pressure Aging test results Currents in the tested chambers Malter currents Integrated charges

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Aging test of MWPC at Casaccia

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  1. Aging test of MWPC at Casaccia • The Calliope gamma facility @ ENEA Casaccia • Setup of the test: • Description of chamber prototypes • Gas system • Temperature and atmospheric pressure • Aging test results • Currents in the tested chambers • Malter currents • Integrated charges • Visual inspection of the chambers after the test • Conclusions C.Forti ( INFN – LNF ) - CERN, meeting on ageing February, 9th, 2004

  2. BOTTLES REF T1 T2 P T GEM GAS RACK PC HV 4.5 m Co60 window CONTROL ROOM Calliope facility @ ENEA Casaccia • REF = reference MWPC • T1 = test MWPC (TEST1) • T2 = test MWPC’s • (TEST2,CERN1,CERN2) • T = temperature sensor • P = atmospheric pressure • sensor • gas mixture: • Ar / CO2 /CF4 = 40 / 40 / 20 • Total gas flow ~ 6 l/hr • Source Co60 (~1015 Bequerel)

  3. Source pool window irradiated panels

  4. Layout of gas connections OPEN MODE Exhaust REF(A,B) TEST 2 CERN 2 Ar CO2 CLOSED LOOP CF4 Fresh gas GAS SYSTEM Circulating gas REF(C,D) CERN 1 TEST 1

  5. Gas system • The goal of this test was also to validate the LHCb gas system. • One bottle for each gas (Ar, CO2,CF4) • Gas from bottle to Mass flowmeter (connected to MKS) • Total gas flow: 40/40/20 cc/min = 6 l/hr to mixer (little cylinder) • Typical flows: Open Mode ~ 4.8 l/hr (~2 volumes/hour of chamber TEST2) • Closed loop: fresh gas ~ 1.35 l/hr circulating gas ~ 5.2 l/hr • Fresh gas should be 10% of circulating gas, but below 25% the system is unstable. • Content of O2 was measured and did not change, water content was unknown. • The oil in the Open Mode bubbler is dark brown, while it’s clean for the Closed loop (this is not sufficient to state that purifier is working properly). All pipes in Copper except: connections to chambers in Rilsan+brass connectors; Output pipes from last chamber to exhaust in Rilsan.

  6. LNF chambers

  7. June,11: Start system with two open loops. June,13: Closed loop starts. June,16: Purifier included in the loop ~1 day needed to stabilize June,17: Closed loop reached steady state. June,28-29:Mixture out of control (low CO2 content) in MWPC’s. Problemnot observed in GEM mixture CO2 bottle could not be the cause Opened more the CO2 bottle and problem (apparently) disappeared. June,30: Ar/CO2/CF4 = 40/18/20 at 8:30 AM. Huge currents in all MWPC’s. Opened more the CO2 bottle and problem (apparently) disappeared. July,1-3: Still mixture problems. July,3:Changed channel of control unit of CO2 flowmeter 40/40/20 OK Gas purifierchanged. July,5-6: Again low CO2 content in gas mixture !! July,7: MWPC mixture 40/10/20; Problem also in GEM’s: 153/18/136 (cc/min) instead of 153/51/136  CO2 bottle and mass flowmeter are changed. Gas mixture unstable. July,8th: Closed loop off 2 open loopsto reach fast a stable situation July,13: Test is finished. In the last days of the test, the observed currents were systematicallylower (of ~25%) respect to currents at begin. This suggests thatthe CO2 content before the change of the bottle was probably lowerthan expected. We found later that the CO2 pressure reducer was defective. Gas system setup during the test

  8. Atmospheric pressure and temperature vs. Time The absolute values of T and P are known within +- 1 K for T and+- 7 mbar for P. We cannot state that T and P are precise estimates of the gas temperature and pressure inside the two chambers. Even if overall variations of T and P are small, (~1.3 % for T and ~1.2 % for P) we have normalized thecurrents in test gaps to the currents in reference gaps, in order toremove the T and P (and the mixture) dependence.

  9. Ratio of currents BTF(A,B,C)/D Currents in TEST2 gaps (chamber in open mode) Absolute currents in A,B,C,D During ~12 days CO2 content in the mixture was anomalously low and led tovery big values of the currents. After change of CO2 bottle (day>25), currents stabilizeat a lower value.The huge current is probably the cause of broken wire in gap C(item discussed later). The ratios on the right are stable within ~4%.

  10. Ratios of currents TEST2(A,B,C) / REF(A+B) Currents REF(A,B) At day ~25, a wire in REF(B) was broken,probably due to overcurrent. For day >25, current ratios refer to gap A alone, not tothe average in (A,B). Even though currents in REF (A,B) are very unregular, due to heavyfluctuations of CO2 pecentage, the current ratios are stable(within ~10%)

  11. Currents in TEST1 (closed loop, low dose rate) Currents in A,B,C,D Ratios (B,C,D)/A Effect on currents of low CO2 content is evident. Behavior of currents after change of CO2 bottle (day >25), is unclear: gaps C and D are stable for ~2 daysand then show a sharp decrease to a lower value, while currents in gaps A (reference) and B have about the same valuethan before the “CO2 problem”.

  12. Ratios of currents TEST1(B,C,D)/TEST1(A) We calculated the ratio of currents in B,C,D respect to reference current in A. During the test, the high voltage of gaps B and C were changed: from 3.2 to 3.15 kV in B (at day~13.5); from 3.15 to 3.2 kVin C (at day~10.5). At day ~13.5 we also removed limiting resistors of 470 kW from gaps A,B,D (in C there was no resistor). These resistors provoked a voltage drop ~80 V. Currents in gaps B and C were rescaled using correction factors: for day < 13.4, current in B was multiplied by 1.141; for day < 10.4, current in C was multiplied by 1.271. It is hard to understand the changes of the current ratios when limiting resistors were removed. If we limit ourselves to data collected for day >13.5, we remark that current ratios are quite stable except forthe last measurement. We do not have any explanation forthis last measurement: currents in gaps C and D sharply decrease at day ~27.5 butthis decrease is not observed neither in gaps A and B,nor in the gaps of the CERN chambers. We can only state that a sharp and simultaneous decreaseof the currents in two different gaps cannot be attributedto an aging process.

  13. Malter currents Currents after ~14 days with source off Power supply used for testhad sensitivity 0.1 mA and different offset for eachchannel, between 0 and 1 mA. So, to measure dark currents, expected to be < 1 mA in each gap, we switchedoff each gap and powered it with a NIM N471A, withsensitivity 1 nA. All TEST2 currents are fastly decreasing, for example: TEST2(D): 90  28 nA in ~ 1 hr, @ HV=2.75 kV. Only gap REF(D) draws a permanent current, but thiseffectcannot be related to aging,because the dose rate on this chamber is negligible. In further measurements in LNF, after theend of the test, we did not find any self-sustaining rest current in thechambers.

  14. Integrated charges in TEST1 and TEST2 Q (mC/cm of wire) in 10 LHC years L=2 1032; equiv. mip rate from TDR; safety factors: 2 (M1) and 5 (M2-M5); charge = 0.44 pC/hit (at relativistic rise) corresponding to gain=5 104; Correction factor 0.5 in M1-R1/R2/R3 and M2-3/R1(double cathode readout) and 2 in M2-M5 (due to photons).

  15. Visual inspection of the chamber in Open Mode.( I ) Etching of the FR4frame The FR4 is etched where there is no electric field. This effect is visible also in reference gap D  due to gas (CF4 ?) Line of FR4 etching

  16. ( II ) No traces of discharges or deposits under the wires.

  17. ( III ) Effect of the discharge activity near gas input (gap A) Gas Input Input gas contains some water (at begin of test) ?

  18. ( IV ) “Bubbles” under the guard strips of the pads Not visible in reference gap D  current is needed

  19. ( V ) Preferred directions of pad etching. Most of the “bubbles” under ground guard traces are included between the couples of parallel lines in the picture.

  20. A good wire of the chamber ( VI ) The broken wire The two ends of the wire for few millimetersarecarbonized. We should consider that in this chamber the wires had no HV limitingresistor, in order to perform the accelerated aging test. In this way a single wire could have drawn a very large current (order of mA). We believe that the broken wire is due to such an effectand not to the effect of aging. The broken wire

  21. Materials used, gas mixture, etc… Material list: FR4; Gold (over cathodes); Wire=Gold plated W; Glue for wires: Adekit A145 (epoxy) Glue for HV bars and closing bars: Adekit A140 (epoxy) but in the tested prototypes we used 3M DP460 (acrylic) Gas mixture: Ar/CO2/CF4 = 40/40/20. Further tests with less CF4 would be useful to see the effect on cathode etching. Gas amplification: ~1.5 105 to accelerate the test, will be 5÷7.5 104 in the experiment (bigap efficiency >95% in 20 ns is required). All pipes in Copper except: connections to chambers in Rilsan+brass connectors; Output pipes from last chamber to exhaust in Rilsan.

  22. Summary and conclusions ( I ) 2 MWPC’s prototypes exposed to gammas from 60Co for ~1 month TEST1: low dose rate and in closed gas loop (pitch=1.5 mm) TEST2: high dose rate and open mode (pitch = 2 mm) Int. charge: TEST1 ~ 100 mC/cm (~3 yrs M1R2) TEST2 ~ 290 ÷ 440 mC/cm (~ 9÷13 yrs M1R2) • Several problems with gas mixture during the test: • Low CO2 content for day ~ 13 ÷ 25 (at least) • Lower currents in all chambers after change of CO2 bottle • Undefined purity of mixture and H2O content Interpretation of data for TEST1 is not easy. Currents are quite stable,except when the CO2 percentage was out of control and for the very end ofthe test, in which 2 gaps exhibit a sharp unexplained decrease in thecurrent. It is evident that this effect (sharp and simultaneous on both gaps) cannot be explained in terms of aging.

  23. Results for chamber TEST2 in gas open mode are verysatisfactory: the currents, normalized to reference gaps, arequite stable (within ~10%) over one month of test, even though the gas mixture was not well defined for a relevant time fraction of the test. Summary and conclusions ( II ) The visual inspection proves that the broken wire in one gap of TEST2 is probably due to an overcurrent, not to aging. Etching on pads due to current (not present in the reference gap) Attack of FR4 frame due to gas (present also in the reference gap). Considering that TEST2 gaps are connected in series and that gas enters the chamber in gap A and goes out from gap D, we do not find any systematic effect due to gas pollution. Further tests needed to validate the gas system (with better control of O2 and H2O content), trying also a mixture with less CF4 to evaluate its effect on cathode etching.

  24. Spare transparencies follow

  25. ~2.55 kV ~2.45 kV Single vs Double Cathode Readout : Double Gap Double Cathode Pad readout: thr ~ 8.1 fC Single Cathode Pad readout:thr ~ 6.8 fC -For double gap the starting point of the plateau is set by the request to have e>95%; -Plateau for double cathode begins ~ 100 V earlier: - e ~ 95% @ 2.45 kV for double cathode - e ~ 95% @ 2.55 kV for single cathode  good ! -The cluster size is much more critical for double cathode:  bad ! -let’s see if we are inside specifications….

  26. Test chamber 1 and reference chamber • 4-gaps with active area ~ 500 cm2 • wire pitch1.5 mm(8 pads x 17 wires) • Operating voltage ~ 3.0-3.2 V • Currents: TEST1 ~200 mA/gapREF ~20 mA/gap LNF chambers REF TEST 1

  27. LNF chambers Chamber in open mode (TEST2) was exposed to high dose rate~0.305 Gy/hr. Gaps A,B,C were permanently switched on at HV=2.75 kV. Gap D is reference: from 2.05 kV to 2.75 kV for ~5 hours every ~3 days. HV=2.75 kV corresponds to gas gain G ~1.5 105, while in theexperiment we will set HV~2.6 kV, providing G ~5104 and ensuring an efficiency >95%and average hit multiplicity < 1.2for each bigap, well within the requirements. The choice HV=2.75 kV is required to accelerate the test, up to the hottest region in which MWPC could be adopted (M1R2),for which the test acceleration factor is ~24. The chamber in closed loop (TEST1) was exposed to a dose rate~0.072 Gray/hr, about 4 times smaller than for TEST2. The gaps B,C,D were permanently switched on. Typical high voltage values were: HV=3.15 kV in gap B, HV=3.2 kV in C and D. Gap A is the reference. Since TEST1 had a smallerwire pitch (1.5 mm) respect to TEST2 (2 mm), the same gas gain as in TEST2gaps was obtained for a high voltage ~400 V higher in TEST1, so at 3.2 kV in TEST1 the gain is again ~1÷2 105.

  28. MWPC-LNF chambers • REF = REFERENCE CHAMBER (1.5 mm pitch) • Total Wire length ~ 3330 cm I ~ 20 mA/gap Active area 500 cm2 • Gaps A,B in OPEN Mode; Gaps C,D in Closed Loop. • TEST1 = TEST CHAMBER 1 (1.5 mm pitch) • HV=3.2 kV (reference A ON for ~5 hrs every 2-3 days) • Total Wire length ~ 3330 cm I ~ 160 mA/gap Active area 500 cm2 • Gaps A, B, C, D in Closed Loop. • TEST2 = TEST CHAMBER 2 (2 mm pitch) active • HV=2.75 kV (reference D ON for ~5 hrs every 2-3 days) • Total Wire length ~ 6030 cm I ~ 1200-1500 mA/gap Area 1200 cm2 • Gaps A, B, C, D in OPEN Mode.

  29. MWPC-CH1 MWPC-CH1 Layout of chamber positions and gas Closed loop Open mode GEM (open mode) P1 P2 0.1-0.2 Gy/hr P3 0.3-0.6 Gy/hr P4 10-20 Gy/hr position MWPC-CH3 MWPC-BTF GEM-LNF-TEST1 AB CD GEM-CA-TEST2 GEM-LNF-MONIT GEM-LNF-TEST2 MWPC-CERN1 GEM-CA-TEST1 MWPC-CERN2 3 m rack

  30. Saturation of gas gain Saturation < 10 %

  31. Currents in BTF reference gap D

  32. Currents in BTF reference gap D

  33. Currents in CH3 reference gap A

  34. TEST1(B,C,D) / REF(C) Currents REF (C, D) Ratios of currents TEST1(B,C,D) / REF(C) REF(D) is anomalous: larger I than inother gaps and presence ofresidual current when Radioactivesource is off (~1200 nA at 3.15 kV). HV in gap C changed (from 3.15 to 3.05 kV) at day~13.6 current ratio isrescaled Except when CO2 content is out ofcontrol,the current ratio for TEST1(B) is stable within ~15 %,while the for C and D we find a sharp decrease at the end of thetest, as already observed in TEST1 currents.

  35. CERN chambers (*) Gaps C,D are asymmetric (wire is at 3.65 and 1.35 mm from cathode)

  36. Currents in CERN1 (closed loop)

  37. Currents in CERN 2 (open mode)

  38. CERN 1 ratios

  39. CERN 2 ratios

  40. I_ref (closed) / I_ref(open)

  41. Integrated charges in CERN 1

  42. Integrated charges in CERN 2

  43. Dark currents (first measurement: 16-jun) CH1 (C,D) 3.15  3.05 kV CH3 (B) 3.2  3.15 kV The power supply does not allow to measure currents < 1 uA

  44. Integrated charges @ G=105 Integrated charge (C/cm of wire) in 10 LHC equivalentyears for each detector region, asuming: luminosity=2 1032; equiv. mip rate from TDR; safety factor is 2 (in M1) and 5 (in M2-M5); charge per hit is Q=0.88 pC/hit (considered to be measured at relativistic rise) corresponding to a gas gain=105; The charge is corrected by a factor 0.5 in M1-R1/R2/R3 and M2-3/R1(assuming double cathode readout) and 2 in M2-M5 (due to photons).

  45. Integrated charges @ G=5 104 Integrated charge (mC/cm of wire) in 10 LHC equivalentyears for each detector region, asuming: luminosity=2 1032; equiv. mip rate from TDR; safety factor is 2 (in M1) and 5 (in M2-M5); charge per hit is Q=0.44 pC/hit (considered to be measured at relativistic rise) corresponding to gain=5 104; The charge is corrected by a factor 0.5 in M1-R1/R2/R3 and M2-3/R1(assuming double cathode readout) and 2 in M2-M5 (due to photons).

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