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

Calliope facility @ ENEA Casaccia

REF

T2

GEM

Co60

PCHV

CONTROLROOM

4.5

m

P

T

win

dow

T1

GA

S

RA

CK

BO

TT

LE

S

• 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)

Source pool

windowirradiated panels

Ar

GAS SYSTEM

CO2

CF4

REF(A,B) TEST 2 CERN 2

REF(C,D)TEST 1CERN 1

Fresh gas

Circulating gas

CLOSED LOOP

OPEN MODE Exhaust

Layout of gas connections

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+brassconnectors; Output pipes from last chamber to exhaust in Rilsan.

LNF chambers

REF (A,B) REF (C,D) TEST1 TEST2

Area (cm 2) 500 500 500 1200

Volume (l) 0.5 0.5 1 2.4

Wire pitch (mm) 1.5 1.5 1.5 2

Gas mode open closed closed open

I (A/gap) 20 20 200 1200÷1400

HV (kV) 3.05-3.15 3.0-3.15 3.15-3.2 2.75

Gas gain/105 1÷1.5 ~1.5

Dose rate (Gy/hr) negligible negligible 0.072 0.305

Reference gap A D

Gas system setup during the test

June,11: Start system with two open loops.June,13: Closed loop starts.June,16: Purifier included in the loop ~1 day needed to stabilizeJune,17: Closed loop reached steady state.June,28-29: Mixture out of control (low CO2 content) in MWPC’s. Problem not 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 purifier changed. 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 loops to reach fast a stable situation July,13: Test is finished. In the last days of the test, the observed currents were systematically lower (of ~25%) respect to currents at begin. This suggests that the CO2 content before the change of the bottle was probably lower than expected.We found later that the CO2 pressure reducer was defective.

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 estimatesof 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 the currents in test gaps to the currents in reference gaps, in order to remove the T and P (and the mixture) dependence.

Currents in TEST2 gaps (chamber in open mode)

Ratio of currents BTF(A,B,C)/D

Absolute currents in A,B,C,D

During ~12 days CO2 content in the mixture was anomalously low and led to very big values of the currents. After change of CO2 bottle (day>25), currents stabilize at 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%.

Currents REF(A,B)

Ratios of currents TEST2(A,B,C) / 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 to the average in (A,B). Even though currents in REF (A,B) are very unregular, due to heavy fluctuations of CO2 pecentage, the current ratios are stable (within ~10%)

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 days and then show a sharp decrease to a lower value, while currents in gaps A (reference) and B have about the same value than before the “CO2 problem”.

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 kV in C (at day ~10.5).

At day ~13.5 we also removed limiting resistors of 470 k 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 for the last measurement.

We do not have any explanation for this last measurement: currents in gaps C and D sharply decrease at day ~27.5 but this 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 decrease of the currents in two different gaps cannot be attributed to an aging process.

Malter currents

I(nA) REF TEST1 TEST2

A 0 5 @ 3.2 70

B 0 9 70

C 0 9 38

D 1200 7 28

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 this effect cannot be related to aging, because the dose rate on this chamber is negligible. In further measurements in LNF, after the end of the test, we did not find anyself-sustaining rest current in the chambers.

Currents after ~14 days with source offPower supply used for test hadsensitivity 0.1 A and differentoffset for each channel, between 0and 1 A. So, to measure darkcurrents, expected to be < 1 Ain each gap, we switched off eachgap and powered it with aNIM N471A, with sensitivity 1 nA.

Integrated charges in TEST1 and TEST2

M1 M2 M3 M4 M5

R1 810 132 35.0 46.0 31.0

R2 328 185 25.0 16.0 13.0

R3 141 46.0 7.0 6.0 5.0

R4 44.0 4.3 1.5 1.0 0.8

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).

Q (mC/cm of wire) in 10 LHC years

 

Line of FR4 etching

Visual inspection of the chamber in Open Mode.( I ) Etching of the FR4 frame

The FR4 is etched where there is no electric field.This effect is visible also in reference gap D due to gas (CF4 ?)

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

Gas Input

( III ) Effect of the discharge activity near gas input (gap A)

Input gas contains some water (at begin of test) ?

( IV ) “Bubbles” under the guard strips of the pads

Not visible in reference gap D current is needed

( 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.

( VI ) The broken wire

The two ends of the wire for few millimeters are carbonized.

We should consider that in this chamber the wires had no HV limiting resistor, 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 effect and not to the effect of aging.

A good wire of the chamber

The broken wire

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 thetested prototypes we used 3M DP460 (acrylic)

Gas mixture: Ar/CO2/CF4 = 40/40/20. Further tests with less CF4would 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+brassconnectors; Output pipes from last chamber to exhaust in Rilsan.

Summary and conclusions ( I )

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

2 MWPC’s prototypes exposed to gammas from 60Co for ~1 monthTEST1: 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)

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 of the test, in which 2 gaps exhibit a sharp unexplained decrease in the current. It is evident that this effect (sharp and simultaneous on both gaps) cannot be explained in terms of aging.

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.

Results for chamber TEST2 in gas open mode are very satisfactory: the currents, normalized to reference gaps, are quite 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.

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.

Spare transparencies follow

Single vs Double Cathode Readout : Single vs Double Cathode Readout : Double GapDouble Gap

Double Cathode Pad readout: thr ~ 8.1 fCSingle Cathode Pad readout:thr ~ 6.8 fC

-Plateau for double cathode begins ~ 100 V earlier: - @ 2.45 kV for double cathode - @ 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….

~2.45 kV~2.55 kV

-For double gap the starting point of the plateau is set by the request to have

LNF chambers

TEST 1

REF

Test chamber 1 and reference chamber

• 4-gaps with active area ~ 500 cm2

• wire pitch 1.5 mm (8 pads x 17 wires)• Operating voltage ~ 3.0-3.2 V• Currents: TEST1 ~ 200 A/gap REF ~ 20 A/gap

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.

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 smaller wire pitch (1.5 mm) respect to TEST2 (2 mm),the same gas gain as in TEST2 gaps was obtained for a high voltage ~400 Vhigher in TEST1, so at 3.2 kV in TEST1 the gain is again ~1÷2 105.

HV=2.75 kV corresponds to gas gain G ~1.5 105, while in the experiment we will set HV~2.6 kV, providing G ~5 104 and ensuring an efficiency > 95% andaverage hit multiplicity < 1.2 for each bigap, well within the requirements. The choice HV=2.75 kV is required to accelerate the test, up to the hottestregion in which MWPC could be adopted (M1R2), for which the testacceleration factor is ~24.

MWPC-LNF chambers

• REF = REFERENCE CHAMBER (1.5 mm pitch)Total Wire length ~ 3330 cm I ~ 20 A/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 A/gap Active area 500 cm2

Gaps A, B, C, D in Closed Loop.

• TEST2 = TEST CHAMBER 2 (2 mm pitch) activeHV=2.75 kV (reference D ON for ~5 hrs every 2-3 days)Total Wire length ~ 6030 cm I ~ 1200-1500 A/gap Area 1200 cm2

Gaps A, B, C, D in OPEN Mode.

Layout of chamber positions and gas

MWPC-CH3 MWPC-BTF GEM-LNF-TEST1

GEM-LNF-TEST2

GEM-CA-TEST1

GEM-LNF-MONIT

MWPC-CH1

MWPC-CH1

P1 P2 0.1-0.2 Gy/hr

P3 0.3-0.6 Gy/hr

P4 10-20 Gy/hr

GEM-CA-TEST2

MWPC-CERN1

MWPC-CERN2

3 m rack

Closed loop Open mode GEM (open mode)

AB

CD

position

Saturation of gas gain

HV % Sat. HV % Sat. HV % Sat.

A 3.2 9 2.75 7

B 3.2 33.3 3.15 7 2.75 6

C 3.2 6.3 3.2 8.3

D 3.2 31.4 3.2 9.9 2.75 8

CH3 CH3 w/o Resis. BTF

Saturation < 10 %

Currents in BTF reference gap D

Currents in BTF reference gap D

Currents in CH3 reference gap A

Ratios of currents TEST1(B,C,D) / REF(C)Currents REF (C, D) TEST1(B,C,D) / REF(C)

REF(D) is anomalous: larger I than in other gaps and presence of residualcurrent when Radioactive source 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 is rescaled

Except when CO2 content is out of control, the current ratio for TEST1(B) isstable within ~15 %, while the for C and D we find a sharp decrease at the endof the test, as already observed in TEST1 currents.

CERN chambers

CERN1 CERN2

Area (cm2) 875 480

Volume (l) 1.75 0.96

Wire pitch (mm) B=1.5

A,C,D=2

1.5

Gas mode closed open

I (A/gap) 990 670

HV (kV) B=3

A,C,D=2.65

HV(A) ~ 3.14HV(B) ~ 3.115HV(C) ~ +-2.125 (*)HV(D) ~ +-2.15 (*)

Dose rate (Gy/hr) 0.421 0.421

Reference gap B D

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

Currents in CERN1 (closed loop)

Currents in CERN 2 (open mode)

CERN 1 ratiosCERN 1 I(A,C,D)/B(ref)

0,92

0,94

0,96

0,98

1

1,02

1,04

0 5 10 15 20 25

Days

A.U.

Series1

Series2

Series3

CERN 2 ratiosCERN 2 I(A,B,D)/C(ref)

0,8

0,9

1

1,1

1,2

1,3

0 5 10 15 20 25

Days

A.U.

Series1

Series2

Series3

I_ref (closed) / I_ref(open)Ratio reference gaps I(D1)/I(C2)

1,3

1,4

1,5

1,6

1,7

1,8

1,9

0 5 10 15 20 25

Days

A.U.

Integrated charges in CERN 1

Integrated charges in CERN 2

Dark currents (first measurement: 16-jun)

ON OFF ON OFF ON OFF

A 21.4 0.6 112 0.4 1300 0.8

B 19.6 0.2 135 1.0 1480 0.6

C 27.2 1.0 141 0.6 1280 0.6

D 29.2 8.2 127 0.6 1110 0.2

CH1 CH3 BTF

CH1 (C,D) 3.15 3.05 kV CH3 (B) 3.2 3.15 kVThe power supply does not allow to measure currents < 1 uA

Integrated charges @ G=105

M1 M2 M3 M4 M5

R1 1.62 0.264 0.070 0.092 0.062

R2 0.655 0.370 0.050 0.031 0.025

R3 0.282 0.092 0.014 0.011 0.009

R4 0.0880 0.0085 0.0030 0.0020 0.0015

Integrated charge (C/cm of wire) in 10 LHC equivalent years 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).

Integrated charges @ G=5 104

Q(mC/cm) M1 M2 M3 M4 M5

R1 810 132 35.0 46.0 31.0

R2 328 185 25.0 16.0 13.0

R3 141 46.0 7.0 6.0 5.0

R4 44.0 4.3 1.5 1.0 0.8

Integrated charge (mC/cm of wire) in 10 LHC equivalent years 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).