First results of an aging test of a full scale MWPC ... · normal operational voltage is of about 1year ... Anode-cathode distance 2.5 mm ... (the chamber firing with Malter current)-ground

First results of an aging test of a full scaleMWPC prototype for the LHCb muonsystem

Aging Phenomena in Gaseous Detectors

LHCb Muon Collaboration

Vsevolod Souvorov, CERN


Contents1.LHCb detector overview2.Aging requirement3. The aging CERN facility4.Chamber parameters5. Chamber status before test6. Aging procedure7. Results8. Conclusion.

• 5 Muon stations with 2 independent layers/station

•redundancy

• 435m2 of detector area with 1380 chambers

M1 M2 M3 M4 M5


The chamber position in the detector

Maximal particle rate 37.5 kHz/cm2

The LHCb spectrometer

Aging constraintsM1 M2 M3 M4 M5

R 1460 kHzt.b.d.

37.5 kHzMWPC

10 kHzMWPC

6.5 kHzMWPC

4.4 kHzMWPC

R 2186 kHzt.b.d.

26.5 kHzMWPC

3.3 kHzMWPC

2.2 kHzMWPC

1.8 kHzMWPC

R 380 kHzMWPC

6.5 kHzMWPC

1.0 kHzMWPC

750 HzRPC

650 HzRPC

R425 kHzMWPC

1.2 kHzMWPC

415 HzMWPC

250 HzRPC

225 HzRPC

Required Rate Capability per cm2

Technology Choice

The maximal expected count rate for MWP-chambers is about 37 kHz/cm2

(Muon station M2 Region 1). The accumulated charge density for 10 years ofLHCb operation under these conditions would be 0.5 C/cm on the wires and 1.7C/cm2 on the cathodes . To collect this charge , the chamber has been exposed tothe very intense Cs137 source of the CERN Gamma Irradiation Facility(GIF).


Particle rates andsystem technologies inLHCb Muon System.

The chamber position


GIF Facility Overview

137 CsEγ =662 keVStrength = 675 GBq in Feb 2001

Count rate ~30 kHz/cm2

MWPC

A1 B2

Beam

B1 A2


The main problem of test

due to small wire spacing value (1.5 mm) the test duration atnormal operational voltage is of about 1year (to collect 0.5 C/cmcharge).

to finish the test for reasonable time we need

not only to put chamber to the source as near as possible

and to increase twice the gap current (from ~280 till ~500 uA)

The basic chamber parameters.

Parameter Design valueNo. of gaps 4Gap width 5 mmAnode-cathode distance 2.5 mmWire spacing 1.5 mmWire diameter 30 µmOperating voltage 3-3.2 kV

Wire surface field 260 kV/cm

Cathode surface field 8 kV/cmTotal wire length per gap (m) ∼99.5Chamber volume (cm 3) 3000Sensitive area (cm 2) 1500Gas flow rate (l/hour) 2.75Gas gain ~10 5

Materials used for the chamber constructionStesalitFr4 (fire resistant fiber glass epoxy with coppercladsGoldAraldite 2011Adekit A 145/50Naturel rubber gasketOne side glued kapton foil.

HV

10k

100k

100k

100kWire-pad OR

100k

LVDS

LVDSChamber cross-section with the fourgaps A1, A2, B2, B1.


Gas mixture

Ar/CO2/CF4 (40%, 50%, 10%)

Gas purityAr 46CO2 40CF4 45

The open loop gas system is used to supply the chamber.The gas pipes stainless steel for supply copper for exhaust

The gas flow rate and the gas mixture composition are controlledremotely with electronic mass flow controllers. To mix thecomponents it was used the standard CERN mixing rack.

The chamber gaps are connected with gas in seriesfrom B1 till A1


The chamber status before the test

Gap A1 A2 B2 B1Current (nA) 27.8/0.5 28.3/0.3 26.8/0.1 35.4/1.1Point number 81 52 29 27Range (nA) 18.6 8 2.5 21

Gain uniformity measured with source

The inner A2, B2 gaps are more uniform than outer A1,B1.

A1/B1 A2/B2 A2/B1A1 0.68(0.78)A2 1.01(1.06) 1.19(1.17)

The cathode pads to ground resistances are in the range 200-500 Gohm.The gas leaks are of about 7 cm3/min or ~10-3 of the chamber volume. The O2admixture is at the level of 400 ppm.

Gas gain of the gaps measured with the GIF beam and source (in brackets)


Monitored chamber parameters .

-the relative gas gain through the ratios of the gap A1,A2,B2currents to the reference B1 current. The B1 gap is under HV onlyvery short time per day and always flashed with the fresh gas.

-dark currents including self-sustaining current following the beamoff (the chamber firing with Malter current)

-ground / cathode pad resistances

-the pressure, temperature and humidity are controlled manuallyand remotely.

The dark currents are measured with current monitors with aresolution of about 1 nA.


The test runningGap currents vs time

100

150200

250

300

0 30 60 90 120 150 180

T (days)

I (uA

)

A2B2B1

A1

The linear current density is less than 0.03 uA/cm.

The test starts 7th of February.

The A2 gap test is finished after the wire break (4.06). The A1 gap isturned off (16.07)due to HV trip. After collecting in the gap B2 chargeequivalent to the 5 LHCb years the test was stopped (27.07) to repair thechamber.


Linear charge vs time

00.05

0.10.15

0.20.25

0.3

0 50 100 150 200

T (days)

q (C

/cm

)

A1

B2A2

Test results.The linear charge collecting

Gap A1 A2 B2Total charge (C) 1470 1700 2540Linear density (C/cm) 0.15 0.17 0.255Cathode density(C/cm2) 0.49 0.57 0.83LHCb years 3 3.4 5.1

The final charges collected and connected values


The A1,A2,B2 are the relative I(Xi)/I(B1) currents (Xi A1,A2,B2). All thecurves are a set of some flat parts. The jumps between parts have the samesign (+/-) in the every fixed point for the all the curves and can not beattributed to the aging.


Relative current vs time

0.7

0.8

0.9

1

1.1

1.2

1.3

0 20 40 60 80 100 120 140 160 180

T (days)

I (X)

/ I (

B1)

A2

B2

A1a b c ed f

The de/ef step connects with the spontaneous change of the B1 gain at thepoint e.

B2,B1 currents as function of gas density

4.84.9

55.15.25.3

-0.03 -0.02 -0.01 0 0.01 0.02

ln (n/no)

ln (I

)

B2(df)

B1(de)

B1(ef)

There are only one dependence on the gas density for the gap B2 and two ones for theB1.The de/ef jump of the B1 gain is practically equal to the B2 step. The same behavior isfixed for the gap A1.


Date 04.02.01 10.04.01 20.08.01 23.08.01

A1/B1 0.79/0.04 0.72/0.03 0.65/0.04 0.67/0.04

A2/B1 0.8/0.03 0.82/0.03 0.8/0.03 0.74/0.03

B2/B1 0.76/0.03 0.76/0.03 0.77/0.02 0.75/0.03

The gain comparison with the Americium source

It is not seen any deterioration of the A2, B2 gaps. The tendency of the gaindecrease for the A1 gap is not confident due to lack of accuracy.

The pad to ground resistances decrease 10-100 times in very short time (~0.7LHCb years) with floating after in the reached range due to humidityvariations. These changes stay well out of the practical limit.

The initial and final dark currents shown a minor increasing which can notprevent the chamber successful work in muon detector .

Gap A1 A2 B2 B1Initial current(nA) 1.3 1.2 1.6 1.2Final current(nA) 3 9 3 1.5-3.


The self-sustaining current following the GIF turn off is observed and the currentranges , number of tests, the ‘positive’ issues, and corresponding probabilities are given.

Gap A1 A2 B2Current (uA) 0.5 0.4-3 0.5-1Test number 182 140 218‘Yes’ number 1 11 14Probability 0.00 0.078 0.064

The B2 current appears 10 times in the107 first tests and 4 times in the second ones. Theprobability of such division is ~9% from the probability of the equal share. One can say thatthe developing of Malter process is not observed. The observed currents are a result of notvery good cleaning the cathode surfaces.



The cathode surface of the wired cathode plane. The brownish region near thewire supporting bar is result of HV instability of wire plane. The same region islocated in the A2 gap near wire supporting bar where the wire breaks down.

Aging Phenomena in Gaseous DetectorsThe cathode deposit is observed in all the cathodes. The cathode of the gap A2 isshown. The deposit border is at the distance 5-10 mm from the gap border. Thedeposit layer has the tracks of cleaning procedure.

The brownish spot onthe border between two pads is probablyresult of sparking.

Aging Phenomena in Gaseous DetectorsThe A1 wired plane corner. The chemical activity of the gas appears as theetching of the FR4 surface. The treated surface shows the basic structure of thematerial due to removing the thin surface layer.

The etching is seenoutside the gapfollowing the gas leakflow.

The etching is absentin the reference gap.


Anode wires

stayed fairly clean

there is some minor deposits (blackdots) sporadically scattered near twobrownish spots on the cathodes. Thetotal spoiled area is negligible (<10 -3).

Conclusion1.The analysis of the data after the dose corresponding to 5 LHCbyears does not show any deterioration, which would prohibit the useof these chambers in LHCb experiment in these time range.

2. Gold plating the cathode surfaces is a good protection of thecathode aging.

3. The bad cleaning procedure can provoke growing the deposit.

4.The interaction of the gas mixture with the FR4 needs specialstudying.

5.The breakdown the one wire and the HV instability of the A1 gapis indication that the the HV operational conditions should beoptimized.


Documents

First results of an aging test of a full scale MWPC ... · normal operational voltage is of about 1year ... Anode-cathode distance 2.5 mm ... (the chamber firing with Malter current)-ground