Andrea Di Simone CERN PH/ATC and INFN-CNAF On behalf of ATLAS RPC groups:

11Andrea Di SimoneAndrea Di Simone – INFN Roma2

Andrea Di SimoneCERN PH/ATC and INFN-CNAF

On behalf of ATLAS RPC groups:Lecce, Napoli, Protvino, Roma2

Ageing test of ATLAS RPCs at X5Ageing test of ATLAS RPCs at X5

22Andrea Di Simone – CERN PH/ATC and INFN-CNAF

Ageing effects in bakelite RPCs

Experimental setup

Plate resistivity increase

Current monitoring

Damage recovery

Conclusions

Outline


Ageing effects in bakelite RPCs

Long time operation of resistive plate chambers is known to produce two main ageing effects:

gradual increase of the electrode resistivity (i.e. reduced rate capability) under very high working currents. This effect, however, is known not to be relevant for the ATLAS experiment:

previous tests showed that after an ageing of ~10 ATLAS years, chamber performance remains above the ATLAS requirements.

degradation of the inner surface of the plates due to operation with fluorine-rich gas mixtures, leading to an increase of the noise in the detector


Plate resistivityThe plate resistivity is known to be related to environmental parameters such as temperature and relative humidity:

Higher T Lower ; Higher RH Lower

Plates kept under high current densities (hundreds of A/m2) for long periods, show a gradual increase in resistivity which is found to be faster when the plates are operated at lower RH values. This effect is selective wrt the working voltage polarity, i.e. any change in environmental RH is more effective when applied to the anode side of the plate.

both the gas mixture and the external environment need to be humidified in order to operate the anode sides of the two plates in proper RH conditions.


Experimental setup

Beam

3 standard productionchambers(BML-D) in the area.6 gaps under ageing test.

137Cs source (20 Ci);660 keV photons


Ageing status

Ageing Progress

0

50

100

150

200

250

12/1

1/2

002

12/0

1/2

003

12/0

3/2

003

12/0

5/2

003

12/0

7/2

003

12/0

9/2

003

12/1

1/2

003

12/0

1/2

004

12/0

3/2

004

12/0

5/2

004

12/0

7/2

004

12/0

9/2

004

12/1

1/2

004

12/0

1/2

005

12/0

3/2

005

12/0

5/2

005

12/0

7/2

005

12/0

9/2

005

Date

tota

l charg

e (m

C/c

m̂2)

Gap 1

Gap 2

Gap 3

Gap 4

Gap 5

Gap 6


Plate resistivity measurements (1)

First method: the chambers are filled with Ar, and operated above 2kV, where the voltage drop across the gas remains constant. In these conditions, the I-V curve is linear and the ratio V/I gives the value of the resistance of the bakelite.

0

50

100

150

200

250

300

350

400

450

500

0 500 1000 1500 2000 2500 3000

Standard voltage (V)

Gap

cu

rren

t (u

A)

gap 1

gap 2

gap 3

gap 4

gap 5

gap 6

Linear increasedominated bybakeliteresitivity

I-V characteristicin pure Ar


Plate resistivity measurements (2)

Second method: the efficiency plateaus with full and closed source are compared. The voltage difference between the two plateaus is due to the gap current, which produces a voltage drop across the bakelite plates. From the voltage drop and the measured current we calculate the plates resistivity.

0

0.2

0.4

0.6

0.8

1

1.2

8500 9000 9500 10000


Eff

icie

ncy

0

0.2

0.4

0.6

0.8

1

1.2

8500 9000 9500 10000


Eff

icie

ncy

No correctionAfter correction for resistivity

No source

full source

Vgas=Vgap-

RbakIgap


Resistivity evolution

0

50

100

150

200

250

300

350

400

11/10/2002 19/01/2003 29/04/2003 07/08/2003 15/11/2003 23/02/2004 02/06/2004 10/09/2004

(G

c

m) @

20

°C

0

50

100

150

200

Inte

grat

ed C

harg

e (m

C/c

m^2

) R

H

gap 1 Argon gap 2 Argon gap 3 Argon gap 4 Argon gap 5 Argon

gap 6 Argon Gap1 eff. plateau Gap2 eff. plateau Gap3 eff. plateau Gap4 eff. plateau

Gap5 eff. plateau Gap6 eff. plateau mC/cm^2 RH fresh gas

Plate resistivity evolution

ext RH control ON OFF


Plate resistivity evolution

Resistivity evolution

0

50

100

150

200

250

300

350

400

11/10/2002 29/04/2003 15/11/2003 02/06/2004 19/12/2004 07/07/2005

rho (G

Ohm

cm

) a

20 °

C

0

50

100

150

200

250

Inte

gra

ted C

har

ge

(mC

/cm̂

2) - R

H

gap 1 Argongap 2 Argongap 3 Argongap 4 Argongap 5 Argongap 6 ArgonGap1 eff. plateauGap2 eff. plateauGap3 eff. plateauGap4 eff. plateauGap5 eff. plateauGap6 eff. plateautemperaturemC/cm 2̂RH fresh gas

ext RH control 50%

OFF ext RH control 40-50%

ext RH


Plate resistivity evolution (2) Each detector layer consist

of two gas gaps with the gas flowing serially from the lower to the upper onesOnly the 6 lower gaps were

kept at the working point The upper ones are normally

kept at HV=0 After ~2 years of operation,

the plate resistivities of the upper chambers are consistent with the initial values

The operating current is the primary cause of the observed increase in plate resistivity


Current monitoring

Chamber currents have been continuously monitored during the test:

currents at working point

ohmic currents @ 5kV

Both current have proved to be an important tool for diagnostics of the gap operation:

Ohmic currents are an indicator of the presence of pollutants on the plate surface, rather than of an actual damage of the surface, and are very sensitive to any problem related to the recirculation system’s filters.

Working currents are sensitive to gas mixture problems and to filter exhaustion


Current monitoring (2)

Working current evolution

0

5

10

15

20

25

30

35

40

21/10/02 29/1/03 9/5/03 17/8/03 25/11/03 4/3/04

Cur

rent

(mic

ro A

)

5

10

15

20

25

30

Tem

pera

ture

(°C)

gap 1 open flow gap 2 open flow gap 3 closed loop gap 4 closed loopgap 5 closed loop gap 6 closed loop T

1 volume/2 h 1 volume/0.5 h (recirculation)

50% external RH

Wrong mixtureFilter exausted


Current monitoring (3)


Detector noise and surface damage

Fluorine rich gas mixtures produce, under electrical discharge, F- ions which can damage the inner surface of the gas gaps. This results in an increase of the detector noise.

This type of damage can, to some extent, be recuperated by operating the chamber at lower voltage, large gas flow and possibly with isobutane enriched mixtures (see G. Aielli's presentation, session N29).

We illustrate in the following a significant example of damage/recovery.


Surface damage (1)

A major malfunctioning of the recirculated gas system occurred at an integrated charge corresponding to 7 ATLAS years (safety factor 5). At the same time, the DCS system has not been able to shut down the HV.

The chambers have continued operating at working point, under full irradiation, without any gas flow.

This lead to a damage to the internal surface of the plates, detectable from an increase (by a factor 2) of the working currents at closed source.Moreover, the presence of pollutants on the surface caused an increase by a factor 4 of the ohmic currents of the gaps.


Surface damage (2)

Ohmic current increase

0

1

2

3

4

5

6

1000 2000 3000 4000 5000 6000 7000 8000 9000

time (minutes)

ga

p c

urr

en

t (u

A)

gap 1

gap 2

gap 3

gap 4

gap 5

gap 6


Surface damage (3)

Working current increase

50

70

90

110

130

150

170

190

1000 2000 3000 4000 5000 6000 7000 8000 9000

time (minutes)

gap

cu

rren

t (u

A)

gap 1

gap 2

gap 3

gap 4

gap 5

gap 6

(full source)


Damage recovery

Increasing the isobutane concentration in the gas mixture has shown in the past to be very effective in the recovery of damaged bakelite RPCs.

The isobutane component was raised from 5% to 15%

Besides the recovery process, the performance of the chambers under this new gas mixture has also been studied.


Damage recovery - results

Chambers were kept at 7kV

We observed a decrease of the working currents on all the chambers

The ohmic current showed also a steady and regular decrease.

The ageing at normal working point has now restarted.

If no current increases are observed, the standard ATLAS mixture will be restored

0

5

10

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35

40

13_02_04 03_04_04 23_05_04 12_07_04 31_08_04

15

17

19

21

23

25

27

Serie1Serie2Serie3Serie4Serie5Serie6Serie7

15%isobutane

0

0.5

1

1.5

2

2.5

13_02_04 03_04_04 23_05_04 12_07_04 31_08_04

15

17

19

21

23

25

27

29Serie1Serie2Serie3Serie4Serie5Serie6Serie7

15%isobutane

Workingcurrents

Ohmiccurrents

I(A

)I(A

)

T (

°C)

T (

°C)

gap 1gap 2gap 3gap 4gap 5gap 6T



Damage recovery – results (2)

0

5

10

15

20

25

30

35

40

13_02_2004 03_04_2004 23_05_2004 12_07_2004 31_08_200415

17

19

21

23

25

27

Serie1Serie2Serie3Serie4Serie5Serie6Serie7

15%isobutane

0

0.2

0.4

0.6

0.8

1

1.2

13_02_2004 03_04_2004 23_05_2004 12_07_2004 31_08_2004

15

17

19

21

23

25

27Serie1Serie2Serie3Serie4Serie5Serie6Serie7

15%isobutane

Current evolution isotherms

Working currents Ohmic currents

I(A

)

T (

°C)

I(A

)

T (

°C)




Conclusions

RPC operation with proper relative humidity in both the gas and the environment limits (and could eliminate) the increase of the plate resistivity under high operating currents, which is one of the dominant ageing effects in bakelite RPCs

All along the test, chamber performance (efficiency, cluster size, rate capability) remained largely above the ATLAS requirements.

After a serious damage to the inner gap surface due to a problem with the gas system, the gaps have been completely recovered using an isobutane enriched gas mixture.

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Andrea Di Simone CERN PH/ATC and INFN-CNAF On behalf of ATLAS RPC groups:

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