Control of the RPC ageing effects in the ATLAS chambers at the

GIF - X5 CERN irradiation facility

Siena 2004

Siena, 24/05/2004

By G. Aielli on behalf of the Atlas RPC collaboration: Lecce, Napoli and Roma2

Ageing test at X5 basics 6 gas volumes (3 ATLAS BMLD units) under test since the

end of 2002 at the GIF-X5 of CERN Typical counting rate under full source 700 Hz/cm2

(expected rate in the barrel 10-20 Hz/cm2) 1 Atlas equivalent ageing year 30 mC/cm2 integrated

charge including a safety factor >5 (109 expected counts/cm2 x 30 pC/count)

The Gamma Irradiation Facility is located downstream of the X5 beam final dump. Inside this zone, a 137Cs source (20 Ci)produces an intense flux of 660 keV photons which can be (nominally) reduced up to a factor 10000 using a system of lead filters.

Setup for the external RH control

Experimental setup Three production chambers (BML-D) are installed on the beam line, equipped with a movable trigger system made of 3 layers of plastic scintillators suitable also for a cosmic rays run.

The 3 chambers have 2 detector layers, each made of 2 gas volumes. Each gas volume has 2 readout panels to read both coordinates. One track can be reconstructed using up to 6 point

Signals from the chambers are read out by the TDCs developed for the KLOE experiment, and acquired with a LabView program.

The DCS system (also implemented in LabView) records LV and HV values, gap currents, gas composition, together with all relevant environmental data such as pressure, temperature, relative humidity.

The HV is automatically corrected for the gas density effect to a standard condition of 980 mbar and 20°C. The gas relative humidity can be set by the user and is also monitored by the DCS.

Main historical eventsCLOSED LOOP OPERATION

At the beginning of July 03 the gas closed loop was introduced on 4 out of the 6 tested gas gaps. The gas recirculated fraction has been 50% until 28/9/03 when it was brought up to 95% The recirculation permitted to enhance the gas flow rate up to 1 change/0.5 hours. High change speed is needed for removing the impurities and to distribute the humidity (time constant of few hours) Two metallic filters and one molecular sieve remove the impurities from the gas before recirculating it.Chemical analysis of the recirculated gas didn't show any excess of “anomalous" components with respect to the open flow operation until the filters are not exhausted.

HUMIDIFICATIONSince last January all the gas volumes are operated with average 30% RH gas mixture to test the effect on the long term stability of the bakelite resistivity. Since no negative effects were detected in the previous test, from the beginning of october the RH in the gas mixture is about 50% In the same time also the external RH stabilization at 55% has been introduced, as a result: the inversion of the resistivity trend

Ageing Status Integrated Charge up to 6 May 2004 [10 Atlas Years = 0.30 C/cm^2 including a safety factor >5] Up to now the average ageing is around 6-7 Atlas Years. All

the gas gaps under test show very good detection efficiency even at fully open source.

Ageing Progress

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Counting rateOpen vs. closed source

Sample tracks source open

Efficiency plateau at various rates (phi view) Cosmic rays data april 2004

The plateau spread depends on the gas volume resistivity

Resistivity measurementsWe use two methods to measure the resistivity of the bakelite plates during the test:

The chambers work in pure Ar in self triggering streamer mode so that the plasma resistance become negligible compared to the electrodes resistance. Above this value, an I-V curve gives the value of the resistivity of the bakelite.

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Linear increasedominated bybakeliteresitivity

I-V characteristicin pure Ar

The efficiency plateaus with full source and with source off are compared. The shift of the plateau with full source is due to the gap current, which produces a voltage drop across the bakelite plates. From the voltage drop and the current measured we calculate the plates resistivity.

This approximation is as good as vgas has a well defined value inside the gas gap: the fluctuations are small with respect to the average value

Resistivity measurements (2)

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

ext RH control 50%

ON OFF

Resistivity evolution vs RH

Systematic effect in resistivity measurement Ar vs. Plateau

We hypotize an inhomogeneous effect of the humidification and a systematic effect due to the sampling area of the trigger

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Noise current history The noise is due

to the inner surface pollution and permanent damaging due to the impurities from the gas degradation (in particular HF)

The pink curve (open flow) was fully recovered enhancing the flow.

The current increase is much amplified by the temperature and by insufficient gas flow

The high RH does not have direct effect on the noise

In the last part only the closed loop chambers are affected

Ohmic current evolution

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Working current evolution

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LastTrends

Working current

Ohmic current

What are we learning on ageing? Two main ageing effects are under study: the increase of

the electrode plate resistivity which reduces the rate capability and the increase of the noise

The gas high flow rate together with the proper RH helps to solve both problems: This high flow rate is practically reached thanks to the closed loop gas system.

The two options are safe and effective only together The results clearly indicate how RPCs have to be handled to

survive a very long working time in hostile environment: temperature < 25°C RH=50% gas flow rate 1 change/hour

One drawback is that most of the system criticality is moved on the gas system: recirculation mechanism, gas purifiers and humidification control. On the long time scale the recirculated chambers accumulated most of the shocks

A problem in one of these systems can lead to very dangerous situations so a very tight and redundant control is needed by the DCS

Block of the recirculation gas rack

“Ohmic” current increase

25/04/04 about 21:00 flux stopped ABS factor 1 HV=9600 each 8 hours HV=7000 for 1 hour Chamber stopped after 3 days

Further healing Gas system restarted V-A characteristic measured after 6 hours of flow Pure Ar open loop at 8-10 l/h per gap HV=2000 V for 3 days -> currents about 5 A cooling

down The steps represents the source switch off time The discontinuity happened after an Ar complete scanning Last enhancement: Ar+5% i-C4H10 improves the F-

cleaning

F- measurement in the exhausted gas

out

Recirculation out

F- readout

Gas rack

Target 50%H2O+ 50%TISAB

F- measured in the output gas

We put in evidence the effect of the chamber turned on with Argon 1.3

moles/20ml/10s HV=2000 I=2A

5.9 moles/20ml/10s HV=2000 I=35A

This technique is applied after the experience made on small samples

The all history The first plot is made with standard mixture and shows an

initial very high rate and a deceleration. The rate is stable afterwards (with Ar) and strongly

depends on the current The rate decreases very slowly (reservoir unlimited) The cleaning mechanism is still under investigation with

various hypothesis: UV effect, mechanical effect of the argon plasma, electrochemical effect due to the conduction…

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Plateau drift effect The comparison of the plateau curves relative to the same readout panel in different tests reveals a drift of the order of ± 100V This does not affect the resistivity calculationGas stability problem? HV CAEN drift?This effect could be correlated to the temperature.In general it is difficult to control the absolute plateau stability over long times.

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Drift effect on the 6 gas volumes

The effect is not homogeneous over all the gas gaps but it is qualitatively similar.

Conclusions We are approaching 2 years of continuous ageing

(with some technical stops) In such a time it was possible to collect a statistical

sample of errors unwanted effects and in general what-was-not-foreseen cases that are precious to estimate the system criticality and its weak points

We had 3 units under test but in ATLAS we have 1000 with a certain spread in the initial quality and environmental conditions.

On the other side: the environment is much less critical than the GIF we learned how to control the main ageing effects foreseen

Now what remains: to enhance the safety margin reducing the unwanted error probability by implementing a tight and smart DCS