control of the rpc ageing effects in the atlas chambers at the gif - x5 cern irradiation facility...
TRANSCRIPT
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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Date
tota
l c
ha
rge
(m
C/c
m^
2)
Gap 1
Gap 2
Gap 3
Gap 4
Gap 5
Gap 6
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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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
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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8500 9000 9500 10000
Standard voltage (V)
Eff
icie
ncy
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Standard voltage (V)
Eff
icie
ncy
No correctionAfter correction for resistivity
No source
full source
Vgas
=Vgap
- Rbak
Igap
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11/10/02 19/01/03 29/04/03 07/08/03 15/11/03 23/02/04
rho
(G
Oh
m c
m)
a 20
°C
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Inte
gra
ted
Ch
arg
e (m
C/c
m^
2) -
RH
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
65 43 21
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21_09_2002
30_12_2002
09_04_2003
18_07_2003
26_10_2003
03_02_2004
13_05_2004
gap1
gap2
gap3
gap4
gap5
gap6
Gas umidificatoUmidificatore
Gra
die
nte
tem
pera
tura
Humidification
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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rren
t (m
icro
A)
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per
atu
re (
°C)
gap 1 open flowgap 2 open flowgap 3 closed loopgap 4 closed loopgap 5 closed loopgap 6 closed loopT
Working current evolution
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Cu
rren
t (m
icro
A)
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Te
mp
era
ture
(°C
)
1 ric/2 h (gas shearing with CMS) 1 ric/.5 h + recirculation
50% gas RH control
Closed loop lock-up
Filte
r ex
haus
ted
Wro
ng m
ixtu
re
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25gap1gap2gap3gap4gap5gap6temperature
openflow new filter
new molecularsieve
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openflow new filter
new molecularsieve
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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11.5
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44.5
5
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Time (x10 S)
[F-]
mic
ro M
ol/2
0 m
l
15001600170018001900200021002200230024002500
Ap
plie
d v
olt
ag
e (
V)
[F-]
Applied Voltage
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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run04/04 s=0run04/04 s=1run06/03 s=0run06/03 s=1run12/03 s=0run06/03 s=1
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run04/04 s=0run04/04 s=1run06/03 s=0run06/03 s=1run12/03 s=0run06/03 s=1
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