N11-4 Ageing Test of ATLAS RPCs at CERN`s Gamma
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1. 25 Filter exausted 25 7 4 im 5 ka DF 5 15 4 T T15 8 5 10 JAIN 5 2 3 i 10 g 5 21 10 02 29 1 03 9 5 03 17 8 03 25 11 03 4 3 04 Fig 5 Working current evolution Gap numeration is given here starting from the nearest to the source gap 1 to the farthest gap 6 The gas flow in volumes per hour is reported together with the start of the recirculation test External RH control is indicated as well Ohmic current evolution gap 1 open flow gap 5 closed loop gap2open flow gap 3 closed loop gap 4 closed loop gap 6 closed loop T 0 7 9 ut 44 recirculation stop y 30 oO P 1 Se Current micro A Randi Lo a Temperature C a i Fs A L ATTAT 3 at Ek f A AN y F M KVIA ita 0 aN 21 10 02 29 01 03 09 05 03 17 08 03 25 11 03 04 03 04 Fig 6 Ohmic current evolution Gap numeration is given here starting from the nearest to the source gap 1 to the farthest gap 6 increase which is amplified in summer is however completely reversible as shown in the plot Working currents have also proven to be an effective tool to detect any malfunctioning of the gas recirculation system The two arrows in figure 5 point out peaks of current due to two of such events the first is due to a wrong gas mixture and the second one to the exhaustion of one of the2. defined as the total current driven by the gas as read on a resistance put on the ground connection of the gap is continuously monitored by the DCS system It is a reliable estimator of the number of avalanches in the gap and therefore of the noise level when measured at low irradiation Gap current measurements as a function of the operating voltage where taken almost daily all along the test pointing out two main causes for the increase of the noise in the detector e temperature increases have shown to be responsible for increases in the gap currents at working point e any malfunctioning of the gas system such as insufficient total flow or lack of maintenance of the recirculation system s filters had a serious impact on the gas gaps causing a rapid increase of the gap currents Figure 5 shows the variation of the working currents of the six gaps along the test as well as the operating temperature Current measurements are taken at closed source It is clear the correlation between temperature and current variations in particular for Stationl gap2 gap 2 in the plot The current 0 7803 8701 5 04 20 00 C 2004 IEEE Working current evolution gap 1 open flow gap 2 open flow gap 3closedloop gt gap 4 closed loop gap5closedloop gap 6 closed loop T 40 50 extemal RH ve ae 30 35 T Ht Ls Wrong mixture 30 ay
3. filters of the recirculation system In addition to working current a useful parameter for the diagnostics of the plate surface status is the measurement of ohmic current defined as the gap current at low voltage where no gas multiplication is possible Ohmic currents are only due to conduction through the internal surface and can therefore indicate the presence of pollutants on the plate surface before any actual damage of the surface can occur The typical HV range for ohmic currents measurements is 5 7kV Figure 6 shows the evolution of the ohmic currents for the gaps under study A comparison with figure 5 clearly shows that ohmic currents have very small absolute values but show larger relative increases as a consequence of gas problems For example the effect of filter exhaustion is much bigger on the ohmic currents than on the working currents Wrong gas mixture didn t have any influence on the ohmic currents as expected while a recirculation stop with no major effects on the working currents lead indeed to a high increase in the ohmic currents V DAMAGE RECOVERY The X5 GIF test was also an important benchmark for testing suitable techniques of damage recovery The most significant case of recovery followed an accident in which after an integrated charge corresponding to about 7 ATLAS years with a safety factor of 5 a malfunctioning of the recirculation system caused the suspension of the gas flow while a problem with the D
4. test Figure 2 shows the efficiency vs HV plots for Station 1 gap 1 at different source intensities The high current driven by the gaps under source irradiation causes a voltage drop across the plates Due to this drop the plateaus show a shift towards higher voltages and a slower raise compared with the plateaus taken without irradiation This effect is enhanced in winter by low operating temperatures 10 15 C which increase the plate resistivity IV AGEING EFFECTS IN RESISTIVE PLATE CHAMBERS Long time operation of resistive plate chambers is known see for example 10 to produce two main ageing effects which degrade the detector performance e changes in the chemical composition of the plates may increase their resistivity thus reducing the rate capability e the quality of the inner surface of the plates is degraded by continuous operation with fluoride rich gas mixtures leading to an increase of the noise in the detector Both effects have been extensively studied during this test and appropriate techniques have been introduced to reduce to an acceptable level all the losses in performace related to the detector ageing A Plate resistivity evolution It has long been known 6 that the electrical properties of the plastic laminate depend on the environment temperature and relative humidity Materials such as phenolic melaminic laminates have a negative temperature coefficient 1 e their re sistivity decreases for inc
5. Ageing Test of ATLAS RPCs at CERN s Gamma Irradiation Facility G Aielli M Alviggi V Ammosov2 M Bianco M Biglietti P Camarri V Canale M Caprio R Cardarelli G Carlino G Chiodini F Conventi R de Asmundis M Della Pietra D Della Volpe A Di Ciaccio A Di Simone E Gorini F Grancagnolo P Tengo B Liberti A Nisati Fr Pastore S Patricelli R Perrino M Primavera R Santonico G Sekhniaidze E Solfaroli S Spagnolo Yu Sviridov R Vari S Veneziano V Zaets University of Rome Tor Vergata and INFN Sezione di Roma II University of Naples Federico II and INFN Sezione di Napoli d THEP Protvino University of Lecce and INFN Sezione di Lecce e INFN Sezione di Roma I Abstract An ageing test of three ATLAS production RPC stations is in progress at X5 GIF the CERN irradiation facility The chamber efficiency as a function of the operating voltage is measured at different source intensities up to a maximum counting rate of about 700 Hz cm All along the test plate resistivity has been monitored using two different methods by measuring the I V characteristics in pure Ar and by comparing the efficiency vs high voltage curves at different source intensities The increase in plate resistivity which is one of the dominant ageing effects for RPCs with phenolic melaminic electrodes has been shown to be strongly dependent on envir
6. CS system prevented the HV to be turned off The result was a three days operation period under full irradiation with no gas flow As a consequence the chambers showed a very high increase in both ohmic and working currents which prevented the chambers from reaching full efficiency under full irradiation At closed source however chamber performace was still satisfactory The recovery procedure consisted in the following steps e the chambers were fluxed with pure Argon at 3 vol umes hour for about five weeks and a sensitive decrease of the gap currents was observed Nevertheless the noise level at working point was still too high to allow efficient operation of the chambers under full irradiation being still ohmic and working currents greater than their normal values by a factor 3 e for the following two months the chambers were fluxed with an isobutane enriched mixture C2 H Fy i C4 A190 S Fe 84 7 15 0 3 keeping their HV at a low value 7kV The decision to increase the isobutane concentration is based on the observation made in previous tests about the strong effect of isobutane in recovering a damaged plate surface Indeed during the operation with the new mixture both ohmic and working currents returned to their normal values as shown in figure 7 where the values of the currents are plotted as a function of time For clarity only the measurements taken at the same temperature are shown VI CONCLUSION The measu
7. DCs working in common stop mode see 5 which record up to 16 hits per event per channel in a 2us gate Both the leading and falling edges of the signals are recorded The data acquisition is performed using a LabView application The DCS system monitors both the low and and high voltages as well as the gap currents The chambers are operated with the standard ATLAS gas mixture C2 H F4 i C4H10 S Fe 94 7 5 0 3 Gas composition together with all relevant en vironmental data such as pressure temperature and relative humidity are controlled as well The gas relative humidity also monitored by the DCS is set in the range of 30 50 by bubbling in water a fraction of the total gas flow In July 2003 the gas closed loop was introduced on 4 out of the 6 tested gas gaps in order to simulate the real working conditions in ATLAS The output gas of the chambers goes through a system of filters and then is sent again in the gaps together with a fraction of fresh gas The two remaining gaps were left in open flow for comparison The total gas flow was set at 40 l h and the fraction of recirculated gas has been gradually increased up to 95 Gas samples were taken at the input and output of the chambers and analyzed in order to find any polluting component related to the chamber operation II CHAMBER PERFORMANCE During the test the chamber efficiencies are periodically monitored using cosmic rays triggered by the scintillators Higher statis
8. IMENTAL SETUP The Gamma Irradiation Facility GIF located downstream of the final dump of the X5 beam uses a 37Cs source of 20C i to produce a large flux of 660 keV y rays A system of lead filters allows to reduce the flux up to a factor of 10 The X5 muon beam can also be sent into the area A more detailed description of this facility and of the characteristics of the y flux can be found in 1 Three production ATLAS RPC chambers type BML D with gas volumes of 1 4m are installed in the GIF area along the beam line A schematical view of the setup is shown in figure 1 The chambers have 2 0 7803 8701 5 04 20 00 C 2004 IEEE detector layers which are read out by strips oriented in both the 7 and y directions The chambers are perpendicular to the X5 beam line with the long side about 4m oriented in the vertical direction For details on the chamber structure see 3 The trigger is provided by the coincidence of three scintillator layers of 33x40 cm each made of a three slabs hodoscope During the beam runs the three layers are aligned along the beam line while for the cosmic rays runs they are arranged as a telescope with the axis oriented at 40 degrees with respect to the vertical direction in order to maximize the trigger rate Signals from the frontend electronics are sent to a standard ATLAS splitter board 4 as in the final architecture foreseen for the trigger and readout electronics and subsequently to T
9. i 23 S o6 12 E e 0 4 19 Ohmic gt currents 0 15 13 02 2004 03 04 2004 23 05 2004 12 07 2004 31 08 2004 Fig 7 temperature are plotted was shown to be a very early and safe indicator of gas poison ing These increases were completely reversible provided that the gas flow and composition are promptly restored to correct values The present gas recirculation system has been proven to be so far at the level of 4 ATLAS years of ageing adequate to ATLAS provided that the filters are properly maintained and changed when necessary Gap current isotherms The values of ohmic and gap current of the six gaps are plotted as a function of time Only measurements taken at the same Damage recovery procedures such as Argon fluxing and low voltage operation with isobutane enriched mixtures were successfully applied after a critical stop of the gas system which caused major damages to the internal plate surface ATLAS RPCs have thus shown to be robust enough to tolerate extremely harmful events on the condition that proper recovery techniques are applied REFERENCES 1 S Agosteo et al A facility for the test of large area muon chambers at high rate CERN EP 2000 031 February 16th 2000 2 A Ferrari and P Sala ATLAS Internal Note MUON NO 162 1997 3 ATLAS Muon Collaboration ATLAS Muon Spectrometer Technical De sign Report CERN LHCC 97 22 4 ATLAS Collaboration ATLAS First Level Trigger Technical De
10. imited using PET foils The air relative humidity near the chambers is hence controlled by means of two humidifiers Two different methods were followed for the resistivity measurements HV drop correction This method is based on the comparison of the efficiency plateaus with and without source irradiation The current driven under irradiation is much higher than the one just due to the muon beam or cosmic rays This current produces a significant voltage drop across the plates and the voltage applied to the gas can be written see 7 as Vga V Boil gap 1 where Vjas iS the effective voltage on the gas gap V is the power supply voltage and I is the current driven by the gap This drop causes the efficiency plateaus under irradiation to be shifted at higher HV values as shown in figure 2 This shift allows to evaluate the plate resistivity when the gap current is also measured I V characterstic in pure Argon The standard ATLAS gas mixture is replaced by pure Ar As shown in figure 3 the I V curve is characterized in this case by a transition region around V 2 kV with a fast current increase For higher voltages a linear current increase is observed We assume that above the transition voltage the drop across the Argon remains constant so that the slope AV AJ in the linear region gives the plates total resistance Figure 4 shows the evolution of the plate resistivity as a function of time for the six gaps under test The rela
11. onmental relative humidity Indeed after setting to values between 40 and 50 the relative humidity of both the operating gas and the air surrounding the chambers a significant decrease of the plate resistivity has been observed I INTRODUCTION The accelerator background at LHC will be dominated by soft neutrons and gammas that are generated by interactions of beam protons at very small angle 2 Such a heavy background calls for severe requirements on the trigger detector in terms of rate capability and time resolution which is crucial for bunch crossing identification Moreover for almost all the chambers installed in the ATLAS cavern it will be impossible to perform any hardware reparation or substitution once the commissioning of the detector is terminated It is therefore crucial that the chambers do not show any abnormal ageing effect which could degrade their performace during the 10 years of the ATLAS operation For this reason an ageing test of three production ATLAS RPCs is being performed at CERN The chambers are irradiated with y rays in order to test their rate capability and to integrate the total charge of 0 3C cm corresponding to 10 ATLAS years at the rate of 100Hz cm which includes a safety factor of 5 39 062023644 An Corresponding author Tel E mail drea DiSimone roma2 infn it Fig 1 Experimental setup at X5 GIF with the trigger configuration used for cosmic rays runs II EXPER
12. reasing temperature Moreover when the plates are exposed to dry air circulation their resistivity gradually increases even by a large factor This effect is completely reversible once the sample is left at room RH for a sufficient time On the other hand plates kept under high current densities hundreds of zA m7 for long periods also show a gradual increase in resistivity which is found to be faster when the plates are operated at lower RH values This effect of the relative humidity has been shown 8 to be selective in the sense that its effect is much more effective if the humidification is performed on the anode side of each plate These observations suggest that both the gas mixture and the external environment need to be humidified in order to guarantee that the anodes of the two electrode plates are operated in the proper RH conditions To achieve the necessary control on the external relative humidity the area surrounding 0 7803 8701 5 04 20 00 C 2004 IEEE G D j S 4l Bd ees eee a Ui a 3 a o8L Sit eta tote ba ads taled otc tate ae oe ee eee L A L E i AE E E E T E L A 0 4 ei ENE Peien rs no source eee eee ae Ho e full source L zo 0 8 5 9 9 5 10 10 5 HV kV Fig 2 Efficiency versus high voltage at different source intensities as measured with the muon X5 beam The plot refers to Station 1 gap 1 y after 5 ATLAS years of ageing the three chambers under test has been del
13. rements performed at X5 GIF on three ATLAS production RPCs provided important information on the ageing effects of the gas gaps As known from previous tests 10 the plate resistivity increased significantly at the beginning of the test mainly due to the dry environment Once the relative humidity of the experimental area has been put under control and set around 50 the resistivity showed a significant decrease The performance of all tested chambers remains much above the ATLAS requirements Indeed after an integrated charge corresponding to 7 ATLAS years including a safety factor of 5 all the gaps under test showed a very good detection efficiency even at fully opened source with a counting rate of about 600Hz cm Both ohmic and closed source working currents were used to monitor the quality of the internal plate surface Any problem related to the gas system has shown to generate an increase on both currents In particular the increase on the ohmic currents 0 7803 8701 5 04 20 00 C 2004 IEEE 40 e gap 27 35 gap 2 bk gap 3 30 eens 25 gap 5 gap 6 25 TT 29 T 20 9 2 21 15 i 19 Workin 10 O g currents 17 5 0 15 13 02 2004 03 04 2004 23 05 2004 12 07 2004 31 08 2004 1 2 3 27 gap 15 isobutane gap 2 gap 3 Zp 25 x lt gap 4 NZ gt gap 5 e 0 8 a
14. sign Re port CERN LHCC 97 22 1998 5 E Gennari et al 32 channel TDC VME board User s Manual http sunset roma2 infn it tdc tdcboard ps R Cardarelli et al Avalanche and streamer mode operation of resistive plate chambers NIM Nuclear Instruments and Methods in physics research A382 1996 7 G Aielli Ph D thesis University of Rome Tor Vergata February 2001 8 G Atelli et al Electrical conduction properties of phenolic melaminic laminates presented at the VII Workshop on Resistive Plate Chambers and related detectors Clermont Ferrand October 20 22 2003 9 G Aielli et al FT tons Production in RPC Operated with Fluorine Compound Gases presented at NSS 2004 Rome October 16 22 2004 G Aielli et al Performance of a large size RPC equipped with the final ATLAS front end electronics at X5 GIF facility NIM Nuclear Instruments and Methods in physiscs research A456 2000 77 6 10 0 7803 8701 5 04 20 00 C 2004 IEEE
15. tics efficiency measurements are made whenever the X5 muon beam is available For each event the muon trajectory is reconstructed The scintillator slabs of the trigger system of size 11x40cm would only allow a very modest tracking The tracking capability is therefore improved using the hits recorded by the RPCs as described below 1 hits recorded by the RPC under test are ignored in the tracking 2 only hits registered in a time window of 25ns around the beam peak are considered useful for the track reconstruc tion in order to reduce the probability to use an accidental photon hit in the tracking 3 for each electronic channel i e for each strip a dead time of 50ns is applied by the analysis algorithm in order to avoid any double counting due for example to after pulses 4 tracks are reconstructed only for events showing a single cluster in at least three layers out of the five available This is useful to improve the quality of the reconstructed track eliminating for example di muon events 5 a track in both the 7 and directions is required 6 the layer is considered efficient if it shows a cluster aligned to the track within 1 strip The steps described above are repeated for each gas gap Sin gle counting rates and average cluster sizes are also measured for each layer Chamber efficiencies at closed source measured periodically both with cosmic rays and with the X5 beam did not show any loss along the
16. tive humidity of the gas mixture is shown as well Starting from October 2003 both the inner gas and the environment surround ing the chambers were put under a constant 50 RH value The resulting decrease in resistivity is clearly visible in the plot This effect was shown to be reversible a second period without humidification of the external air has produced again an increase in the plate resistivity lt q c Co i gt S Q 1000 1500 2000 2500 3000 Standard voltage V Fig 3 I V characteristics in pure Argon Resistivity evolution e gap 1 Argon a gap 2 Argon 4 gap 3 Argon gt gap 4 Argon gap 5 Argon gap 6 Argon Gapi eff plateau Gap2eff plateau Gap3 eff plateau x Gap4 eff plateau x Gap5 eff plateau s Gap6 eff plateau mC cm 2 RH fresh gas 400 ext RH control ON OFF 200 350 _ 300 150 Z N 2504 Sz 200 100 oOo O Ss G 150 g S 3 ae 100 3 50 50 oF K a wa E f m l ran 7 LT 0 T T T T L T iy T T 0 11 10 2002 19 01 2003 29 04 2003 07 08 2003 15 11 2003 23 02 2004 02 06 2004 10 09 2004 Fig 4 Plate resistivity evolution The controlled environment RH period is marked together with the gas RH all along the test B Surface damage and noise level monitoring The gap current
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