The main results of the quality assurance tests performed on the Resistive
Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are
reported and discussed.
  Since July 2004, about 270 RPC units has been certified at INFN Lecce site
and delivered to CERN, for being integrated in the final muon station of the
ATLAS barrel region.
  We show the key RPC characteristics which qualify the performance of this
detector technology as muon trigger chamber in the harsh LHC enviroments.
  These are dark current, chamber efficiency, noise rate, gas volume
tomography, and gas leakage.Comment: Comments: 6 pages, 1 table, 9 figures Proceedings of XXV Physics in
  Collision-Prague, Czech Republic, 6-9 July 200

Bianco, M.

Borjanovic, I.

Cataldi, G.

Cazzato, A.

Chiodini, G.

Coluccia, M. R.

Creti, P.

Gorini, E.

Grancagnolo, F.

Perrino, R.

Primavera, M.

Spagnolo, S.

Tassielli, G.

Ventura, A.

English

arXiv

Bianco, M

Borjanovic, I

Cataldi, G

Cazzato, A

Chiodini, G

Coluccia, M R

Creti, P

Gorini, E

Grancagnolo, F

Perrino, R

Primavera, M

Spagnolo, S

Tassielli, G

Ventura, A

CERN Document Server

ATLAS RPC QA results at INFN Lecce

The main results of the quality assurance tests performed on the Resistive Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are reported and discussed. These are dark current, gas volume tomography, gas tightness, efficiency, and noise rate

BIANCO, MICHELE

I. Borjanovic

G. Cataldi

A. Cazzato

G. Chiodini

COLUCCIA, MARIA RITA

P. Creti

GORINI, Edoardo

F. Grancagnolo

R. Perrino

PRIMAVERA, Margherita

SPAGNOLO, Stefania Antonia

TASSIELLI, GIOVANNI FRANCESCO

VENTURA, Andrea

Archivio Istituzionale della Ricerca- Università del Salento

ATLAS RPC QA results at INFN LecceM. Bianco  ,†, I. Borjanovic , G. Cataldi†, A. Cazzato  ,†, G. Chiodini†,M.R. Coluccia  ,†, P. Creti†, E. Gorini  ,†, F. Grancagnolo†, R. Perrino†, M.Primavera†, S. Spagnolo  ,†, G. Tassielli,  ,† and A. Ventura†Dipartimento di Fisica - via Arnesano 73100, Lecce - Italy†INFN - via Arnesano 73100, Lecce - ItalyAbstract. The main results of the quality assurance tests performed on the Resistive Plate Chamberused by the ATLAS experiment at LHC as muon trigger chambers are reported and discussed. Theseare dark current, gas volume tomography, gas tightness, efficiency, and noise rate.Keywords: ATLAS, RPC, Quality Assurance, Cosmic ray test.PACS: PACS numbers: 29.40.Cs.INTRODUCTIONResistive Plate Chamber RPC [1] will be used as the muon trigger detector, in the barrelregion of the ATLAS experiment at LHC [2]. A total number of 1116 RPC moduleswill be installed, for a total surface area of about 3800 m2. The extreme difficulty inaccessing the ATLAS detectors, after installation is complete, imposes a high standardquality assurance for these modules. For this purpose three cosmic ray teststands havebeen built at INFN Napoli[3], Lecce [4], and Roma 2. Here, we report on the resultsfrom the Lecce site.ATLAS RPC MODULESIn the ATLAS muon spectrometer a large variety of sizes and shapes of stations and,hence, RPC counters is foreseen. All counters, however, share the standard internalstructure, described here in the following.A unit consists of a doublet of RPC detector layers enclosed in rigid lateral profilesand two support panels. A thin honeycomb-paper panel with aluminum skin separatesthe two layers realizing two independent Faraday cages. In figure [1] a cartoon of a RPClayer and a read-out panel are shown. A RPC layer contains a 2 mm thick active gaslayer inside a planar bakelite gas volume (  1010Ωcm) externally painted with graphite.The gas volume is surrounded by two pick-up strip panels segmented in two orthogonalviews and separated by insulating plastic foils. The gas volume high voltage electrodeis connected directly to the power supply, but the other one is connected to ground by ashunt resistor, in such a way that the dark current can be monitored.The RPC dimension can be as large as 4.8 m in one direction and 1.1 m in the other.Depending on the module size, a bakelite gas layer can consist of a single gas volumeor of two adjacent gas volumes, and, most of the times, a read-out strip plane consists of285Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspFIGURE 1. Drawing of ATLAS RPC layer (a) and read-out strip panel (b).two adjacent strip panels, instead of one. This results in modules which have 2 or 4 gasvolumes and 4 or 8 read-out strip panels.QUALITY ASSURANCE PROCEDUREThe quality control of the RPC chamber is accomplished with a series of accuratemeasurements and tests intended to verify the correctness of the assembly and detectorperformance. They represent the very first full chamber characterization and allow toextract statistical information useful to monitor the assembly line and give useful feed-back for improvements.The quality control procedure consists of main certification tests and subsidiary con-trol tests. The main tests regard leakage current versus high voltage curves, chamberefficiency and noise versus high voltage and front-end voltage threshold, and, finally,chamber 2D tomography. Instead, subsidiary control tests regard pulse test, gas volumeleak test, front-end current absorption, and gas volume leakage current temporal drift.QUALITY ASSURANCE RESULTSGas volume testsThe radiation reliance and aging properties of the ATLAS RPC has been proven andcarefully investigated at X5 gamma irradiation facility at CERN [5]. Nevertheless, thequality control of the gas volumes is crucial. The dark current versus high voltage curveis measured during the chamber conditioning and at the end of the tests. Gas volumesshowing a dark current with a large ohmic or exponential part are rejected. In addition thedark current of each gas volume is monitored continuously to look for anomalous driftor current glitches (maybe due to local discharge or defective high voltage connectors).In figure [2] on the left the gas volume dark current distribution of ATLAS RPC testedin Lecce is shown at nominal conditions. On the same figure, on the right, is plotted thedistribution of the difference between the high voltage power supply current and the gasvolume current. From this plot is possible to infer a high level of electrical insulation286Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspFIGURE 2. Gas volume dark current distribution (on the left) and power supply current and dark currentdifference distribution (on the right). The data are taken from the ATLAS RPC tested in Lecce with a highvoltage of 10kV.FIGURE 3. Example of tomography plot of a small size gas volume with cut-out. The dark regular spotarray are due to the spacers dead area.between gas volumes and other parts, such as mechanical supports and read-out strippanels (  100GΩ) .The 2D efficiency of each gas volume is measured with high accuracy and this is themost time consuming test which takes about 24 hours of running. The muon cosmic raytomography is a very powerful tool to discover gas volume assembly defects (due forexample to glue spots, badly glued spacers, and inner surface contaminations) whichcan compromise the aging performance of the unit. A noise rate 2D map is also taken, inorder to exclude the presence of hot spots, which can become degenerative in the longterm period. A gas volume tomography of a special unit is reported in figure [3]. Thisparticular gas volume has a cut-out in order to leave room for the barrel toroid feet. Otherspecial counters have gas volumes with cut-out for the alignment laser rays of the MDTprecision muon chambers. Figure [3] shows that the high quality of the gas volumes isalso achieved when shapes more complicated than the rectangular one are realized.287Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspEfficiency0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1Entries/0.00201020304050607080Strip panel efficiencyCluster size1 1.5 2 2.5 3 3.5 4 4.5 5Entries/0.1020406080100120140160180200 Strip panel average cluster sizeFIGURE 4. Total efficiency distribution (on the left) and average cluster size distribution (on the right)for different high voltage values of 10kV (...), 10.1kV (.-.), and (-) 10.2 kV. The data are taken from theATLAS RPC tested in Lecce with an equivalent input voltage threshold of about 0.5 mV.Gas tightness test, which is performed independently for each layer, consists in mon-itoring the differential pressure for about 2 hours, after imposing about 3 mbar of over-pressure and closing the gas inlet and outlet. With our instrumental sensitivity of about10  1 mbar we can reject modules with a gas leak larger than about 10  4mbar ls .Read-out strip panel testsThe read-out strip panel efficiency (given by the convolution of gas volume efficiencyand strip panel electronic efficiency) is measured as a function of the applied highvoltage and front-end voltage threshold. This allows to establish the detector workingpoint and the parameter range where the chamber performs well. In figure [4] thedistribution of the read-out strip panel efficiency (dead channels included) and theaverage cluster sizes (number of adjacent firing strips) are shown. The RPC countershave in average an efficiency greater than 97%, as expected by the detector active areacoverage and required by the trigger design.The read-out strip panel noise rate is also measured at different high voltage andvoltage threshold. In figure [5] the corresponding distribution is shown on the left. Theaverage noise rate is less than 1 Hzcm2 , which is an order of magnitude smaller than theexpected counting rate due to the photon and neutron background in the ATLAS exper-iment at the designed LHC luminosity. In the same figure on the right, we reported thedistribution of the ratio between the gas volume dark current (with the ohmic componentsubtracted) and the total strip panel noise rate. By assuming that the exponential part ofthe dark current is due to noise counts, we can estimate an average saturated avalanchecharge of about 15 pC.288Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspNoise rate [Hz/cm2]0 1 2 3 4 5 6 7 8 9 10Entries/(0.1Hz/cm2)010203040506070 Strip panel average noise rateIgap/Noise[pC]0 5 10 15 20 25 30 35 40 45Entries/(2pC)010203040506070Dark current - Noise ratioFIGURE 5. Noise rate distribution (on the left) for different high voltage values of 10kV (...), 10.1kV(.-.), and (-) 10.2 kV. Current-to-Noise ratio distribution at a high voltage value of 10kV (on the right).The data are taken from the ATLAS RPC tested in Lecce with an equivalent input voltage threshold ofabout 0.5 mV.TABLE 1. RPC modules failure rate observed during QA tests in Lecce.Component failure Number of modules out of 272 Fraction of modulesReadout strip panel 37 14%Gas volume spot 9 3.5%Gas volume high dark current 7 2.6%Gas leak 9 3.5%HV connector 2 0.9%Gas volume shunt resistor 3 1.3%Production components yieldTable [1] reports the number of counters which failed the quality assurance test andthe defective component responsible for the rejection. A rejection rate of about 6% is dueto the gas volumes, which are discarded, 14% to the read-out panels, which are repaired,and, finally, 6% to assembly errors. Taking into account the number of componentsinside a module (we tested about 2000 strip panels and 1000 gas volumes), we have ayield of 98.5% for gas volume and 98.3% for the strip panel (without considering thefactory single component pre-selection).CONCLUSIONSThe cosmic rays teststand for the ATLAS RPCs in Lecce is routinely in operationsince July 2004, and up to now, july 2005, it has certified about 270 RPC modules,corresponding to about 25% of the ATLAS Muon Spectrometer. The test results and theirstatistical distributions show that the RPC properties are stable and uniform, satisfyingthe ATLAS experiment requirements. The end of the Quality Assurance Tests is foreseen289Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspfor the end of October 2005.ACKNOWLEDGMENTSWe thank R. Assiro, E.M. Fasanelli, G. Fiore, R. Gerardi, A. Miccoli, S. Podladkin andF. Ricciardi for the generous technical assistance they provide us in the construction andoperational phase of the cosmic ray test-stand.REFERENCES1. R. Santonico and R. Cardarelli, Nucl. Instrum. and Meth. 187, 377 (1981).2. ATLAS detector and physics performance. Technical Design Report (May 1999).3. M. Alviggi et al, Nucl. Instrum. and Meth. A518, 79 (2004).4. G. Cataldi et al., ATLAS RPC Cosmic ray Teststand at INFN Lecce - proceedings of XXIV Physics inCollision - Boston, June 27-29, 2004. arXiv:physics/0409075.5. G. Aielli et al., Nucl. Instrum. and Meth. A 533, 98 (2004).290Downloaded 19 Apr 2006 to 137.138.5.66. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

The main results of the quality assurance tests performed on the Resistive Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are reported and discussed. Since July 2004, about 270 RPC units has been certified at INFN Lecce site and delivered to CERN, for being integrated in the final muon station of the ATLAS barrel region. We show the key RPC characteristics which qualify the performance of this detector technology as muon trigger chamber in the harsh LHC enviroments. These are dark current, chamber efficiency, noise rate, gas volume tomography, and gas leakage.The main results of the quality assurance tests performed on the Resistive Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are reported and discussed. Since July 2004, about 270 RPC units has been certified at INFN Lecce site and delivered to CERN, for being integrated in the final muon station of the ATLAS barrel region. We show the key RPC characteristics which qualify the performance of this detector technology as muon trigger chamber in the harsh LHC enviroments. These are dark current, chamber efficiency, noise rate, gas volume tomography, and gas leakage.The main results of the quality assurance tests performed on the Resistive Plate Chamber used by the ATLAS experiment at LHC as muon trigger chambers are reported and discussed. These are dark current, gas volume tomography, gas tightness, efficiency, and noise rate