Colas, P.

Giomataris, I.

Jeanjean, J.

Lepeltier, V.

Martin, J.

Olivier, A.

Rebourgeard, P.

Ronan, M.

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Micromegas TPC R&DP. Colas, I. Giomataris, J. Martin, A. Olivier, P. Rebourgeard, J. Jeanjean, V.Lepeltier, M. RonanTo cite this version:P. Colas, I. Giomataris, J. Martin, A. Olivier, P. Rebourgeard, et al.. Micromegas TPC R&D.International Workshop On Linear Colliders LCWS 2002, Aug 2002, Jeju Island, South Korea.pp.1-10, 2003. <in2p3-00022317>HAL Id: in2p3-00022317http://hal.in2p3.fr/in2p3-00022317Submitted on 27 Mar 2003HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.MICROMEGAS TPC RDP COLAS I GIOMATARIS J MARTIN A OLIVIER Ph REBOURGEARDDSMDapnia CEASaclay F Gif sur Yvette CEDEX FranceJ JEANJEAN V LEPELTIERLAL Orsay F Orsay FranceM RONANLBNL Berkeley CA 	 USAStudies of the properties and performances of small gap wire chambers and Micromegas detectors for LCTPC readout are presented Gas amplication driftand attachment studies and aging eects are reported for Micromegas deviceswith several dierent gases and mixtures Preliminary results on positive ion feedback in a  T magnetic eld are summarized The design of a small MicromegasTPC prototype with ne metallic micromesh structures of dierent pitches andanode pad readout for testing in a high magnetic eld is discussed IntroductionLarge Time Projection Chamber TPC detectors are being designed for afuture Linear Collider LC These next generation TPC detectors will havefull pad coverage to provide excellent overall pattern recognition and trackingConventional TPC s using wires for electron multiplication have large cathodepad response corrections and E X B eects TPC RD on new MicroPatternGas Detectors MPGD which provide gas electron amplication within microstructures is being performed to obtain better resolution and to allow nerreadout segmentation In October 	 the DESY Physics Research Committee PRC recommanded the approval of the LC TPC program and encourages the collaboration to perform in High magnetic elds tests of the different endplate geometries GEM Micromegas and the standard wire chambers The work described here intends to fulll part of this goal namelythe micromegas and wire chamber studiesThe advantages of Micromegas for reading out a TPC are listed in Section	 In Section  we report on the various tests and measurements carried outto assess the feasability of a micromegas TPC and in Sections  and  wedescribe the measurement of the feedback in the magnetic eld respectivelyfor a wire chamber TPC and a micromegas TPC In Section  we report onthe prototype under construction by the BerkeleyOrsaySaclay collaborationfor a cosmic ray test in a magnetic eld in Saclay Principle and advantages of MICROMEGASIn a micromegas detector a  to  m thick mesh divides the gas volume intwo parts a loweld  typically 	 Vcm  drift region and a higheld  kVcm amplication region typically  micron thickA Micromegas TPC is a Micromegas detector MICROMEsh GASeousdetectorwhere the conversion and drift region extends over a large distanceallowing the coordinate along the drift eld to be measured through the drifttime of the charge bucket Equivalently a Micromegas TPC is a standard TPCwhere the wire chamber has been replaced by a Micromegas device There isa slight dierence between these two schemes as far as readout electronics isconcerned whereas the electron signal on the pads is positive it is the imagecharge of the avalanche which takes place on the wires it is negative in thecase of Micromegas where the avalanche takes place very close to the padsMicropattern Gaseous Detector TPCs which use GEMsor Micromegasreadout planes oer several advantages with respect to standard wire TPCsThey appear more and more to be a necessary improvement of the TPC technics in view of its application to the Linear Collider First of all in a wirechamber the electric eld is radial near the wires generating a spread of theelectron cluster of more than hundred microns by EB eect along the wiredirection In a micropattern detector the region where the eld is not parallelto the magnetic eld is much reduced Also due to the high density of tracksin the jets produced at high energy and due to the high background a highgranularity and an excellent 	track resolution a few millimeters are requiredFor instance for a 	 GeV  jet in  prongs the tracks are not separated bymore than  mm Clearly the  to  cm pads used in LEP detectors wouldbe too large for disentangling neighbouring tracks at the LC Micropatterndetectors allow a much more granular pad structureMicromegas presents advantages over other micropattern detectors Thegain shows a maximum as a function of the gap for a given voltage applied Instandard conditions of operation this optimal gap is a few tens of micronsThis makes the gain independent of the gap to rst order and gives a goodpotential for dEdx resolution provided that the gap and the gas compositioncan be matched to an optimum Micromegas has also proved to be usable witha gas mixture containing CFas a quencher Such a hydrogenless gas wouldrespond much less to the neutron backgrounds than isobutane methane andother hydrocarbons Such a neutrontolerant gas cannot be used withouteciency loss in structures where elds of the order of a few kVcm exist overa mm or so as is the case in wire chambers and GEMs Indeed the electronspectrum overlaps the attachment resonance for such elds Another advantage	of Micromegas for a TPC is the natural ion feedback suppression it providesthanks to the funnel eect studied in Sections   and  In Micromegas thefast ion signal about  ns does not suer from balistic decit  it is entirelytransmitted through a fast charge amplier that allows operation at quite a lowgain less than  for the Linear Collider TPC In a MWPC because of thebalistic decit and in GEM because of partial transmission of the electronsthe loss can be as high as an order of magnitude which has to be compensatedby an operation at higher gain and thereby more ion feedback Micromegasdetectors are easy to build and as large as  cmhave functionned Lastbut not least Micromegas are successfully used in two large experiments atCERN COMPASSand NA breaking records of rate and time resolutionfor gaseous detectors Study of various properties of a Micromegas TPCA number of experiments have been conducted to perform studies of variousaspects of a LC TPC The drift velocity of electrons has been measured inseveral gases using a laser to produce photoelectrons from two grids distant bya few millimeters the mesh and the drift cathode The method and the resultsof the measurements have been described elsewhere ArgonCFmixtureshave been shown to have a maximum velocity of up to  cms at elds aslow as 	 Vcm Simulations supported by our measurements show thatthere is no attachment for elds below  Vcm An aging test has beenperformed using this gas with a Xray gun A total of 	 mCmm well abovethe requirements for a linear collider have been accumulated without decreaseof the signalExtensive studies have been carried out of the ion feedback In a Micromegas TPC the electric eld ratio between the amplication eld  to kVcm and the drift eld typically 	 Vcm is usually high about Under such conditions most of the eld lines originating at the anode end atthe micromesh As the diusion is negligible for ions the ions produced onthese eld lines do not go back to the drift volume and most of them will beneutralised on the mesh If the extension of the avalanche is not too large compared to the hole pitch the ion feedback is optimal and equal to the inverse ofthe eld ratio This is called the funnel eect Study of the ion feedback in a smallgap wire chamber TPC in amagnetic eldClassical wire chamber TPCs have been operated for years at various eecolliders in magnetic elds up to  tesla To be a backup solution for the LCTPC such a device must have small enough distances between the sense wiresand between the wire plane and the pads so that the inuence of the chargecreated near a wire is limited to a small number of narrow pads We thus builta small gap wire chamber  cm where the 	m diameter sense wiresat a high positive potential sitted 	 mm from the pads Its pitch was  mmA grounded wire grid at 	 mm from the sense wire plane separated the driftvolume from the detection volume The conversion and drift space was  mmthick The drift electrode was set to a negative potential of typically 	 Vbut was varied from  V to  VThe primary ionisation was provided by a 	 MegabequerellFe sourceThe gas used was ArCH except in a rst measurement with zero magnetic eld where Arisobutane was used The sense wires were kept at a potential of 	 V in the rst measurement and 	 V in the others The gainin the chamber was of the order of Small-gap wire chamberMagnetic field (T)feedback00.020.040.060.080.10.120.140 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Figure  Positive ion feedback vs magnetic eld for the smallgap wire chamber TPCThe currents in the iwireand idriftin the wire and the drift power supplieswere monitored allowing the measurement of the positive ion feedback f fromthe relation f  idriftiiwire where iis the primary ionisation measuredto be about  nA in conditions where the voltage on the wires was loweredto  V to work in the ionisation chamber mode ie with no gain Thecurrents in this experiment ranged typically from a few nA to  nA A smallcorrection is applied to the voltages as the current goes through a  Mresistor part of a Tlter for the noise This correction ranges from less than V to  V for the sense wire voltage The feedback is shown as a function ofthe magnetic eld in Fig Small-gap wire chamb, B=0, Ar-IsobutanField ratio EA/EDfeedbackSmall-gap wire chamb, B=1T, Ar-CH4feedbackSmall-gap wire chamb, B=2T, Ar-CH4feedback10-210-10 10 20 30 40 50 60 70 80 90 100Figure  Positive ion feedback vs ratio of amplication to drift electric elds for the smallgap wire chamber TPC Data are shown for three values of the magnetic eld  	  and tesla The lower curve shows the ideal case were the feedback is equal to the inverseof the eld ratio Note that the data taken with no magnetic eld were taken with anArgonisobutane 		 mixtureAn eective electric eld ratio can be dened as the ratio of the average eld in the cell Vwires	 mm to the drift eld The dependance of themeasured feedback on this eective electric eld ratio is shown in Fig 	 Forthree values of the magnetic eld The data at B were taken with anArgonisobutane  mixture whereas the data at B T and 	 T usedan Argonmethane  mixture The inverseeld ratio law is well obeyedfor Arisobutane and an increase of 	 on the feedback is observed for Armethane as expected from a lower diusion in this gas Study of the feedback in a micromegas TPC in a magnetic eldWe have built a small TPC with a  cm drift length and a micromegas Themesh pitch is  lines per inch and the gap is  m The grid is in copperas preliminary tests in the magnet showed that nickel grids are deformed bythe magnetic eldThe primary ionisation was provided by a  GigabequerellSr sourceThe gas used was ArCH The gain in the chamber was measured to beup to  Fig  TheSr nucleid emits a 	 MeV  ray and the primaryMicromegas 500 lpi55Fe source90Sr sourceHV (mesh)multiplication factor11010 210 30 50 100 150 200 250 300 350 400Figure  Current driven through the mesh arbitrary units as a function of the voltageapplied on the meshionisation is thus dependent on the magnetic eld as the path length andthe acceptance of the detector depends on it The data are corrected to takeinto account this eect which cancels out in the feedback measurement Thepositive ion feedback f is obtained from the supply currents by the relationf  idrift iiwire idrift where iis the primary ionisation Themeasured feedback is shown as a function of the magnetic eld in Fig  Theline is the expectation from the inverse electriceld ratio law It is not atas due to the combined eect of the resistors mentionned in previous sectionand the dependance of the primary ionisation on the magnetic eld the eldratio depends on the magnetic eld In these measurements the drift eld was Vcm and the amplication voltage was  V which corresponds to again of  These conditions are close to the operating conditions at a linearcolliderMicromegas 500 lpiMagnetic field (T)feedback10-310-210-10 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Figure  Positive ion feedback vs magnetic eld for the micromegas TPCThe positive ion feedback is shown in Fig  as a function of the eld ratioThe departure by a factor of  with respect to the ideal case inverse eld ratiois consistent with the expectation of 	 given the uncertainties on the gapand on the gas parameters This factor of 	 is due to the relatively coarsegrid used in these measurements  micron pitch whereas the optimum of could could be reached for a 	 micron pitchIn conclusion the operation of a micromegas TPC in a magnetic eldhas been demonstrated The feedback does not depend signicantly on themagnetic eld and the expected feedback suppression factors of a few per milshould be obtained in ideal conditionsMicromegas 500 lpi B=0Field ratio EA/EDfeedback Micromegas 500 lpi B=1TfeedbackMicromegas 500 lpi B=2Tfeedbackfeedback10-310-210-10 100 200 300 400 500 600 700Figure  Positive ion feedback vs ratio of amplication to drift electric elds for themicromegas TPC Data are shown for three values of the magnetic eld  	  and  teslaThe lower curve shows the ideal case were the feedback is equal to the inverse of the eldratio	 The cosmic prototypeA large micromegas TPC prototype  cm drift length  cm diameter isunder construction see Fig  It is tted to the bore of the 	 tesla superconducting magnet RMN  a medical NMR magnet recently brought backto operation in Saclay after 	 years of use in the Orsay hospital The magnetic eld is homogeneous within  over the drift area The electric eld isprovided by a cylindrical eld cage  cm in diameter made of a series of circular copper rings interconnected through resistors to provide an homogeneouselectric eld The Very High Voltage supply is recuperated from the DELPHITPC at LEP and will provide an electric drift eld up to  Vcm The cagehas been designed to allow for a laser beam and a radioactive source to be usedfor test purposes The micromegas endplate is removable allowing to test various pad congurations and to study possible improvements as for instance aresistive layer to improve the sharing of the signal on several pads	 It can alsobe replaced by a wire chamber still considered as a backup solution for theFigure  Cosmic ray TPC setupLC TPC In the baseline version of the setup the PC board of the endplatewears  micron high and 	 micron diameter polyimide pillars positionnedevery millimeter used as spacers to support the micromesh The  lpi copper micromesh has been manufactured at the CERN workshop The PC boardcarries anode pads arranged in  lines  lines of  	 mm   mm padsand 	 lines of 	  mm   mm pads The readout pads are connectedthrough the PC boards by metallized vias On the external side of the PCboard connectors are soldered by short wires The front end readout cards atotal of 	 channels are recuperated from the STAR TPC at RHIC andhave been modied to allow for the amplication sampling and digitization ofnegative signals as those provided by a micromegas TPCThe trigger is provided by a coincidence of two layers of scintillators placedbelow and above the magnetThe rst tests of this setup are scheduled for the spring of 	AcknowledgmentsWewould like to thank the Saclay Magnet and Cryogenics department SACMfor having operated the 	T magnet successfully We also wish to thank theengineers and technicians for their help in designing and building the cosmicprototype and mounting the magnet experiment F Bieser R Cizeron PDesaunais E Delagnes A Giganon G Guilhem and P ImbertReferences I Giomataris et al Nucl Instrum Methods A  		 F Sauli Nucl Instrum Methods A   G Charpak et al Nucl Instrum Methods A 		 	 A Magnon et al Nucl Instrum Methods A 		 	 R Settles et al LC TPC R and D DESY PRC  October 	 P Colas et al Nucl Instrum Methods A 		 	 K Sachs these proceedings

Micromegas TPC R&D

Micromegas TPC R&D

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