The TOTEM experiment at LHC has chosen the triple Gas Electron Multiplier
(GEM) technology for its T2 telescope which will provide charged track
reconstruction in the rapidity range 5.3<|eta|<6.5 and a fully inclusive
trigger for diffractive events. GEMs are gas-filled detectors that have the
advantageous decoupling of the charge amplification structure from the charge
collection and readout structure. Furthermore, they combine good spatial
resolution with very high rate capability and a good resistance to radiation.
Results from a detailed T2 GEM simulation and from laboratory tests on a final
design detector performed at CERN are presented.Comment: To appear in the proceedings of 10th Topical Seminar on Innovative
  Particle and Radiation Detectors (IPRD06), Siena, Italy, October 1-5 200

Altunbas

Bonivento

E. Oliveri

G. Latino

L. Ropelewski

N. Turini

S. Lami

Sauli

English

arXiv

Crossref

A triple-GEM telescope for the TOTEM experiment

The TOTEM experiment at LHC has chosen the triple Gas Electron Multiplier (GEM) technology for its T2 telescope which will provide charged track reconstruction in the rapidity range 5.3<|eta|<6.5 and a fully inclusive trigger for diffractive events. GEMs are gas-filled detectors that have the advantageous decoupling of the charge amplification structure from the charge collection and readout structure. Furthermore, they combine good spatial resolution with very high rate capability and a good resistance to radiation. Results from a detailed T2 GEM simulation and from laboratory tests on a final design detector performed at CERN are presented.The TOTEM experiment at LHC has chosen the triple Gas Electron Multiplier (GEM) technology for its T2 telescope which will provide charged track reconstruction in the rapidity range 5.3<|eta|<6.5 and a fully inclusive trigger for diffractive events. GEMs are gas-filled detectors that have the advantageous decoupling of the charge amplification structure from the charge collection and readout structure. Furthermore, they combine good spatial resolution with very high rate capability and a good resistance to radiation. Results from a detailed T2 GEM simulation and from laboratory tests on a final design detector performed at CERN are presented

Lami, S.

Latino, G.

Oliveri, E.

Ropelewski, L.

Turini, N.

CERN Document Server

arXiv:physics/0611178 v1   18 Nov 2006 A triple-GEM telescope for the TOTEM experimentS. Lamia, G. Latinoba∗, E. Oliveriba, L. Ropelewskic, N. TurinibaaPisa INFN, Largo B. Pontecorvo, 3 - 56127 Pisa, ItalybPhysics Department, Siena University, Via Roma, 56 - 53100 Siena, ItalycCERN, EP Division, 1211 Geneva 23, SwitzerlandThe TOTEM experiment at LHC has chosen the triple Gas Electron Multiplier (GEM) technology for its T2telescope which will provide charged track reconstruction in the rapidity range 5.3<|η|<6.5 and a fully inclusivetrigger for diffractive events. GEMs are gas-filled detectors that have the advantageous decoupling of the chargeamplification structure from the charge collection and readout structure. Furthermore, they combine good spatialresolution with very high rate capability and a good resistance to radiation. Results from a detailed T2 GEMsimulation and from laboratory tests on a final design detector performed at CERN are presented.1. INTRODUCTIONThe TOTEM [1] experiment at the LHC col-lider will measure the total pp cross section witha precision of about 1÷2%, the elastic pp crosssection over a wide range in -t, up to 10GeV2,and will study diffractive dissociation processes.Relying on the “luminosity independent method”the evaluation of the total cross section with sucha small error will require simultaneous measure-ments of the pp elastic scattering cross sectiondσ/dt down to −t ∼ 10−3GeV2 (to be extrapo-lated to t= 0) as well as of the pp inelastic interac-tion rate with a good rapidity coverage up to thevery forward region. Roman Pots (RP) stationsat 147m and at 220m on both sides from theInteraction Point (IP), equipped with “edgelessplanar silicon” detectors, will provide the formermeasurement. The latter will be achieved by twoinelastic telescopes, T1 and T2, placed in the for-ward region of the CMS experiment on both sidesof the IP. T1, using “Cathode Strip Chambers”,will cover the rapidity range 3.1<|η|<4.7 whileT2, based on “Triple-GEM” technology, will ex-tend charged track reconstruction to the rapid-ity range 5.3<|η|<6.5. These detectors will alsoallow common CMS/TOTEM diffractive studies∗Corresponding author. Phone: +39-050-2214439. E-mail address: giuseppe.latino@pi.infn.itwith an unprecedented coverage in rapidity. TheT2 telescope will be placed 13.56m away fromIP and the GEMs employed will have an almostsemicircular shape, with an inner radius match-ing the beam pipe. Each arm of T2 will have aset of 20 triple-GEM detectors combined into 10aligned semi-planes mounted on each side of thevacuum pipe (Figure 1).Figure 1. One arm of TOTEM T2 Telescope.2. GEM TECHNOLOGYThe CERN developed GEM technology [2] hasalready been successfully adopted in other ex-periments such as COMPASS and LHCb and12has been considered for the design of TOTEMvery forward T2 telescopes thanks to its charac-teristics: large active areas, good position andtiming resolution, excellent rate capability andradiation hardness. Furthermore, GEM detec-tors are also characterized by the advantageousdecoupling of the charge amplification structurefrom the charge collection and readout structurewhich allows an easy implementation of the de-sign for a given apparatus. The T2 GEMs usethe same baseline design as the one adopted inCOMPASS [3]: each GEM foil consists of thincopper clad polymer foil of 50µm, with copperlayers of 5µm on both sides, chemically perfo-rated with a large number of holes of 70µm indiameter. A potential difference around 500Vapplied between the two copper electrodes gen-erates an electric field of about 100kV/cm in theholes which therefore can act as multiplicationchannels (gains of order 10 ÷ 102) for electronscreated in a gas (Ar/CO2 (70/30 %) for T2) by anionizing particle. The triple-GEM structure, re-alized by separating three foils by thin (2÷3mm)insulator spacers, is adopted in order to reducesparking probabilities while reaching high totalgas gains, of order 104 ÷ 105, in safe conditions.The read-out boards will have two separate layerswith different patterns: one with 256x2 concen-tric circular strips, 80µm wide and with a pitch of400µm, allowing track radial reconstruction withσR down to 70µm, and the other with a matrix of24x65 pads of 2x2 to 7x7mm2 in size from innerto outer circle, providing level-1 trigger informa-tion as well as track azimuthal reconstruction.3. T2 TRIPLE-GEM SIMULATIONA detailed simulation of T2 triple-GEM detec-tor has been developed starting from the existingimplementation for the GEMs used at LHCb [4].The general framework is relying on several pack-ages allowing a complete and detailed “step bystep” simulation, for a given gas mixture and de-tector geometry, for the several underlying pro-cesses: starting from the primary ionization upto the spatial and timing properties of the col-lected signals. The main framework is imple-mented in Garfield; the electric field mapping issimulated withMaxwell; the electron/ion drift ve-locity and diffusion coefficients are evaluated withMagboltz; Townsend and attachment coefficientsare simulated by Imonte; the energy loss by agiven ionizing particle in gas and the cluster pro-duction process are evaluated by Heed. As anexample, Figure 2 reports the simulation for the“weighting field” ~EWK (x) (defined by putting at1V the given readout electrode while keeping allthe others at 0V) for a pad electrode. Signal in-duction is then derived via the Ramo theorem:Ik = −q~v(x) × ~EWK (x).Figure 2. Simulation of the weighting field for aT2 GEM pad electrode.From the reconstruction of the full processchain leading to signal collection, with propermodeling of lateral electron cluster diffusionthrough each GEM foil, the expected signal fora MIP particle has been derived for both stripsand pads for typical values of the electric field inthe drift and induction zones between GEM foils(Ed/t ∼ 3kV/cm). Timing properties, such asa typical signal time delay(duration) of ∼ 60(50)ns, have been found consistent with preliminarytest beam studies on prototypes. Furthermore,the study of signals as a function of distance fromelectron cluster centroid, when combined with ex-pected signal processing by the readout electron-ics, has shown a typical strip cluster size of 2÷3channels (1÷2 for pads), which is consistent withCOMPASS test beam results [3]. Ongoing testbeam activities, performed with final productiondetectors read by final design electronics (digitalreadout via VFAT chip), are expected to allow animproved test and tuning of current simulation.34. TEST ACTIVITIES AT CERNTwo final full size detectors, whose componentswere provided by CERN, have been assembled byan italian private company [5], and then testedat CERN Gas Detector Development Laboratorywith a Cu X-Ray source (Kα/β = 8/8.9 KeV).These activities involved studies on: general func-tionality, absolute gain, strip/pad charge shar-ing, energy resolution, time stability and responseuniformity. In particular, the analysis of signalssimultaneously collected from 8 strip/pad elec-trodes allowed to check the most important de-tector parameters. Figure 3 shows the total ef-fective gain GT2 for both strip and pad readoutchannels as a function of the applied HV: an ex-pected gain of 8÷10×103 for a typical HV valueof -4 kV is observed.1000104-4100-4000-3900-3800-3700-3600-3500Absolute GAIN Calibration2nd Chamber Assembled at G&AStrips Gain 2ndPads Gain 2ndy = 1.8996e-7 * e^(-0.0060998x)   R= 0.99964 y = 8.7311e-8 * e^(-0.0062633x)   R= 0.99985 Absolute GAINH.V.StripsPadsFigure 3. Strip/Pad gain as a function of theapplied high voltage.The study of strip/pad cluster charge sharingshowed the expected correlation between the twoclusters (Figure 4). A slightly higher charge col-lected by strips (about 10÷15%), considering thetypical higher strip cluster size, is consistent withthe design for an optimal setup of the commonreadout chip.The evaluation of energy resolution representsanother important detector test as it is related tothe quality and uniformity of GEM foils. In fact,a not uniform gain over the irradiated zone will2 This parameter can be derived according to equationGT = Itot/(e · n · f), from total readout current (Itot)and X-Ray interaction rate (f) measurements, knowingthe average number of electrons produced by an interact-ing X-Ray (n).Figure 4. Strip/Pad cluster charge sharing.The reduced correlation at higher current val-ues (around the 8KeV peak) is attributed to notcomplete alignment of X-Ray beam to the instru-mented readout channels.results in an anomalous broadening of the peak inthe response spectrum. An energy resolution of∼ 20%, in terms of FWHM for the leading 8 KeVpeak, was found to be well in agreement with theexpected design performance of the detector.Furthermore, time stability of signal has beentested with continuous detector irradiation overmore than one hour and response uniformitychecked by randomly moving the X-Ray sourceover the detector surface.In conclusion detector performances wellwithin expectations have been observed. A moreextensive test on ten production detectors will beperformed at the incoming test beam activities.ACKNOWLEDGMENTSWe are particularly grateful to A. Cardini andD. Pinci of LHCb Collaboration for very preciousinitial input on triple-GEM simulation.REFERENCES1. “TOTEM Technical Design Report”, CERN-LHCC-2004-002; addendum CERN-LHCC-2004-020 (2004).2. F. Sauli, Nucl. Inst. & Meth A386 (1997) 531.3. C. Altunbas et al., Nucl. Inst. & Meth A490(2002) 177.4. W. Bonivento et al., IEEE Trans.Nucl.Sci.49(2002) 1638.5. G&A Engineering, Carsoli (Aquila) - Italy,www.gaengineering.com.

A triple-GEM telescope for the TOTEM experiment

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