The ATLAS Semiconductor Tracker (SCT) has been installed, and fully connected to electrical, optical and cooling services. Commissioning has been performed both with calibration data and cosmic ray events. The cosmics were used to align the detector, measure the hit efficiency and set the timing. The SCT is now ready to take data when the LHC turns on this autumn. At the same time, it is clear that the present ATLAS tracker will need to be renewed for projected luminosity upgrade of the LHC, the SLHC. This is mainly driven by occupancy and radiation hardness issues. The new tracker will likely be entirely made of silicon, with the space of the present SCT largely taken up by detectors with much shorter strips. Several large-scale R&D projects on the sensors and module concepts for this upgrade are running, including sensor and module prototyping. We will report upon the commissioning experience from the SCT, use it to extract valuable lessons for future silicon tracker projects, and give an up-to-date overview of the status and results of the R&D efforts for the ATLAS tracker upgrade

Mitrevski, J

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ATL-INDET-PROC-2008-00215January2009The ATLAS SCT: Commissioning Experience and SLHC UpgradeJ. MitrevskiSanta Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USAfor the ATLAS SCT CollaborationAbstractThe ATLAS Semiconductor Tracker (SCT) has been installed, and fully connected to electrical, optical and cooling services.Commissioning has been performed both with calibration data and cosmic ray events. The cosmics were used to align the detector,measure the hit efficiency and set the timing. The SCT is now ready to take data when the LHC turns on this autumn. At thesame time, it is clear that the present ATLAS tracker will need to be renewed for projected luminosity upgrade of the LHC, theSLHC. This is mainly driven by occupancy and radiation hardness issues. The new tracker will likely be entirely made of silicon,with the space of the present SCT largely taken up by detectors with much shorter strips. Several large-scale R&D projects onthe sensors and module concepts for this upgrade are running, including sensor and module prototyping. We will report upon thecommissioning experience from the SCT, use it to extract valuable lessons for future silicon tracker projects, and give an up-to-dateoverview of the status and results of the R&D efforts for the ATLAS tracker upgrade.Key words: ATLAS, SCT, Commissioning, SLHCPACS: 29.40.Gx, 29.40.Wk1. IntroductionThe CERN Large Hadron Collider (LHC) is a proton-proton collider designed for a center of mass energy of√s =14TeV and an instantaneous luminosity of 1034 cm−2s−1.ATLAS is one of two general-purpose detectors currentlybeing commissioned at the LHC [1]. The ATLAS In-ner Detector is responsible for reconstructing tracks andcontributing to particle identification [2,3]. As shown inEmail address: Jovan.Mitrevski@cern.ch (J. Mitrevski).Fig. 1, it consists of three subdetectors, all within a 2Tmagnetic field provided by the superconducting CentralSolenoid magnet. Closest to the beam line are the pixeldetectors for precise spatial measurements and vertexing.Next comes the Semiconductor Tracker (SCT), the subjectof this paper. And outside the SCT is the Transition Radi-ation Tracker (TRT), which uses gas-filled straw tubes fortracking and fibers or foils to provide transition radiationfor electron identification.Preprint submitted to Elsevier 19 November 2008CryostatPPF1CryostatSolenoid coilz(mm)Beam-pipe   Pixelsupport tubeSCT (end-cap)TRT(end-cap)1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 2 3 4 5 6 7 8Pixel400.5495580650749853.89341091.51299.91399.71771.4 2115.2 2505 2720.200R50.5R88.5R122.5R299R371R443R514R563R1066R1150R229R560R438.8R408R337.6R275R644R10042710848712 PPB1Radius(mm)TRT(barrel)SCT(barrel)Pixel PP13512ID end-plateR34.3Fig. 1. The layout of the ATLAS Inner Detector. Only a quarter ofthe detector is displayed.Fig. 2. Diagram of a barrel module.2. The Semiconductor TrackerThe SCT is a silicon microstrip tracker, made up of fourbarrels and eighteen end-cap disks, nine per end-cap. Fig-ure 1 shows the layout, with the barrels located at a ra-dius of 299mm to 514mm, and the disks located at z =±854mm to z = ±2720mm. Placed on the disks and bar-rels are modules holding p-in-n single-sided microstrip sen-sors. The modules need to be able to withstand a dose of2 × 1014 1 MeV-neutron-equivalent/cm2, which would ac-count for the first ten years of running of the LHC with a50% additional safety factor.Figure 2 shows a diagram of a barrel module [4], 2112 ofwhich are placed on the four barrels. Each module containsfour microstrip sensors made by Hamamatsu [5], 64mm inlength and width, 285µm thick, with AC-coupled strips of80µm pitch. Pairs of sensors are attached end-to-end, re-sulting in 768 strips of 126mm in length per pair. A mod-ule contains two pairs of sensors, one on each side, with arelative stereo angle of 40mrad.Figure 3 shows photographs of the three types of mod-ules used in the end-caps [6], outer, middle, and inner, basedon whether they are part of the outer, middle or inner ringof modules on the disk. There are 1976 modules total in thetwo end-caps. Due to the shape of the sensors, the pitch ofthe strips is variable, ranging from 57µm to 94µm. Likethe barrel modules, the outer and middle (except on Disk8) modules are made of four sensors, with two sensors at-tached end-to-end on each side, resulting in strips about12 cm long. A pair of sensors is placed on each sides of amodule, with a relative stereo angle of 40mrad betweenthem. The inner modules, as well as a variant of the middlemodules used in Disk 8, only use a single sensor on eachFig. 3. Photograph of the three types of end-cap modules. Left toright are an outer module, middle module, and inner module. Disk8 also contains middle modules but with only two instead of foursensors.Corrected threshold (fC)0.7 0.8 0.9 1 1.1 1.2 1.3 1.4Efficiency0.90.910.920.930.940.950.960.970.980.991Noise occupancy10-610-510-410-310-210-11Corrected threshold (fC)0.7 0.8 0.9 1 1.1 1.2 1.3 1.4Efficiency0.90.910.920.930.940.950.960.970.980.991Noise occupancy10-610-510-410-310-210-11Fig. 4. The efficiency (circles) and noise occupancy (traingles) forSCT barrel modules. On the left is the result before irradiation, whileon the right is the result after a dose of about 3× 1014 protons/cm2from a 24GeV proton test beam.side, again with a stereo angle. The sensors were manufac-tured by Hamamatsu [5] and CiS [7].A Cu/polyimide hybrid flex circuit holds twelveABCD3TA ASICs [8], fabricated using DMILL technologyfor radiation hardness, to read out the strips. Each chipreads out 128 channels, so six are placed on each side ofthe module on the hybrid to read the 768 strips for thesensors on that side. The ASICs implement a binary read-out scheme, meaning that there is either a hit for a stripor not. Thus, choosing the appropriate threshold is key:too low, occupancy will increase; too high, efficiency willdecrease. The specifications require a noise occupancy be-low 5 × 10−4 with a detector efficiency greater than 99%.Figure 4 shows that this is possible even after irradiation.The nominal threshold is 1 fC, and the green bands in theright plot show the range of thresholds for which the re-quirements would still be met. The ABCD3TA allows forknown charges to be inserted to the readout for calibration,and the threshold settings can be trimmed per channel.Communication for the data acquisition system (DAQ)is optical, with one fiber for clock and commands to themodule and two for readout from the module. The commu-nication is made to be fault-tolerant. On the hybrid, badABDC3TAASICs can be bypassed. If the input signal pathis broken, clock and control can be taken from a neighbor-ing module, and for a broken output path, most of the datacan be routed to the other of the two fibers.Power is supplied individually to each module from float-ing power supplies. There is a high voltage power supply to2bias the sensors, with a range from 0 to 500V. The initialoperating voltage will be 150V, but it will need to be in-creased after the sensors suffer radiation damage. Low volt-age power supplies provide analog power, digital power, andpower to the PIN diodes and VCSELs used to receive andsend data along the fibers. The power system is monitoredand controlled using a Supervisory Control and Data Ac-quisition (SCADA) system based on PVSS-II fromETM [9]and a finite state machine (FSM).In order to limit the reverse annealing of the sensors, thetemperature needs to be kept at around −7C. This is ac-complished with an evaporative cooling system that usesC3F8 [10]. The detector is filled with dry N2 gas to pre-vent condensation and because it is inert. The humidityis monitored using radiation-hard Xeritron sensors, alongwith non-radiation-hard Honeywell sensors for studies andmonitoring during early running, interfaced to the PVSS-II based SCADA system. The temperature of the modules,of the cooling loops through which the C3F8 flows, of thegas, and of the mechanical structure are similarly moni-tored using NTCs. In addition to software interlocks andmonitoring provided by PVSS-II and the FSM, there is ahardware-based interlock system that cuts the power of themodules on a cooling loop if the temperature is measuredto be outside a certain range.3. SCT CommissioningThe sensors were tested by the manufacturer, and theywere checked upon reception on a sampling basis. The mod-ules were assembled at various locations, being put throughan extensive suite of tests at various times in the produc-tion sequence. The performance of both irradiated and non-irradiated modules was evaluated in beam-tests [11], an ex-ample of which was given in Fig. 4.Between June 2004 and August 2005 the barrel mod-ules were mounted on carbon-fiber barrel structures us-ing robots, and the four completed barrels were shippedto CERN. Similarly, modules were mounted on the end-cap disks, with the completed disks being sent to CERN.In a surface building at CERN, the SCT assemblies wereplaced inside the corresponding TRT assemblies, and com-bined SCT and TRT tests were performed [12]. Then in Au-gust 2006, the SCT+TRT barrel detectors were installed asone package in the ATLAS cavern, with continuity checksand a sign-off taking place in April and May 2007. TheSCT+TRT end-cap assemblies were installed in the cavernin May and June 2007, with sign-offs coming in Januaryand February 2008.Subsequently, the SCT was run combined with the TRT,and the SCT barrel was run together with much of the restof ATLAS in Milestone week 6 (M6) in March 2008. Afterthat, inner detector commissioning focused on signing offthe pixel detectors that, along with some problems encoun-tered with the cooling system, resulted in a fewmonths dur-ing which the SCT was not run. Work on the SCT duringinnershort middlemiddleouterFig. 5. The ENC for End-cap C modules at warm running. The dis-tributions of the different module types are labeled on the histogram,with short middle referring to the special middle modules used inDisk 8.that time continued, consisting of developing the softwareand other things that did not require cooling the detector.Finally, on 12 August 2008, the SCT started runningagain, with an aggressive schedule to be prepared for theplanned LHC turn-on set for 10 September 2008. An ini-tial task was to test cooling the full detector stably. All buttwo cooling loops, one to Quadrant 2, Disk 9 and one toQuadrant 1, Disk 1, both of End-cap C, are presently run-ning. Another early task was to configure the optical com-munication to the modules, including setting redundantmode correctly where there are problems. The discrimina-tor thresholds also needed to be tuned. One problem re-cently encountered is a high rate of deaths of the VCSELsused to transmit commands and clock to the modules. Thesuspected culprit is ESD damage during the handling ofthe optical hardware. Luckily these VCSELs are located incrates in the service caverns so they can be replaced.A common task in many sensor tests is the thresholdscan. For a given charge injection value, a threshold scanrepeatedly injects the charge and reads if there was a hitusing a range of discriminator thresholds. This hit efficiencyversus threshold is fit with a complementary error function.From plotting the discriminator thresholds at which the hitefficiency is 50% as a function of the injected charge, thegain of the channel can be determined. From the width ofthe turn-off of the complementary error function, the noisecan be determined, and this can be divided by the gain todetermine the equivalent noise charge (ENC). Plots of theENC for End-cap C measured during the sign-off are givenin Fig. 5. Note that these are for warm running (around25C); there is less noise when the modules are cooled. End-cap A shows similar behavior, and the barrel modules showsimilar behavior to the long end-cap strips. For comparison,a MIP is expected to deposit a charge of 3.6 fC.In addition to measuring module performance, combinedrunning has been useful to set up the timing and test theDAQ system. It is also useful to test the oﬄine software,monitoring, reconstruction. Finally, cosmics have been usedfor alignment. This was performed during the surface tests,and also during the M6 running, though statistics for thatwere lower.3pixelsshort stripslong stripsFig. 6. A proposed layout of the ATLAS Inner Detector in the SLHC.Cooling InCooling OutPS cableHV cableDCS, Interlock`TTC, Data& DCSfibersOptoSMCDCSinterlockSMCHybridModule #1 Module #2 Module #10Service busFig. 7. Proposed supermodule (top) and stave (bottom).4. The Super LHCThe Super LHC (SLHC), planned to start in 2018, willhave a peak luminosity ten times that of the LHC. The TRTwould have too high of an occupancy, and increased SCToccupancy would also degrade performance. The SCT andpixel detectors would have also suffered radiation damageby then. Therefore, a new inner detector is need [13]. Thecurrent plan is for an all-silicon system, in the barrel con-sisting of four pixel layers, three layers of short strips, 25 to35mm in length, and two layers of long strips 90 to 120mmin length. Figure 6 shows the current straw-man proposal.For radiation hardness, the proposal is to use n-in-p sen-sors. Test sensors have been built.The plan is to combine multiple modules into a super-module or stave, shown in Fig. 7, instead of installing theindividual modules onto the barrel, as was done in the SCT.This will allow for simpler assembly with more work go-ing on in parallel, and it will potentially make it easier toreplace faulty modules.The power distribution system will also have to be up-dated. The current independent power scheme, where eachmodule has its own power supplies, does not scale enoughfor the SLHC. One potential scheme is to have serial power,where the modules are connected in series, with each mod-ule stepping down the voltage. A prototype was built thatsupplies 30 hybrids with 4V each by providing 120V to thechain. Another scheme, DC-DC conversion, increases effi-ciency by sending higher voltage, which needs to be steppeddown at the module. For high voltage distribution, optionsinclude decreasing granularity and allowing for faulty sen-sors to be bypassed, and local high voltage generation witha step-up transformer.The front end electronics have to be upgraded to sup-port the new n-in-p sensors, as well as to upgrade fromthe now old DMILL technology. Therefore, a new deepsub-micron front-end ASIC, called the ABCn, has been de-signed and submitted for fabrication, with engineering sam-ples expected in October 2008. The final design should be in0.13µm technology, although 0.25µm technology has beenused for prototyping. A binary readout is maintained. Thecommunication bandwidth has been increased, and the var-ious proposed power distribution schemes are supported.The cooling capacity will also have to be increased. Thecurrent proposal is to use CO2 cooling, though other op-tions are also being considered.5. ConclusionThe LHC is currently getting started. The SCT is readyto take part in the initial running, as a result of many yearsof work by many people. The commissioning is continuing,with the running of the detector being tuned, and mucheffort continuing to be spent to make sure that the datataken by the detector can be used to enable the physicsgoals of the ATLAS experiment.There have been yearly workshops working on the up-grade starting from 2005. Ten countries are working on theupgrade of the tracker. Even though the LHC is just nowstarting, considering that it is planned to install the up-grade in 2016 to 2018, the schedule is very tight.References[1] G. Aad, et al., The ATLAS Experiment at the CERN LargeHadron Collider, JINST 3 (2008) S08003.[2] ATLAS Collaboration, ATLAS Inner Detector: Technical DesignReport. Vol. 1, CERN-LHCC-97-16.[3] ATLAS Collaboration, ATLAS Inner Detector: Technical DesignReport. Vol. 2, CERN-LHCC-97-17.[4] A. Abdesselam, et al., The barrel modules of the ATLASsemiconductor tracker, Nucl. Instrum. Meth. A568 (2006) 642–671.[5] Hamamatsu Photonics Co. Ltd., Japan.[6] A. Abdesselam, et al., The ATLAS semiconductor tracker end-cap module, Nucl. Instrum. Meth. A575 (2007) 353–389.[7] CiS Institut fu¨r Mikrosensorik gGmbH, Germany.[8] F. Campabadal, et al., Design and performance of theABCD3TA ASIC for readout of silicon strip detectors in theATLAS semiconductor tracker, Nucl. Instrum. Meth. A552(2005) 292–328.[9] http://www.pvss.com.[10] D. Attree, et al., The evaporative cooling system for the ATLASinner detector, JINST 3 (2008) P07003.[11] F. Campabadal, et al., Beam tests of ATLAS SCT silicon stripdetector modules, Nucl. Instrum. Meth. A538 (2005) 384–407.[12] E. Abat, et al., Combined performance tests before installationof the ATLAS Semiconductor and Transition Radiation TrackingDetectors, JINST 3 (2008) P08003.[13] J. Tsang, N. Hissey, Layout requirements and options for a newInner Tracker for the ATLAS Upgrade, ATL-P-EP-0001.4

The ATLAS SCT: Commissioning Experience and SLHC Upgrade

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