The LHCb VErtex LOcator (VELO) is designed to reconstruct primary and secondary vertices in b-hadron decays. It is a silicon microstrip detector situated closest to the interaction region. It provides fast tracking information to the trigger system. The design of the detector and the commissioning of its various sub-components are described here

Tobin, M

CERN Document Server

PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO)Mark Tobin∗†University of LiverpoolE-mail: Mark.Tobin@cern.chThe LHCb VErtex LOcator (VELO) is designed to reconstruct primary and secondary verticesin b-hadron decays. It is a silicon microstrip detector situated closest to the interaction region.It provides fast tracking information to the trigger system. The design of the detector and thecommissioning of its various sub-components are described here.17th International Workshop on Vertex detectorsJuly 28-August 1, 2008Utö Island, Sweden∗Speaker.†On behalf of the LHCb VELO groupc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobin1. IntroductionThe LHCb experiment[1] is a single arm spectrometer designed to study heavy flavour physicsin decays of B-mesons It will constrain the unitarity triangles of the CKM matrix[2] by measuringCP-violation and probe for physics beyond the Standard Model by studying rare decays.The LHCb VErtex LOcator[3] (VELO) is required to reconstruct primary and secondary inter-actions close to the beam in an extreme radiation environment. It is the primary tracking detector ofthe experiment and provides fast reconstruction information to the trigger. An additional require-ment placed on the VELO by the LHC is that the detector has to be 30mm from the interactionregion during the injection of the beam.2. Vertex LocatorThe LHCb Vertex Locator is shown in figure 1 and consists of 42 modules arranged perpen-dicular to the beam around the interaction region. It is divided into two retractable halves containedin a secondary vacuum (<10−4mbar) and is separated from the primary vacuum (<10−8mbar) bya 300µm aluminium foil. The maximum allowed differential pressure is 5mbar. The shape of thefoil is corrugated such that the halves partially overlap when the detector is in its closed position ata distance of 5mm from the interaction region.Figure 1: Overview of the VELO system.A VELO module consists of two 300µm thick semi-circular silicon sensors with 2048 strips;one measures the radial co-ordinate, R, and the other measures the azimuthal angle, Φ. The sen-sors are n+-on-n oxygenated silicon which are optimised to operate in the non uniform radiationenvironment close to the LHC beams. The maximum fluence is expected to be 1.3.1014neq/cm2 inthe innermost region for a luminosity of 2fb−1. This corresponds to one normal year of operation.The R-sensor is segmented in 45◦ quadrants and has a pitch that increases linearly from 40µmat the inner radius (8.13mm) to 101.6µm at the outer radius (42mm). TheΦ sensor has radial strips2PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobinwith a small stereo angle split into 683 short inner strips and 1365 long outer strips. The pitch of theΦ sensors varies from 35 to 101µm and the strips in each region are skewed in opposite directionsby 10◦ and 20◦.The sensors are mounted on a double sided hybrid consisting of a substrate with a ThermalPyrolytic Graphite (TPG) core cloaked in carbon fibre onto which a kapton circuit is laminated oneach side. The 2048 strips are routed via a double metal layer on the sensor to the hybrid wherethere are 16 Beetle front-end chips[4]. The nominal power consumption of 20W is carried awayfrom the front end chips by the TPG. A two-phase CO2 system[5] keeps the sensors at −7◦. Thevarious components of a module are shown in figure 2.Figure 2: Principle components of a module (left) and a photograph of a mounted module (right). Thetemperature of the hybrid can be monitored at two different positions using NTC resistors.The modules and mechanics were designed to minimise the amount of material in the geo-metrical acceptance. The average radiation length seen by particles passing through the VELO isshown in figure 3 (a) as a function of azimuthal angle and pseudo-rapidity. The average contribu-tion to the total radiation length for each component of the VELO is shown in figure 3 (b). Theaverage thickness of the VELO is 17.5% of a radiation length where the largest contribution comesfrom from the RF-foil. The shape of the foil was chosen to minimize the material between theinteraction point and the first measuring plane.3. Installation and Commissioning of Readout ElectronicsThe VELO electronics chain for one silicon sensor is shown in figure 4. A description ofits various sub-components and the tests performed during installation are given in the followingsections.3.1 Repeater boardsEach sensor is connected via kapton cables to a repeater board located outside the vacuumtank. Each board contains four driver cards which send the analogue data signals across 60m3PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobin(a)(b)Figure 3: (a) The average radiation length seen by particles passing through the VELO as a function ofazimuthal angle, φ , and pseudo-rapidity, η , with 1.6< η <4.9. (b) The average contribution to the totalradiation length from each component of the VELO.Figure 4: The VELO readout chain for one side of one module.cables to the digitiser card; a low voltage card to power the driver cards and the Front End chips;and an Experimental Control System (ECS) card to communicate with the control boards (see nextsection). The repeater board receives and transmits differential signals, eg data, Timing and Fastcontrol (TFC) and FE chip configuration, to the electronic circuits of the hybrid. Monitoring signalsare also sent out to the detector control system.3.2 Control BoardsThe control board[9] distributes timing signals and fast commands from the Readout Supervi-sor to the L0 electronics. They distribute commands via I2C from the Experimental Control Systemto Beetles chips on the hybrid and control the low voltage distribution for the repeater boards. Eachcontrol board supports six hybrids. The control boards are located on a platform in the cavern 25m4PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobinaway from the vacuum tank.3.3 Low Voltage (LV) SystemThe low voltage system provides the power to the LV card on the repeater board. This in turnprovides the power to the front end chips, four analogue driver cards and an ECS card mounted onthe repeater board. The LV system is a multi-channel power supply system based on the CAENEASY3000 Embedded Assembly System[6]. The power to four repeater boards is provided by oneA3009 12 channel 8V board. The boards are housed in crates in the counting room 60m away fromthe detector and each channel has a sensing circuit to compensate for the voltage drop across thecables.The boards were tested to check the operation and linearity of the supplies before installation.The number of times an A3009 module had to be returned to the manufacturer under guarantee isshown in figure 5 (a).(a) (b)Figure 5: Problems encountered during testing of Low Voltage system. (a) shows the number of times aCAEN A3009 module was returned to the manufacturer under guarantee and (b) shows the CAEN moduleis powered when the interlock was fired.Various safety tests were performed. A problem was discovered in the firmware of the powersupply boards. It was found that the module could remain powered when communication betweenthe board and the mainframe that controls it was lost. Figure 5 (b) shows one board powered withthe interlock circuit open. This state could lead to rapid heating of the hybrid and a firmware fixwas provided by the manufacturer.3.4 High Voltage SystemThe high voltage system provides the bias voltage to the sensors. The system uses six ISegEHQ F607 n-F modules1 each of which can control up to 16 sensors. The modules are housed in1Manufactured by ISEG Spezialelektronik GmbH.5PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobina non-interruptible power supply 60m from the detector. A detailed description of the system isavailable in [7].The system was tested in four parts; normal operation of the high voltage system; verificationof its stability under operation; discussion of its behaviour in failure modes; and details of operationat low voltage. The results are described in detail in [8]. An example problem found with themodules was that when a second channel in the module was started to ramp to its nominal voltageany other channel that was in the process of ramping jumped to its target voltage. The OPC serverwould crash and have to restarted at least once a week. These problems were fixed after 5 iterationsof the OPC server.3.5 TELL1 BoardsThe TELL1 readout board[10] collects analogue data directly from the Beetle chips. Thedata from one sensor is digitised by four analogue receiver cards (A-Rx) on the board. One A-Rxcard provides 16 channels of 10-bit ADCs which sample the analogue data from 4 Beetle chips at40MHz. The TELL1 then performs data processing in four pre-processing FPGAs. The steps inthe data processing are pedestal subtraction, removal of cross-talk, channel re-ordering, commonmode noise suppression and clustering. The zero suppressed data are then sent to the trigger farm.The power consumption and the output voltage of the A-Rx cards were tested before theywere mounted on the TELL1. The digital performance of the A-Rx cards was tested when theywere installed in the counting house. The sampling time for each channel of the A-Rx cards wasadjusted using phase programmable clocks to compensate for different cable lengths.The uniformity of the TELL1s was tested using a dummy module2, a repeater board, a controlboard and a full set of cables. The noise spectra for each TELL1 was studied to find problematiclinks and broken TELL1s. There were 17 boards with problems which prevented data taking andwere repaired. Some of the boards had a problem with the vias3 which caused them to break whenthermally cycled.3.6 Readout Slice TestsThe electrical components of each module position, referred to as a readout slice, were con-nected to the dummy module. A series of tests were performed: the optimal sampling point forthe ADCs on the TELL1 was set to account for differences in the length of the readout cables; theconnections between the repeater boards and the TELL1 were checked by injecting known patternsinto the detector readout; and problematic channels and links were found by looking at the noiseand pedestal spectra of the data. The various tests used in the commissioning of the data acquisitionare described in [13]. Problematic readout boards were exchanged and sent for repair.4. Commissioning of VELOThe modules were assembled in Liverpool before being shipped to CERN where they weremounted on the base plates of each half. The 2 halves were installed in the vacuum tank in October2The dummy module was a fully populated hybrid with no silicon sensor attached.3Connections between the different layers of the TELL1 printed circuit board.6PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobin2007. During the assembly process the modules had six visual inspections, six metrologies, sevenelectrical tests and 4 vacuum tests.The geometry of the VELO is such that it would be extremely difficult to commission thedetector using tracks from cosmic rays. Final commissioning can only be made when real beamcollisions occur in the interaction region. A study of the expected performance of the dectector wasmade using a test beam and it is described in the section 4.1. The commissioning of the detectorafter installation is described in section 4.2 and was performed using sensor noise studies whenoperating single modules, detector halves and the full VELO system.4.1 Alignment Challenge and Detector Commissioning in Beam TestIn November 2006, a partially equipped VELO half was tested in a 180 GeV pion beam at theCERN SPS. A total of 10 modules were installed, together with a small scale cooling system, anddifferent combinations of 6 modules were read out using the final readout chain. The software fordata acquisition; detector control; track and vertex reconstruction; and online monitoring of the datawere tested. More than 50 million events were collected in various configurations which includeddata taken from targets displaced from the beam axis to investigate the tracking performance of theVELO in the open position. The data was used to study the performance of the VELO alignmentalgorithm[11].The signal to noise ratio of the R (Φ) sensors was found to be 20-24 (24-29) depending onthe sensor region. The sensor resolution was found to be 9.5+0.3× (pitch−40)µm for R andΦ sensors for normal incidence tracks. A detailed description of the experimental setup and theperformance of the alignment algorithm is given in [12].4.2 Detector Commissioning after InstallationAfter the installation of the detector halves, data was taken in two configurations: firstly,the vacuum vessel was filled with ultra-pure Neon and the modules in one half were poweredup individually and collectively; secondly, the detector volume was evacuated to a pressure of10−5mbar and the modules in both halves of the detector were operated collectively. In all tests thesensors were biased to 100V.4.2.1 Single Module OperationThe first VELO module was operated under a Neon atmosphere in March 2008 using its finalelectronic readout chain. This test was repeated for all module positions in one VELO half beforebeing repeated for the all modules in the opposite half. A typical distribution of the raw noise forR and Phi sensors is shown in figures 6 (a) and (b) respectively.4.2.2 System TestsThe A side of the VELO was operated under Neon atmosphere with 15 modules fully poweredon the 15th May 2008. A reduced cooling capacity under the Neon atmosphere meant that themodules were operated at 25◦C. The C side of the VELO was operated for the first time on the 10thJune. Many problems were found with faulty TELL1 boards. The broken boards were replacedand their failure was found to be due to a problem in the manufacturing process (c.f. section 3.5).7PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark Tobin(a) (b)Figure 6: The raw noise (red) and noise after common mode subtraction (black) as a function of stripnumber for a typical (a) R sensor and (b) Φ sensor.The VELO vacuum tank and the beam pipe were evacuated until a pressure of 10−5mbarwas reached in the secondary vacuum. The cooling was operated fully loaded at -25◦C where thetemperature of the modules was -5◦C4. The full VELO system was operated under vacuum for thefirst time at 01:40 on the 28th June 2008.An initial data sample of 300k non-zero suppressed events was taken. The number of fullyoperation channels was found to be 99.2%. A comparison of the noise for a single module whenoperated as part of the full system and standalone is shown in figure 7. There is a clear correlationbetween the noise values and only small variations were seen. The biggest difference in noise wasfound to be 0.045 ADC counts or 2.3%. No increase in noise was seen when all modules werepowered simultaneously. The results from this study are described in [14].5. ConclusionsThe various components of the VELO were installed and extensively tested over the past year.There were problems with the delivery and the reliability of the Low Voltage modules. The HighVoltage system suffered from instabilities in the OPC server which were fixed by the manufacturer.The TELL1 readout boards also suffered from problems in the manufacturing process and manyboards had to be replaced and repaired. The full VELO system was operated successfully undervacuum for the first time. The number of channels which were fully operational was 99.2%. TheVELO was ready for the first LHC beams.4The cooling was set to -5◦C and the modules were operated at 14◦C for all subsequent data taking to minimise thethermal cycling of modules during the commissioning.8PoS(VERTEX 2008)009Commissioning the LHCb VErtex LOcator (VELO) Mark TobinFigure 7: Comparison between the noise after common mode suppression for a module when operatedstandalone and as part of the full system.References[1] The LHCb Detector at the LHCThe LHCb CollaborationPublished in Journal of Instrumentation (INST), 2008 JINST 3 S08005[2] C. Amsler et al., Physics Letters B667, 1 (2008)[3] LHCb VELO Technical Design Report, CERN/LHCC 2001-011 (2001)[4] N. van Bakel et al., CERN/LHCC 2001-046 (2001)[5] Martin van Beuzekom, Ann Van Lysebetten, Bart Verlaat, CO2 cooling experience (LHCb), PoS(Vertex 2007) 009[6] CAEN Technical Information Manual, MOD. A3009/A3009B 12 CH 8V/9A/45W Power SupplyBoard, http://www.caen.it[7] B. Rakotomiaramanana, High Voltage Connectivity, https://edms.cern.ch/document/836439/2[8] B. Rakotomiaramanana et al., Characterisation of the VELO High Voltage System,CERN-LHCb-2008-009 (2008)[9] L.Eklund, Control and Monitoring of VELO and Pile-Up Level 0 Electronics, EDMS documenthttps://edms.cern.ch/document/596194.[10] G. Haefeli et al., The LHCb DAQ interface board TELL1, Nucl. Inst. Meth. A560 (2006) 494[11] S. Viret, C. Parkes, M. Gersabeck, "Alignment procedure of the LHCb Vertex Detector", Nucl. Instr.and Meth. A596 (2008) 157-163, doi:10.1016/j.nima.2008.07.153 [arXiv:0807.5067][12] M. Gersabeck et al., Performance of the LHCb Vertex Detector Alignment Algorithm determinedwith Beam Test Data,Nucl. Instr. and Meth. A (2008), doi:10.1016/j.nima.2008.07.154 [arXiv:0807.5069][13] O. Behrendt et al., VELO DAQ commissioning tests, LHCb-INT-2009-004.[14] A. Papadeliset al., First study of the VELO detector noise performance at Point 8,LHCb-INT-2009-005.9

Commissioning the LHCb VErtex LOcator (VELO)

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server