- Email: [email protected]
The Front-End Electronics of the straw tube tracker for the LHCb experiment
Nuclear Instruments and Methods in Physics Research A 623 (2010) 469–471
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The Front-End Electronics of the straw tube tracker for the LHCb experiment Antonio Pellegrino ,1 NIKHEF, Science Park 105, 1098 XG Amsterdam, The Netherlands
a r t i c l e in fo
abstract
Available online 16 March 2010
The LHCb experiment is a single-arm spectrometer, designed to study CP violation in B-decays at the Large Hadron Collider (LHC). It is crucial to accurately and efficiently track the charged decay products, in the high-density particle environment of the LHC. For this, the Outer Tracker has been constructed, consisting of 55,000 straw tubes, distributed over a sensitive area of 12 double layers of 6 5 m2 each. The detector is foreseen to operate up to 100 kHz/cm per straw in the region closest to the beam. The task of the Front-End Electronics is to provide the precise (0.5 ns) drift-time measurement, at an average occupancy of 5% and at a 1 MHz trigger rate. The tracking procedure requires high-efficiency (low thresholds), while at the same time putting stringent limits on the noise level. The modular detector structure reflects on the FE electronics: 128 channels are read out by one FE ‘‘Box’’. The mass production and installation of 450 FE-Boxes is completed. Quality checks have been performed in several stages, at the level of individual boards and at the global level with dedicated test systems mimicking the real detector and capable of simulating all the readout functionalities. At the time of the conference, all FE electronics has been commissioned in situ with test-pulses, cosmic rays and the with first beam events from LHC. No dead channels and very few noisy channels have been found. An upgrade is currently under study, aiming at digitizing and reading out events at each beam-crossing. & 2010 Elsevier B.V. All rights reserved.
Keywords: Outer-Tracker Straw-tubes Drift-chambers Front-End Electronics
1. Introduction The LHCb detector is a single arm spectrometer [1]. Its tracking system is divided in a silicon detector close to the interaction region, a dipole magnet, and a tracking system behind the magnet. By measuring the deflection of a charged particle by the magnetic field, the particle momentum is determined. The tracking system behind the magnet is divided in two parts: a gaseous straw tube detector, the Outer Tracker (OT), covering most of the LHCb acceptance, and a silicon detector at high rapidity in the highest particle flux region. The OT has a modular design: 168 long F-modules (500 34 cm2), and 96 short S-modules above and below the beam-pipe. A module consists of two staggered layers of 64 straws, electrically floating at the center and read out at the two ends (2 128 channels) [2]. The structure of the Front-End (FE) Electronics matches the module design: the straw signals are analyzed and digitized by a set of specially designed PCBs, hosted in a metallic housing that fits to a module end. As shown in Fig. 1, straw tube modules and their FE Electronics are attached to mobile metallic structures (C-Frames) that provide mechanical support, as well as the distribution of the
Tel.: + 41 76 4872572; fax: + 31 20 5925155.
E-mail address: [email protected] On behalf of the LHCb Outer Tracker Collaboration.
1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.039
various services to the modules and the FE Boxes: Low-Voltage (LV), High-Voltage (HV), slow (ECS) and fast control (TFC), etc.
2. Front-End Electronics A FE Box services 128 straw tubes: each readout channel consists of a preamplifier/shaper and a discriminator, a Time-toDigital Converter (TDC) and a data serializer and optical link transmitter. So, each FE Box contains all the electronics necessary to read out the hit signals from the straws, determine their times with respect to the LHC clock, and ship them to the off-detector electronics if a positive trigger (L0) decision arrives (Fig. 2). These functionalities are implemented in a modular way through various boards (see Fig. 3): the HV Board (decoupling the analog signal on the anode wire from the HV), the ASDBLR Board (pre-amplifying and discriminating the analog hit signals), the OTIS Board (measuring the hit timing), and the GOL/AUX Board (supplying bias voltages and serializing data for optical transmission). About 460 FE Boxes (including prototypes and spares) were produced, including several thousands PCBs of each type. This large number and the demanding quality criteria required a complex system of quality assurance and tracing to be developed. Quality checks were performed on all individual PCBs and on the finally assembled FE Box. Each item was assigned a serial number and traced in a database system.
470
A. Pellegrino / Nuclear Instruments and Methods in Physics Research A 623 (2010) 469–471
Fig. 4. HV Board. Top-left: embedded capacitor. Fig. 1. Modules and FE Boxes assembly, supported by metallic C-Frames, providing all services (LV, HV, ECS, TFC, etc.).
Fig. 2. Schematic view of the OT Electronics.
Fig. 3. Open view of a FE Box.
The HV Board is the interface between the FE Box and the anode wires. It hosts 32 capacitors (330 pF) to decouple the positive HV to the anode wires from the small hit charges to the pre-amplifiers. The PCB adopts a novel design (see Fig. 4), with the capacitors buried inside the PCB layers to reduce the environmental impact on the leakage current. All HV Boards were checked with a dedicated burn-in test setup: 64 HV Boards at a time were supplied 2500 V (nominal operational voltage is 1550 V) at 701 for 48 hours, while the leakage current (typically of the order of 1 nA per board) and the capacitance were monitored. After this test, about 10% of the HV boards were rejected, mainly due to high or unstable leakage currents. The hit signals from the anode wires are amplified, shaped and discriminated with the 8-channels ASDBLR chip [3], produced with the radiation-hard DMILL process, and featuring a fast peaking time (about 7 ns) and baseline restoration to eliminate
the long ion tail. Two ASDBLR chips are assembled on one PCB; each FE Box hosts 8 ASDBLR Boards. All ASDBLR chips were tested with an automatic probe station and carefully selected to achieve high threshold uniformity (better than 50 mV, roughly corresponding to 0.5 fC). The OTIS (Outer tracker Time Information System) is a 32-channels clock-driven TDC chip especially designed for OT and produced with 0:25 mm CMOS process. It operates synchronous to the 40 MHz bunch crossing clock and provides intermediate data storage in a 4 ms pipeline; if a positive L0 decision arrives (up to a 1.1 MHz rate), it transfers the corresponding event data to the GOL serializer chip. In addition, it provides the threshold voltages to the four ASDBLR chips connected to it. Each FE Box hosts 4 OTIS boards. The output of four OTIS is sent to one Gigabit Optical Link chip (GOL) mounted on the GOL/AUX Board. This board provides also the bias to the OTIS and the ASDBLR by means of radiation-hard voltage regulators, and distributes the ECS and TFC signals to the FE Box. Optical fibers carry the data to the TELL1 buffer boards in the counting house at a rate of 1.3 Gb/s. Once the quality of the individual boards was checked and all data stored for tracing, FE Boxes were assembled and then checked with a special FE-Tester, a programmable pulser with a time resolution of 150 ps capable of testing all the functionalities of the readout, mimicking the real detector. The heart of the setup is a PCB with an Altera programmable logic chip, while the FE Box interface is realized by a ‘‘Flipper’’ PCB, capable of controlling individually all input signals to the 128 FE Box channels. A series of tests is performed: a threshold scan for fixed input charge, followed by an input scan at fixed threshold, to determine the half-efficiency points of all channels (see Fig. 5); the TDC time is probed by means of a delay scan; the noise is studied as function of the threshold. All defects (noisy channels, dead channels, etc.) found were repaired prior to installation.
3. Installation and commissioning The detector and the FE Electronics were installed between 2006 and 2008. The commissioning is still on-going, mainly based on a built-in test-pulse facility, injecting pulses with adjustable heights and time phases into the ASDBLRs. Dedicated stand-alone data-taking procedures, integrated within the ECS and combined with monitoring and analysis tasks, have been developed. Moreover, the capability of the LHCb Calorimeter and MUON Systems to trigger on cosmic rays events is an invaluable tool for commissioning: cosmic events produce clean tracks through OT [2] and are used to train the complete online and offline monitoring chain. Track residual minimization in cosmic events allows the determination of the time phases of the various detector channels (T0), as shown in Fig. 6. Differences were found
A. Pellegrino / Nuclear Instruments and Methods in Physics Research A 623 (2010) 469–471
471
Fig. 6. Summary of the T0 phases of the OT FE Boxes as determined from cosmic rays data.
Fig. 5. Single-channel hit efficiency as a function of threshold: the curve denotes an errf fit to the data.
to be quite small (of the order of 2–3 ns) and of no consequence for the detector efficiency during data-taking: they will therefore only be corrected offline to achieve optimal tracking resolution. During the first days (September 2008) of the LHC commissioning with single beams, the OT detector was operated on nominal voltage (1550 V) for short periods and events triggered by the LHCb Calorimeter system (multiplicity trigger) were recorded. This first data confirmed that the Outer Tracker detector is fully operational and will be ready for data taking from the early days of the LHCb Experiment.
4. Upgrade plans The LHCb experiment is planning an upgrade based on a trigger-less readout [4]. With the present OT FE design, this would
lead to a data throughput of about 40 Gb/s per FE Box. Therefore, an R&D program has started to digitize and zero-suppress drifttime data at the FE level, in an FPGA-based design. A first prototype (adopting an ACTEL ProAsic3E) of a clock-driven, deadtime free TDC has been built and is currently tested. Preliminary results show that a differential non-linearity (DNL) of about 1.2 ns can be reached.
References [1] LHCb Collaboration, LHCb reoptimized detector design and performance: Technical Design Report, [CERN-LHCC-2003-030], CERN Geneva, September 2003. [2] A. Pellegrino, Installation and commissioning of a high-efficiency and high-resolution straw tube tracker for the LHCb experiment, these proceedings. [3] M. Newcomer, et al., Radiation hardness: design approach and measurements of the ASDBLR ASIC for the ATLAS TRT, [Nuclear Science Symposium Conference Record], 2002 IEEE, vol. 1, 10–16 November 2002. [4] LHCb Collaboration, Expression of interest for an LHCb upgrade, [CERN-LHCC2008-007], CERN Geneva, April 2008.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The Front-End Electronics of the straw tube tracker for the LHCb experiment Antonio Pellegrino ,1 NIKHEF, Science Park 105, 1098 XG Amsterdam, The Netherlands
a r t i c l e in fo
abstract
Available online 16 March 2010
The LHCb experiment is a single-arm spectrometer, designed to study CP violation in B-decays at the Large Hadron Collider (LHC). It is crucial to accurately and efficiently track the charged decay products, in the high-density particle environment of the LHC. For this, the Outer Tracker has been constructed, consisting of 55,000 straw tubes, distributed over a sensitive area of 12 double layers of 6 5 m2 each. The detector is foreseen to operate up to 100 kHz/cm per straw in the region closest to the beam. The task of the Front-End Electronics is to provide the precise (0.5 ns) drift-time measurement, at an average occupancy of 5% and at a 1 MHz trigger rate. The tracking procedure requires high-efficiency (low thresholds), while at the same time putting stringent limits on the noise level. The modular detector structure reflects on the FE electronics: 128 channels are read out by one FE ‘‘Box’’. The mass production and installation of 450 FE-Boxes is completed. Quality checks have been performed in several stages, at the level of individual boards and at the global level with dedicated test systems mimicking the real detector and capable of simulating all the readout functionalities. At the time of the conference, all FE electronics has been commissioned in situ with test-pulses, cosmic rays and the with first beam events from LHC. No dead channels and very few noisy channels have been found. An upgrade is currently under study, aiming at digitizing and reading out events at each beam-crossing. & 2010 Elsevier B.V. All rights reserved.
Keywords: Outer-Tracker Straw-tubes Drift-chambers Front-End Electronics
1. Introduction The LHCb detector is a single arm spectrometer [1]. Its tracking system is divided in a silicon detector close to the interaction region, a dipole magnet, and a tracking system behind the magnet. By measuring the deflection of a charged particle by the magnetic field, the particle momentum is determined. The tracking system behind the magnet is divided in two parts: a gaseous straw tube detector, the Outer Tracker (OT), covering most of the LHCb acceptance, and a silicon detector at high rapidity in the highest particle flux region. The OT has a modular design: 168 long F-modules (500 34 cm2), and 96 short S-modules above and below the beam-pipe. A module consists of two staggered layers of 64 straws, electrically floating at the center and read out at the two ends (2 128 channels) [2]. The structure of the Front-End (FE) Electronics matches the module design: the straw signals are analyzed and digitized by a set of specially designed PCBs, hosted in a metallic housing that fits to a module end. As shown in Fig. 1, straw tube modules and their FE Electronics are attached to mobile metallic structures (C-Frames) that provide mechanical support, as well as the distribution of the
Tel.: + 41 76 4872572; fax: + 31 20 5925155.
E-mail address: [email protected] On behalf of the LHCb Outer Tracker Collaboration.
1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.039
various services to the modules and the FE Boxes: Low-Voltage (LV), High-Voltage (HV), slow (ECS) and fast control (TFC), etc.
2. Front-End Electronics A FE Box services 128 straw tubes: each readout channel consists of a preamplifier/shaper and a discriminator, a Time-toDigital Converter (TDC) and a data serializer and optical link transmitter. So, each FE Box contains all the electronics necessary to read out the hit signals from the straws, determine their times with respect to the LHC clock, and ship them to the off-detector electronics if a positive trigger (L0) decision arrives (Fig. 2). These functionalities are implemented in a modular way through various boards (see Fig. 3): the HV Board (decoupling the analog signal on the anode wire from the HV), the ASDBLR Board (pre-amplifying and discriminating the analog hit signals), the OTIS Board (measuring the hit timing), and the GOL/AUX Board (supplying bias voltages and serializing data for optical transmission). About 460 FE Boxes (including prototypes and spares) were produced, including several thousands PCBs of each type. This large number and the demanding quality criteria required a complex system of quality assurance and tracing to be developed. Quality checks were performed on all individual PCBs and on the finally assembled FE Box. Each item was assigned a serial number and traced in a database system.
470
A. Pellegrino / Nuclear Instruments and Methods in Physics Research A 623 (2010) 469–471
Fig. 4. HV Board. Top-left: embedded capacitor. Fig. 1. Modules and FE Boxes assembly, supported by metallic C-Frames, providing all services (LV, HV, ECS, TFC, etc.).
Fig. 2. Schematic view of the OT Electronics.
Fig. 3. Open view of a FE Box.
The HV Board is the interface between the FE Box and the anode wires. It hosts 32 capacitors (330 pF) to decouple the positive HV to the anode wires from the small hit charges to the pre-amplifiers. The PCB adopts a novel design (see Fig. 4), with the capacitors buried inside the PCB layers to reduce the environmental impact on the leakage current. All HV Boards were checked with a dedicated burn-in test setup: 64 HV Boards at a time were supplied 2500 V (nominal operational voltage is 1550 V) at 701 for 48 hours, while the leakage current (typically of the order of 1 nA per board) and the capacitance were monitored. After this test, about 10% of the HV boards were rejected, mainly due to high or unstable leakage currents. The hit signals from the anode wires are amplified, shaped and discriminated with the 8-channels ASDBLR chip [3], produced with the radiation-hard DMILL process, and featuring a fast peaking time (about 7 ns) and baseline restoration to eliminate
the long ion tail. Two ASDBLR chips are assembled on one PCB; each FE Box hosts 8 ASDBLR Boards. All ASDBLR chips were tested with an automatic probe station and carefully selected to achieve high threshold uniformity (better than 50 mV, roughly corresponding to 0.5 fC). The OTIS (Outer tracker Time Information System) is a 32-channels clock-driven TDC chip especially designed for OT and produced with 0:25 mm CMOS process. It operates synchronous to the 40 MHz bunch crossing clock and provides intermediate data storage in a 4 ms pipeline; if a positive L0 decision arrives (up to a 1.1 MHz rate), it transfers the corresponding event data to the GOL serializer chip. In addition, it provides the threshold voltages to the four ASDBLR chips connected to it. Each FE Box hosts 4 OTIS boards. The output of four OTIS is sent to one Gigabit Optical Link chip (GOL) mounted on the GOL/AUX Board. This board provides also the bias to the OTIS and the ASDBLR by means of radiation-hard voltage regulators, and distributes the ECS and TFC signals to the FE Box. Optical fibers carry the data to the TELL1 buffer boards in the counting house at a rate of 1.3 Gb/s. Once the quality of the individual boards was checked and all data stored for tracing, FE Boxes were assembled and then checked with a special FE-Tester, a programmable pulser with a time resolution of 150 ps capable of testing all the functionalities of the readout, mimicking the real detector. The heart of the setup is a PCB with an Altera programmable logic chip, while the FE Box interface is realized by a ‘‘Flipper’’ PCB, capable of controlling individually all input signals to the 128 FE Box channels. A series of tests is performed: a threshold scan for fixed input charge, followed by an input scan at fixed threshold, to determine the half-efficiency points of all channels (see Fig. 5); the TDC time is probed by means of a delay scan; the noise is studied as function of the threshold. All defects (noisy channels, dead channels, etc.) found were repaired prior to installation.
3. Installation and commissioning The detector and the FE Electronics were installed between 2006 and 2008. The commissioning is still on-going, mainly based on a built-in test-pulse facility, injecting pulses with adjustable heights and time phases into the ASDBLRs. Dedicated stand-alone data-taking procedures, integrated within the ECS and combined with monitoring and analysis tasks, have been developed. Moreover, the capability of the LHCb Calorimeter and MUON Systems to trigger on cosmic rays events is an invaluable tool for commissioning: cosmic events produce clean tracks through OT [2] and are used to train the complete online and offline monitoring chain. Track residual minimization in cosmic events allows the determination of the time phases of the various detector channels (T0), as shown in Fig. 6. Differences were found
A. Pellegrino / Nuclear Instruments and Methods in Physics Research A 623 (2010) 469–471
471
Fig. 6. Summary of the T0 phases of the OT FE Boxes as determined from cosmic rays data.
Fig. 5. Single-channel hit efficiency as a function of threshold: the curve denotes an errf fit to the data.
to be quite small (of the order of 2–3 ns) and of no consequence for the detector efficiency during data-taking: they will therefore only be corrected offline to achieve optimal tracking resolution. During the first days (September 2008) of the LHC commissioning with single beams, the OT detector was operated on nominal voltage (1550 V) for short periods and events triggered by the LHCb Calorimeter system (multiplicity trigger) were recorded. This first data confirmed that the Outer Tracker detector is fully operational and will be ready for data taking from the early days of the LHCb Experiment.
4. Upgrade plans The LHCb experiment is planning an upgrade based on a trigger-less readout [4]. With the present OT FE design, this would
lead to a data throughput of about 40 Gb/s per FE Box. Therefore, an R&D program has started to digitize and zero-suppress drifttime data at the FE level, in an FPGA-based design. A first prototype (adopting an ACTEL ProAsic3E) of a clock-driven, deadtime free TDC has been built and is currently tested. Preliminary results show that a differential non-linearity (DNL) of about 1.2 ns can be reached.
References [1] LHCb Collaboration, LHCb reoptimized detector design and performance: Technical Design Report, [CERN-LHCC-2003-030], CERN Geneva, September 2003. [2] A. Pellegrino, Installation and commissioning of a high-efficiency and high-resolution straw tube tracker for the LHCb experiment, these proceedings. [3] M. Newcomer, et al., Radiation hardness: design approach and measurements of the ASDBLR ASIC for the ATLAS TRT, [Nuclear Science Symposium Conference Record], 2002 IEEE, vol. 1, 10–16 November 2002. [4] LHCb Collaboration, Expression of interest for an LHCb upgrade, [CERN-LHCC2008-007], CERN Geneva, April 2008.