- Email: [email protected]
LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker Lukas Gruber, On behalf of the LHCb SciFi Tracker Collaboration CERN, EP Department, 1211 Geneva 23, Switzerland
ARTICLE Keywords: Particle tracking Scintillating fibre SciFi SiPM
INFO
ABSTRACT LHCb will undergo a major upgrade during the LHC long shutdown 2 in 2019/2020 to cope with increased instantaneous luminosities and to implement a trigger-less 40 MHz readout. The current inner and outer tracking detectors will be replaced by a single homogeneous detector based on plastic scintillating fibres (SciFi). The SciFi tracker covers an area of 340 m2 by using more than 10,000 km of scintillating fibre with 250 μm diameter, enabling a spatial resolution of better than 100 μm for charged particles. Linear arrays of Silicon Photomultipliers cooled to −40◦ C are placed at the fibre ends. The readout of 524 k channels occurs through custom-designed front-end electronics. The assembly of the detector has started and the project is on track for installation starting end 2019. In view of future upgrades, a R&D programme aiming at the development of very fast and efficient scintillating fibres, based on a novel type of luminophores (NOL), has been launched in parallel to the SciFi production. The performance of the prototype NOL fibres is competitive, in particular the decay time constant is close to 1 ns, i.e. about 50% shorter than the best standard fibre.
1. Introduction The LHCb experiment is preparing for a major upgrade in order to run from 2021 onwards at a five times higher luminosity of 2 × 1033 cm−2 s−1 and a readout rate of 40 MHz. The goal is to collect an integrated luminosity of 50 fb−1 in 10 years of operation. A large scintillating fibre (SciFi) tracker [1,2] is going to replace the current outer and inner trackers, consisting of straw tubes and silicon micro strip detectors, respectively, by using a single detector technology and more than 10,000 km of plastic scintillating fibres. The SciFi tracker requires a hit detection efficiency of 99%, a spatial resolution better than 100 μm in the horizontal bending plane and a low material budget (X/X0 ≤ 1% per detector layer). During operation the SiPMs will be exposed to a total neutron fluence of up to 6 × 1011 neq ∕cm2 and therefore have to be cooled to −40 ◦ C to reduce the dark count rate and retain single photon counting capability. The fibres are exposed to an ionising dose of 35 kGy close to the beam pipe. To reduce spill over into consecutive bunch crossings a fast signal generation and shaping is mandatory. The high demands on the fibre quality led to a R&D initiative aiming at the development of very fast and efficient scintillating fibres, which are based on a novel type of luminophores, called Nanostructured Organosilicon Luminophores (NOL) [3]. The article gives a brief introduction to the SciFi tracker design, an overview about the production process and performance of the various detector components as well as a summary about the current status of assembly and testing. Eventually, the most recent achievements of the NOL fibre development are outlined.
1
2. LHCb SciFi detector design and construction Overall detector layout. The 340 m2 large active area is divided into three tracking stations with four independent stereo layers each (XUVX geometry, U and V with ± 5◦ stereo angle) with a size of about 6 × 5 m2 (see Fig. 1). The chosen scintillating fibres have a circular cross section and a diameter of 250 μm and are arranged in a 6-layer staggered pattern with 275 μm pitch, to form 1.3 mm thick fibre mats (see Fig. 3, right). 8 mats are combined to a single module, the basic unit of the SciFi tracker. The 2.4 m long fibre mats are read out at one end by linear 128-channel SiPM arrays, while the other end of the mats is equipped with mirrors (3M reflective foil). The light yield is 16– 20 photoelectrons (p.e.) for particles hitting close to the mirror side. Six pairs of C-shaped frames (C-frames), arranged on either side of the beam pipe, will carry the 128 modules with photodetectors and front-end electronics as well as all services to and from them. Fibre quality assurance. Blue emitting plastic scintillating fibres of type SCSF-78MJ with 250 μm diameter produced by Kuraray1 were chosen for the LHCb SciFi tracker. The fibres are made from a polystyrene (PS) core (refractive index 𝑛 = 1.59), surrounded by a double cladding structure of polymethylmethacrylate (PMMA, 𝑛 = 1.49) and a fluorinated polymer (𝑛 = 1.42). The thickness of each cladding layer is 3% of the total fibre diameter. The core is doped with an activator and a wavelength shifter (WLS) dye. The SCSF-78MJ fibre has its maximum emission at a wavelength of 𝜆 = 440 nm and an integral attenuation length of about 3.5 m [4].
E-mail address: [email protected]. Kuraray Co. Ltd., Tokyo, Japan, http://kuraraypsf.jp/psf/.
https://doi.org/10.1016/j.nima.2019.03.080 Received 20 March 2019; Accepted 26 March 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Left: picture of the custom designed mat winding machine. Right: cross section of a fibre mat.
Fig. 1. SciFi tracking station, shown in a semi-open configuration.
alignment holes on the winding wheel are filled with glue and form precise pins on the fibre mat, which are later on used as alignment reference for the assembly of 8 fibre mats into one module with a size of about 5 × 0.5 m2 . The alignment with respect to a straight line was found to be better than 50 μm over the total length of 5 m. A structure of honeycomb and carbon fibre panels serve as support. The lightweight design allows to build detector modules with a material budget of only 1% 𝑋0 per module.
Over a period of about 2 years the full supply of 12,000 km of fibres (including spares) underwent a systematic and rigorous quality assurance (QA) programme, including geometrical refinement to deal with rare punctual imperfections. The measurements comprised attenuation length, ionisation light yield, diameter, cladding integrity and radiation tolerance to X-rays. A detailed description of the technical and experimental methods as well as results of the fibre QA can be found in Ref. [4]. In general, the supply was found to be timely and of very high quality and stability, which led to negligible rejection rates. The verification of the fibre diameter and its uniformity is an integral part of the QA. Diameter defects (‘‘bumps’’ or ‘‘necks’’) cause deficits in the light transport (light leaks) of a fibre and potentially reduce its mechanical stability and, in the SciFi tracker, lead to distortions in the 6-layer pattern of the fibre mats, affecting hit efficiency and position resolution. Despite a clean work environment and other process optimisations, the occurrence of such defects could never be fully avoided during fibre production. Empirically, bumps with a diameter larger than 350 μm tend to cause irregular winding patterns and complicate or delay the fibre mat winding. A dedicated fibre scanning machine [5] was built to monitor the fibre diameter and cladding integrity (see Fig. 2). The scanner automatically detected bumps and necks and precisely recorded their shape. Bumps exceeding a diameter of 350 μm were shrunk by drawing the fibre through a hot conical tool [6].
Silicon photomultipliers. The scintillating fibres are readout by customised 128-channel linear SiPM arrays (Hamamatsu MPPC 13552– H2017) with a single channel size of 0.25 × 1.62 mm2 . Each channel consists of 104 individual pixels (57.5 × 62.5 μm2 ), as illustrated in Fig. 4. In total, 4096 SiPM arrays are going to be installed in the SciFi tracker, resulting in about 524k channels. The photodetectors are optimised for high photon detection efficiency (45% peak PDE at 𝛥𝑉 = 3.5 V), low after-pulse and cross-talk and have a thin entrance window (105 μm epoxy). The H2017 features a quenching resistor (𝑅𝑄 ) of 520 kΩ, which allows a large operating range. After R&D [7,8], the total correlated noise probability could be reduced to 7% at the operating point 𝛥𝑉 = 3.5 V. Direct and delayed cross-talk are the dominant sources of correlated noise, while the after-pulse probability is below 0.1%. The recovery time is 85 ns with typically 10% channel-to-channel variations due to dependence on 𝑅𝑄 . At the SiPM location in LHCb SciFi, the main damage is originating from neutron irradiation, which manifests itself in a massive increase of the dark count rate (DCR). As single photon sensitivity must be maintained, the DCR can only be managed by reducing the operational temperature, which reduces the DCR by a factor 2 every 10 K, as can be seen in Fig. 5. At the end of the detector lifetime (6 × 1011 neq ∕cm2 ), a DCR of 14 MHz per channel at −40 ◦ C is expected at the operating point of 𝛥𝑉 = 3.5 V. As an additional challenge, the amount of light seen by the SiPMs will be reduced by about 40% to 10–12 p.e. per hit
Mat and module production. After QA, the fibres are wound into 6-layer mats of 2.4 m length and 13 cm width by using a custom designed winding machine. A special winding wheel with a threaded surface is used to define precisely the position of the first fibre layer (see Fig. 3). Before each layer, an epoxy glue mixed with TiO2 powder is applied to stabilise the mat and reduce optical cross-talk between fibres. Precision
Fig. 2. Picture of the fibre scanning machine at CERN.
2
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 4. 128-channel SiPM array bonded to a Kapton flex-cable and close-up of a single channel and pixel.
Fig. 6. Drawing of a cold-box with cooling bellows connected.
Fig. 7. A picture of the SciFi front-end electronics. Fig. 5. Measured DCR of a single SiPM channel after a neutron fluence of 6 × 1011 neq ∕cm2 for different operating voltages.
due to radiation damage of the fibres, primarily close to the beam pipe (35 kGy at the end of the detector lifetime), corresponding also to the highest track density region. SiPM cooling. The SiPM cooling system of the SciFi tracker is a major engineering challenge. For the full detector the total expected heat load is about 10 kW and the required mass flow amounts to about 5 kg/s, i.e. 18,000 kg/h. The transfer lines from the cooling plant to the detector will be about 100 m long and except for the SiPMs, all detector parts are located and operated in ambient temperature. To thermally insulate the cold photodetector arrays from the surrounding, a gas- and light-tight cold-box is installed around them. Inside the cold-box, the SiPMs are glued on 3D printed Titanium cold-bars through which the cooling liquid (monophase 3M Novec 649 at −50 ◦ C) is circulating. The SiPMs are then connected to the front-end electronics via special Kapton flex-PCBs. On a length of about 20 m, the near detector cooling lines are vacuum insulated. To avoid ice building up inside the coldbox, the boxes are flushed with dry air with a dew point of −70 ◦ C. A schematic of a cold-box is shown in Fig. 6.
Fig. 8. Signal processing chain in the SciFi front-end electronics.
purposes. The SiPM arrays are connected to the PACIFIC boards, where the PACIFIC ASICs are placed and signal digitisation is performed. The custom designed ASIC features 64 channels (current mode input) and is based on 130 nm CMOS TSMC technology. The power consumption is about 10 mW per channel. Each channel consists of a preamplifier, a fast shaper (10 ns shaping time), a double gated integrator (25 ns integration time) to avoid dead time, a track and hold stage and three comparators to apply signal thresholds for digitisation, as illustrated in Fig. 8. The three thresholds allow to identify signal clusters and accurately determine their location while efficiently suppressing noise. The 3-bit outputs are encoded into 2-bit values and serialised (combining several channels), before being sent to the FPGAs located on the cluster boards, where individual channels are combined into clusters. The expected cluster rate at 40 MHz readout is less than 3 MHz per SiPM array. The master boards provide power, fast and slow control, and comprise the optical links for data transmission.
Front-end electronics. The 40 MHz trigger-less readout demands zerosuppression of data at the stage of the front-end electronics for all sub-detectors. In the special case of the SciFi tracker, the increase in DCR of SiPMs and noise cluster rate due to radiation damage requires fast readout, i.e. short integration time, and efficient rejection of noise from SiPM dark counts by clustering. The SciFi front-end electronics consist of a chain of different electronics boards, PACIFIC boards, cluster boards and master boards, as shown in Fig. 7. A light injection system is used for calibration 3
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
3. Detector module performance
of the NOL fibres are on a level comparable to the respective reference fibres, however irradiation tests with hadrons and to higher doses still have to be performed. The new fibres may be also interesting for a future upgrade or replacement of the most irradiated regions of the LHCb SciFi tracker and in general for future fibre detectors.
Two fibre modules equipped with production front-end electronics were tested in a 180 GeV/c mixed proton and pion beam at CERN SPS in July 2018. An external beam telescope was used as a reference for defining reconstructed particle tracks with a resolution much better than the fibre modules. A single hit resolution of 70-80 μm was achieved and the single hit efficiency was found to be 99.5%. The spillover was determined to be about 2%. All obtained results are in agreement with the design requirements and expectations.
6. Summary and conclusion The LHCb SciFi tracker is going to replace the current downstream tracking system and to start its operation in 2021. An area of 340 m2 is covered by using more than 10,000 km of scintillating fibres with 250 μm diameter, providing a spatial resolution of better than 100 μm. SiPM arrays cooled to −40 ◦ C are attached to the fibres and readout through custom designed front-end electronics at 40 MHz. The radiation environment (35 kGy for the fibres, 6 × 1011 neq ∕cm2 for the SiPMs) push both the SciFi and SiPM technology to the limits. Recently developed NOL fibres exhibit very short decay times just above 1 ns and may be an interesting option for future LHCb upgrades and fibre tracking detectors.
4. Production status The SciFi project is on track for the installation during the LHC long shutdown 2 in 2019/2020. To produce the detector elements including spares, 12,000 km of scintillating fibre were tested and wound into 1500 fibre mats. Accordingly, the module production is finished. All photodetectors have been received and the serial production and testing of cold-boxes is ongoing. Similarly, the serial production of front-end electronics is under way. A first prototype C-frame has been built and the assembly of the first serial C-frame has started in March 2019. All C-frames will be fully equipped and functionally tested above ground at LHC point 8 before being installed in the LHCb cavern. The first detector half is expected to be installed at the end of 2019. The installation is planned to be completed in spring 2020, followed by a commissioning period of about half a year and start of operation with first LHC beams from 2021 onwards.
Acknowledgments L. Gruber acknowledges the support of the Marie Skłodowska-Curie Fellowship Programme as part of the EU-funded project ‘‘Cofunding of the CERN Fellowship Programme (COFUND-FP-CERN-2014)’’. References [1] LHCb Collaboration, LHCb Tracker Upgrade Technical Design Report, LHCb TDR 15, CERN-LHCC-2014-001. [2] C. Joram, G. Haefeli, B. Leverington, Scintillating fibre tracking at high luminosity colliders, JINST 10 (2015) C08005, http://dx.doi.org/10.1088/1748-0221/10/08/ C08005. [3] O. Borshchev, et al., Development of a new class of scintillating fibres with very short decay time and high light yield, JINST 12 (2017) P05013, http: //dx.doi.org/10.1088/1748-0221/12/05/P05013. [4] A.B.R. Cavalcante, et al., Refining and testing 12,000 km of scintillating plastic fibre for the LHCb SciFi tracker, JINST 13 (2018) P10025, http://dx.doi.org/10. 1088/1748-0221/13/10/P10025. [5] A. Bachlehner, et al., Scanners for the quality control of scintillating plastic fibres, CERN-LHCb-PUB-2015-009. [6] A.B. Cavalcante, et al., Shrinking of bumps by drawing scintillating fibres through a hot conical tool, CERN-LHCb-PUB-2016-010. [7] Axel Kuonen, Development and Characterisation of Silicon Photomultiplier Multichannel Arrays for the Readout of a Large Scale Scintillating Fibre Tracker (Ph.D. Thesis), 2018, http://dx.doi.org/10.5075/epfl-thesis-8842. [8] Olivier Girard, Development of the Scintillating Fibre Tracker Technology for the LHCb Upgrade and the LHC Beam Profile Monitoring System (Ph.D. Thesis), 2018, http://dx.doi.org/10.5075/epfl-thesis-8851. [9] S.A. Ponomarenko, et al., Nanostructured organosilicon luminophores and their application in highly efficient plastic scintillators, Sci. Rep. 4 (2014) 6549, http: //dx.doi.org/10.1038/srep06549.
5. NOL fibres Scintillating fibres are usually based on a polystyrene polymer matrix containing two types of organic luminophores: an activator and a spectral shifter. Recently a new type of high performance luminophores called Nanostructured Organosilicon Luminophores (NOLs) was suggested [9]. These luminophores are based on covalent bonding of activator and wavelength shifter complexes through Si atoms, allowing for fast and efficient, non-radiative energy transfer by the Förster mechanism. The principle was adapted to fibres and it was shown that the NOL prototype fibres have already reached a competitive level [3], in particular the decay time constants are just above 1 ns, i.e. about 50% shorter than the fastest known fibres,2 which makes them particularly promising for time critical applications. The achieved attenuation lengths and scintillation light yields are still 10%–15% lower compared to the reference fibres from Kuraray, but it can be expected that further optimisation of the luminophore concentrations as well as of the production process will lead to improved performance. Irradiation tests with X-rays to a dose of 1 kGy indicate that the radiation damage
2
Kuraray SCSF-78 and SCSF-3HF fibres were used as reference. 4
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker Lukas Gruber, On behalf of the LHCb SciFi Tracker Collaboration CERN, EP Department, 1211 Geneva 23, Switzerland
ARTICLE Keywords: Particle tracking Scintillating fibre SciFi SiPM
INFO
ABSTRACT LHCb will undergo a major upgrade during the LHC long shutdown 2 in 2019/2020 to cope with increased instantaneous luminosities and to implement a trigger-less 40 MHz readout. The current inner and outer tracking detectors will be replaced by a single homogeneous detector based on plastic scintillating fibres (SciFi). The SciFi tracker covers an area of 340 m2 by using more than 10,000 km of scintillating fibre with 250 μm diameter, enabling a spatial resolution of better than 100 μm for charged particles. Linear arrays of Silicon Photomultipliers cooled to −40◦ C are placed at the fibre ends. The readout of 524 k channels occurs through custom-designed front-end electronics. The assembly of the detector has started and the project is on track for installation starting end 2019. In view of future upgrades, a R&D programme aiming at the development of very fast and efficient scintillating fibres, based on a novel type of luminophores (NOL), has been launched in parallel to the SciFi production. The performance of the prototype NOL fibres is competitive, in particular the decay time constant is close to 1 ns, i.e. about 50% shorter than the best standard fibre.
1. Introduction The LHCb experiment is preparing for a major upgrade in order to run from 2021 onwards at a five times higher luminosity of 2 × 1033 cm−2 s−1 and a readout rate of 40 MHz. The goal is to collect an integrated luminosity of 50 fb−1 in 10 years of operation. A large scintillating fibre (SciFi) tracker [1,2] is going to replace the current outer and inner trackers, consisting of straw tubes and silicon micro strip detectors, respectively, by using a single detector technology and more than 10,000 km of plastic scintillating fibres. The SciFi tracker requires a hit detection efficiency of 99%, a spatial resolution better than 100 μm in the horizontal bending plane and a low material budget (X/X0 ≤ 1% per detector layer). During operation the SiPMs will be exposed to a total neutron fluence of up to 6 × 1011 neq ∕cm2 and therefore have to be cooled to −40 ◦ C to reduce the dark count rate and retain single photon counting capability. The fibres are exposed to an ionising dose of 35 kGy close to the beam pipe. To reduce spill over into consecutive bunch crossings a fast signal generation and shaping is mandatory. The high demands on the fibre quality led to a R&D initiative aiming at the development of very fast and efficient scintillating fibres, which are based on a novel type of luminophores, called Nanostructured Organosilicon Luminophores (NOL) [3]. The article gives a brief introduction to the SciFi tracker design, an overview about the production process and performance of the various detector components as well as a summary about the current status of assembly and testing. Eventually, the most recent achievements of the NOL fibre development are outlined.
1
2. LHCb SciFi detector design and construction Overall detector layout. The 340 m2 large active area is divided into three tracking stations with four independent stereo layers each (XUVX geometry, U and V with ± 5◦ stereo angle) with a size of about 6 × 5 m2 (see Fig. 1). The chosen scintillating fibres have a circular cross section and a diameter of 250 μm and are arranged in a 6-layer staggered pattern with 275 μm pitch, to form 1.3 mm thick fibre mats (see Fig. 3, right). 8 mats are combined to a single module, the basic unit of the SciFi tracker. The 2.4 m long fibre mats are read out at one end by linear 128-channel SiPM arrays, while the other end of the mats is equipped with mirrors (3M reflective foil). The light yield is 16– 20 photoelectrons (p.e.) for particles hitting close to the mirror side. Six pairs of C-shaped frames (C-frames), arranged on either side of the beam pipe, will carry the 128 modules with photodetectors and front-end electronics as well as all services to and from them. Fibre quality assurance. Blue emitting plastic scintillating fibres of type SCSF-78MJ with 250 μm diameter produced by Kuraray1 were chosen for the LHCb SciFi tracker. The fibres are made from a polystyrene (PS) core (refractive index 𝑛 = 1.59), surrounded by a double cladding structure of polymethylmethacrylate (PMMA, 𝑛 = 1.49) and a fluorinated polymer (𝑛 = 1.42). The thickness of each cladding layer is 3% of the total fibre diameter. The core is doped with an activator and a wavelength shifter (WLS) dye. The SCSF-78MJ fibre has its maximum emission at a wavelength of 𝜆 = 440 nm and an integral attenuation length of about 3.5 m [4].
E-mail address: [email protected]. Kuraray Co. Ltd., Tokyo, Japan, http://kuraraypsf.jp/psf/.
https://doi.org/10.1016/j.nima.2019.03.080 Received 20 March 2019; Accepted 26 March 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Left: picture of the custom designed mat winding machine. Right: cross section of a fibre mat.
Fig. 1. SciFi tracking station, shown in a semi-open configuration.
alignment holes on the winding wheel are filled with glue and form precise pins on the fibre mat, which are later on used as alignment reference for the assembly of 8 fibre mats into one module with a size of about 5 × 0.5 m2 . The alignment with respect to a straight line was found to be better than 50 μm over the total length of 5 m. A structure of honeycomb and carbon fibre panels serve as support. The lightweight design allows to build detector modules with a material budget of only 1% 𝑋0 per module.
Over a period of about 2 years the full supply of 12,000 km of fibres (including spares) underwent a systematic and rigorous quality assurance (QA) programme, including geometrical refinement to deal with rare punctual imperfections. The measurements comprised attenuation length, ionisation light yield, diameter, cladding integrity and radiation tolerance to X-rays. A detailed description of the technical and experimental methods as well as results of the fibre QA can be found in Ref. [4]. In general, the supply was found to be timely and of very high quality and stability, which led to negligible rejection rates. The verification of the fibre diameter and its uniformity is an integral part of the QA. Diameter defects (‘‘bumps’’ or ‘‘necks’’) cause deficits in the light transport (light leaks) of a fibre and potentially reduce its mechanical stability and, in the SciFi tracker, lead to distortions in the 6-layer pattern of the fibre mats, affecting hit efficiency and position resolution. Despite a clean work environment and other process optimisations, the occurrence of such defects could never be fully avoided during fibre production. Empirically, bumps with a diameter larger than 350 μm tend to cause irregular winding patterns and complicate or delay the fibre mat winding. A dedicated fibre scanning machine [5] was built to monitor the fibre diameter and cladding integrity (see Fig. 2). The scanner automatically detected bumps and necks and precisely recorded their shape. Bumps exceeding a diameter of 350 μm were shrunk by drawing the fibre through a hot conical tool [6].
Silicon photomultipliers. The scintillating fibres are readout by customised 128-channel linear SiPM arrays (Hamamatsu MPPC 13552– H2017) with a single channel size of 0.25 × 1.62 mm2 . Each channel consists of 104 individual pixels (57.5 × 62.5 μm2 ), as illustrated in Fig. 4. In total, 4096 SiPM arrays are going to be installed in the SciFi tracker, resulting in about 524k channels. The photodetectors are optimised for high photon detection efficiency (45% peak PDE at 𝛥𝑉 = 3.5 V), low after-pulse and cross-talk and have a thin entrance window (105 μm epoxy). The H2017 features a quenching resistor (𝑅𝑄 ) of 520 kΩ, which allows a large operating range. After R&D [7,8], the total correlated noise probability could be reduced to 7% at the operating point 𝛥𝑉 = 3.5 V. Direct and delayed cross-talk are the dominant sources of correlated noise, while the after-pulse probability is below 0.1%. The recovery time is 85 ns with typically 10% channel-to-channel variations due to dependence on 𝑅𝑄 . At the SiPM location in LHCb SciFi, the main damage is originating from neutron irradiation, which manifests itself in a massive increase of the dark count rate (DCR). As single photon sensitivity must be maintained, the DCR can only be managed by reducing the operational temperature, which reduces the DCR by a factor 2 every 10 K, as can be seen in Fig. 5. At the end of the detector lifetime (6 × 1011 neq ∕cm2 ), a DCR of 14 MHz per channel at −40 ◦ C is expected at the operating point of 𝛥𝑉 = 3.5 V. As an additional challenge, the amount of light seen by the SiPMs will be reduced by about 40% to 10–12 p.e. per hit
Mat and module production. After QA, the fibres are wound into 6-layer mats of 2.4 m length and 13 cm width by using a custom designed winding machine. A special winding wheel with a threaded surface is used to define precisely the position of the first fibre layer (see Fig. 3). Before each layer, an epoxy glue mixed with TiO2 powder is applied to stabilise the mat and reduce optical cross-talk between fibres. Precision
Fig. 2. Picture of the fibre scanning machine at CERN.
2
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 4. 128-channel SiPM array bonded to a Kapton flex-cable and close-up of a single channel and pixel.
Fig. 6. Drawing of a cold-box with cooling bellows connected.
Fig. 7. A picture of the SciFi front-end electronics. Fig. 5. Measured DCR of a single SiPM channel after a neutron fluence of 6 × 1011 neq ∕cm2 for different operating voltages.
due to radiation damage of the fibres, primarily close to the beam pipe (35 kGy at the end of the detector lifetime), corresponding also to the highest track density region. SiPM cooling. The SiPM cooling system of the SciFi tracker is a major engineering challenge. For the full detector the total expected heat load is about 10 kW and the required mass flow amounts to about 5 kg/s, i.e. 18,000 kg/h. The transfer lines from the cooling plant to the detector will be about 100 m long and except for the SiPMs, all detector parts are located and operated in ambient temperature. To thermally insulate the cold photodetector arrays from the surrounding, a gas- and light-tight cold-box is installed around them. Inside the cold-box, the SiPMs are glued on 3D printed Titanium cold-bars through which the cooling liquid (monophase 3M Novec 649 at −50 ◦ C) is circulating. The SiPMs are then connected to the front-end electronics via special Kapton flex-PCBs. On a length of about 20 m, the near detector cooling lines are vacuum insulated. To avoid ice building up inside the coldbox, the boxes are flushed with dry air with a dew point of −70 ◦ C. A schematic of a cold-box is shown in Fig. 6.
Fig. 8. Signal processing chain in the SciFi front-end electronics.
purposes. The SiPM arrays are connected to the PACIFIC boards, where the PACIFIC ASICs are placed and signal digitisation is performed. The custom designed ASIC features 64 channels (current mode input) and is based on 130 nm CMOS TSMC technology. The power consumption is about 10 mW per channel. Each channel consists of a preamplifier, a fast shaper (10 ns shaping time), a double gated integrator (25 ns integration time) to avoid dead time, a track and hold stage and three comparators to apply signal thresholds for digitisation, as illustrated in Fig. 8. The three thresholds allow to identify signal clusters and accurately determine their location while efficiently suppressing noise. The 3-bit outputs are encoded into 2-bit values and serialised (combining several channels), before being sent to the FPGAs located on the cluster boards, where individual channels are combined into clusters. The expected cluster rate at 40 MHz readout is less than 3 MHz per SiPM array. The master boards provide power, fast and slow control, and comprise the optical links for data transmission.
Front-end electronics. The 40 MHz trigger-less readout demands zerosuppression of data at the stage of the front-end electronics for all sub-detectors. In the special case of the SciFi tracker, the increase in DCR of SiPMs and noise cluster rate due to radiation damage requires fast readout, i.e. short integration time, and efficient rejection of noise from SiPM dark counts by clustering. The SciFi front-end electronics consist of a chain of different electronics boards, PACIFIC boards, cluster boards and master boards, as shown in Fig. 7. A light injection system is used for calibration 3
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.
L. Gruber and L.S.T. Collaboration
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3. Detector module performance
of the NOL fibres are on a level comparable to the respective reference fibres, however irradiation tests with hadrons and to higher doses still have to be performed. The new fibres may be also interesting for a future upgrade or replacement of the most irradiated regions of the LHCb SciFi tracker and in general for future fibre detectors.
Two fibre modules equipped with production front-end electronics were tested in a 180 GeV/c mixed proton and pion beam at CERN SPS in July 2018. An external beam telescope was used as a reference for defining reconstructed particle tracks with a resolution much better than the fibre modules. A single hit resolution of 70-80 μm was achieved and the single hit efficiency was found to be 99.5%. The spillover was determined to be about 2%. All obtained results are in agreement with the design requirements and expectations.
6. Summary and conclusion The LHCb SciFi tracker is going to replace the current downstream tracking system and to start its operation in 2021. An area of 340 m2 is covered by using more than 10,000 km of scintillating fibres with 250 μm diameter, providing a spatial resolution of better than 100 μm. SiPM arrays cooled to −40 ◦ C are attached to the fibres and readout through custom designed front-end electronics at 40 MHz. The radiation environment (35 kGy for the fibres, 6 × 1011 neq ∕cm2 for the SiPMs) push both the SciFi and SiPM technology to the limits. Recently developed NOL fibres exhibit very short decay times just above 1 ns and may be an interesting option for future LHCb upgrades and fibre tracking detectors.
4. Production status The SciFi project is on track for the installation during the LHC long shutdown 2 in 2019/2020. To produce the detector elements including spares, 12,000 km of scintillating fibre were tested and wound into 1500 fibre mats. Accordingly, the module production is finished. All photodetectors have been received and the serial production and testing of cold-boxes is ongoing. Similarly, the serial production of front-end electronics is under way. A first prototype C-frame has been built and the assembly of the first serial C-frame has started in March 2019. All C-frames will be fully equipped and functionally tested above ground at LHC point 8 before being installed in the LHCb cavern. The first detector half is expected to be installed at the end of 2019. The installation is planned to be completed in spring 2020, followed by a commissioning period of about half a year and start of operation with first LHC beams from 2021 onwards.
Acknowledgments L. Gruber acknowledges the support of the Marie Skłodowska-Curie Fellowship Programme as part of the EU-funded project ‘‘Cofunding of the CERN Fellowship Programme (COFUND-FP-CERN-2014)’’. References [1] LHCb Collaboration, LHCb Tracker Upgrade Technical Design Report, LHCb TDR 15, CERN-LHCC-2014-001. [2] C. Joram, G. Haefeli, B. Leverington, Scintillating fibre tracking at high luminosity colliders, JINST 10 (2015) C08005, http://dx.doi.org/10.1088/1748-0221/10/08/ C08005. [3] O. Borshchev, et al., Development of a new class of scintillating fibres with very short decay time and high light yield, JINST 12 (2017) P05013, http: //dx.doi.org/10.1088/1748-0221/12/05/P05013. [4] A.B.R. Cavalcante, et al., Refining and testing 12,000 km of scintillating plastic fibre for the LHCb SciFi tracker, JINST 13 (2018) P10025, http://dx.doi.org/10. 1088/1748-0221/13/10/P10025. [5] A. Bachlehner, et al., Scanners for the quality control of scintillating plastic fibres, CERN-LHCb-PUB-2015-009. [6] A.B. Cavalcante, et al., Shrinking of bumps by drawing scintillating fibres through a hot conical tool, CERN-LHCb-PUB-2016-010. [7] Axel Kuonen, Development and Characterisation of Silicon Photomultiplier Multichannel Arrays for the Readout of a Large Scale Scintillating Fibre Tracker (Ph.D. Thesis), 2018, http://dx.doi.org/10.5075/epfl-thesis-8842. [8] Olivier Girard, Development of the Scintillating Fibre Tracker Technology for the LHCb Upgrade and the LHC Beam Profile Monitoring System (Ph.D. Thesis), 2018, http://dx.doi.org/10.5075/epfl-thesis-8851. [9] S.A. Ponomarenko, et al., Nanostructured organosilicon luminophores and their application in highly efficient plastic scintillators, Sci. Rep. 4 (2014) 6549, http: //dx.doi.org/10.1038/srep06549.
5. NOL fibres Scintillating fibres are usually based on a polystyrene polymer matrix containing two types of organic luminophores: an activator and a spectral shifter. Recently a new type of high performance luminophores called Nanostructured Organosilicon Luminophores (NOLs) was suggested [9]. These luminophores are based on covalent bonding of activator and wavelength shifter complexes through Si atoms, allowing for fast and efficient, non-radiative energy transfer by the Förster mechanism. The principle was adapted to fibres and it was shown that the NOL prototype fibres have already reached a competitive level [3], in particular the decay time constants are just above 1 ns, i.e. about 50% shorter than the fastest known fibres,2 which makes them particularly promising for time critical applications. The achieved attenuation lengths and scintillation light yields are still 10%–15% lower compared to the reference fibres from Kuraray, but it can be expected that further optimisation of the luminophore concentrations as well as of the production process will lead to improved performance. Irradiation tests with X-rays to a dose of 1 kGy indicate that the radiation damage
2
Kuraray SCSF-78 and SCSF-3HF fibres were used as reference. 4
Please cite this article as: L. Gruber, LHCb SciFi — Upgrading LHCb with a scintillating fibre tracker, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.03.080.