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The LHCb Silicon Tracker
Nuclear Instruments and Methods in Physics Research A 831 (2016) 174–180
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The LHCb Silicon Tracker Mark Tobin, On behalf of the LHCb Silicon Tracker group1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 November 2015 Received in revised form 25 May 2016 Accepted 27 May 2016 Available online 10 June 2016
The LHCb experiment is dedicated to the study of heavy flavour physics at the Large Hadron Collider (LHC). The primary goal of the experiment is to search for indirect evidence of new physics via measurements of CP violation and rare decays of beauty and charm hadrons. The LHCb detector has a largearea silicon micro-strip detector located upstream of a dipole magnet, and three tracking stations with silicon micro-strip detectors in the innermost region downstream of the magnet. These two sub-detectors form the LHCb Silicon Tracker (ST). This paper gives an overview of the performance and operation of the ST during LHC Run 1. Measurements of the observed radiation damage are shown and compared to the expectation from simulation. & 2016 Published by Elsevier B.V.
Keywords: Tracking detectors Silicon micro-strips Radiation damage
1. Introduction The LHCb detector [2] is a single-arm forward spectrometer designed to study the heavy flavour sector using proton–proton collisions at the LHC. The experiment makes measurements of CPviolation using decays of b- and c-hadrons, as well as studying rare decays of these hadrons, to search indirectly for New Physics. The detector covers a large pseudorapidity range, 2 < η < 5, in order to exploit the fact that b- and c-hadrons are typically produced in the forward (or backward) direction close the LHC beampipe. The detector consists of: a high precision tracking system with a silicon micro-strip detector around the proton–proton interaction region; a large area silicon micro-strip detector (TT) before a cold dipole magnet; and three tracking stations downstream of the magnet (T1, T2 and T3) with silicon micro-strips in the region closest to the beam-pipe and straw tubes outside. In addition, there are two ring-imaging Cherenkov detectors, a calorimeter system and muon detectors that are used to distinguish various types of particle. Events are selected using a trigger that consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. The layout of the LHCb detector is shown in Fig. 1 and the performance of the detector during LHC Run 1 is described in Ref. [3]. The experiment recorded an integrated luminosity of around 3 fb 1 during Run 1. The majority of the data was taken during 2011 and 2012 when the centre-of-mass energy was 7 TeV and 8 TeV, respectively. The LHC provided collisions with instantaneous luminosities up to E-mail address: Mark.Tobin@epfl.ch M. Tobin et al., CERN-LHCb-PROC-2016-011, 2016 [1].
1
http://dx.doi.org/10.1016/j.nima.2016.05.115 0168-9002/& 2016 Published by Elsevier B.V.
4 × 1032 cm−2 s−1 and a bunch spacing of 50 ns. The resulting average number of visible interactions per bunch crossing, μvis, was approximately 1.7.
2. Silicon Tracker The Silicon Tracker is composed of two sub-detectors: the Tracker Turicensis (TT) and the Inner Tracker (IT). The TT covers the full acceptance before the magnet while the IT covers a crossshaped region in the centre of the three tracking stations downstream of the magnet. Both detectors use silicon micro-strip detectors with p þ -on-n sensors from Hamamatsu.2 The total active area of the TT and IT is 8 m2 and 4.2 m2, respectively. The TT consists of four detection planes arranged in an (x − u − v − x ) geometry where the strips in each plane are orientated at (0°, þ5°, 5°, 0°) with respect to the vertical direction.3 Each sensor is 500 μm thick and has 512 strips with a pitch of 183 μm. The TT modules have read-out hybrids located outside of the acceptance at the top and bottom of the station. Each read-out sector has one, two, three or four sensors that are bonded together. The sectors with a single sensor are located in the central region close to the beam-pipe and are connected to the read-out hybrid via a flex cable. The two- and three-sensor sectors are placed outside in regions of decreasing occupancy. They are also connected to the read-out hybrid via flex cables. The four2 Hamamatsu Photonics K.K., 325–6, Sunayama-cho, Naka-ku, Hamamatsu City, Shizuoka Pref., 430–8587, Japan. 3 LHCb follows a right-handed co-ordinate system with z along the beam-axis, y vertical and x horizontal.
M. Tobin / Nuclear Instruments and Methods in Physics Research A 831 (2016) 174–180
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Fig. 1. The LHCb detector [2].
Fig. 3. Schematic of one Inner Tracker station [2].
boards use FPGAs to perform pedestal subtraction, common mode noise subtraction and zero suppression of the data. Both detectors are cooled using C6F14 and the cooling plant operates at 0°. The temperature of the sensors is around 8° when the electronics are powered. Fig. 2. Schematic of the third detection layer in TT [2]. The different read-out sectors are indicated by the different shadings.
sensor sectors are located furthest away from the beam-line and are connected directly to the hybrid. The read-out strips vary in length up to 37 cm. There are 280 read-out sectors and 143360 read-out channels. A picture of one layer in the TT is shown in Fig. 2. The Inner Tracker consists of three stations each containing four boxes arranged in a 120 cm by 40 cm cross-shaped region around the beam-pipe. The modules in the boxes either side of the beam-pipe have two sensors bonded together while those above and below have a single sensor. This is shown schematically for one station in Fig. 3. The single sensor modules are 320 μm thick while the two sensor sectors use sensors with a thickness of 410 μm. Both sensor types have 384 strips with a strip pitch of 198 μm. There are four detection layers in each box with seven modules in each layer, and the strips in each layer are orientated at (0°, + 5°, − 5°, 0°) with respect to the vertical direction. In total, there are 336 read-out sectors and 129024 read-out channels. The read-out of both detectors uses the Beetle ASIC [4] to amplify the analogue signals from the strips. These signals are then transmitted via copper cables to digitiser boards contained in service boxes that are located outside the detector acceptance. The signals are digitised and VCSEL diodes are used to transfer the data to the TELL1 [5] read-out boards in the counting house. The TELL1
3. Performance in run 1 3.1. Detector operation The detector operated with a high data taking efficiency during Run 1. The fraction of working channels varied as a function of time and the luminosity-weighted average of the fraction of working channels was 99.7% and 98.6% for the TT and IT, respectively. These numbers differ slightly as repairs can be made to the TT read-out during short technical stops whereas problems with the IT read-out can only be fixed during the LHC shutdowns at the end of each year. Three full read-out sectors were disabled in the IT during 2011 due to problems with the configuration of the readout while two were disabled in 2012. One other major cause of inefficiency was the failure of the VCSEL diodes that are used to transmit optical data. In total, 47 diodes had to be replaced during Run 1. The total number of failures is shown as a function of time in Fig. 4. No correlation with the delivered luminosity was observed and no environmental change was found that could be used to explain the failure of the diodes. The cause of death for these VCSEL diodes is still unknown. 3.2. Signal-to-noise ratio The signal-to-noise ratio (S/N) was measured using the clusters
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Fig. 4. Cumulative number of dead VCSELs as a function of time. The delivered luminosity is also shown.
on tracks that were reconstructed with a momentum greater than 5 GeV/c. It is defined as the ratio between the Most Probable Value of the cluster charge distribution and the rms-noise of the central strip of the cluster. The S/N measured in the TT is shown in Fig. 5 as a function of the four different strip capacitances and it was found to be in the range 12–15. The measured S/N is shown for all IT sectors in Fig. 6. The two- and one-sensor modules in IT have a S/N of 16.5 and 17.5, respectively. A third group of S/N values with a peak around 23 can be seen because some single-sensor modules were constructed using 410 μm thick sensors by mistake. The mean size of a cluster is around 1.9 in TT and 1.6 in IT, and the cluster size increases with the angle of the track. 3.3. Spatial alignment and hit resolution The spatial alignment of the detector was performed using a global χ2 minimisation based on Kalman track residuals [6] with additional requirements on the decay vertices and invariant mass of the particles used [7]. The hit resolution is calculated from the distance between the measured hit position and the extrapolated track position after the measured hit has been removed from the track fit. The resolution is given by the spread of this distribution after correcting for the uncertainty in the track parameters. The hit resolution measured using the 2011 data is 52.6 μm and 50.3 μm for the TT and IT, respectively. The resolution measured in the IT is shown as a function of the sector number in Fig. 7 where the sector number corresponds roughly to the x-direction. The
Fig. 6. Signal-to-noise ratio measured for all read-out sectors in IT. Long and short ladders correspond to one- and two-sensor modules, respectively.
resolution is slightly worse in the central regions closest to the beam-pipe where the track angles are smallest and, consequently, where there is the least amount of charge sharing between strips. The strip pitch is 183 μm in TT and 198 μm in IT, and the measured resolution is slightly better than the resolution that would be obtained from a binary system. 3.4. Hit efficiency The hit efficiency of the sensors was measured. Reconstructed tracks are used to probe whether or not the hits that are expected to be used to construct the track were found. Particle candidates from J/ψ → μ+ μ− decays were used to ensure that a track sample with a high purity was used. The tracks were required to have momentum greater than 10 GeV/c to reduce the effect of multiple scattering and additional cuts were placed on the track quality to minimise the effect of fake tracks on the efficiency measurement. A search is made for hits in a window around the intersection point between the track and each sensor where a hit was expected. The efficiency is defined as the ratio between the number of hits found and the number of hits expected for a given sector. The overall hit efficiency is determined to be greater than 99.7% and 99.8% for TT and IT, respectively. The hit efficiency in each IT sector measured using 2012 data is shown in Fig. 8. There are two modules which are known to be affected by large common mode noise and these sectors have a much lower efficiency than the rest of the detector.
4. Long shutdown 1
Fig. 5. Signal-to-noise ratio measured in TT as a function of the strip capacitance.
The temperature of the detectors was kept at 0° during the LHC Long Shutdown 1 (LS1) which ran from 2013 until 2015. They were only warmed up for maintenance of the cooling system. Regular operation periods were scheduled to monitor the status of the detectors. Several improvements were made to the system to improve its reliability during Run 2. The C6F14 cooling system regularly lost cooling power and required daily interventions by the end of Run 1. A new cooling system was installed to improve the reliability of the system and has been running without problems since April 2014. The old cooling plant remains as a back-up. The control software has been re-written resulting in a 43% reduction in the configuration time of the electronics. The full system was migrated from PVSS to WinCC-OA after Siemens
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Fig. 7. Hit resolution measured using 2011 data as a function of the IT sector number. The sector number corresponds roughly to the x-coordinate. The labels X1, U, V and X2 denote the four detection planes in each detector box. The resolution in the stereo layers (U and V) is worse than that measured in the x-layers ( X1 and X2). This effect was not seen in simulation so is most likely caused by a small remaining misalignment in y. The x-layers are not sensitive to misalignments in y so their measured resolution is better than that in the stereo layers.
Fig. 8. Hit efficiency measured using 2011 data as a function of the IT sector number. The sector number corresponds roughly to the x-coordinate. The labels X1, U, V and X2 denote the four detection planes in each detector box. Two modules have a lower efficiency than the rest of the detector.
acquired the company that supplied the PVSS software.4 Both IT and TT were opened at the start of LS1 while a new beam-pipe was installed. The previous beam-pipe was not at its nominal design position during Run 1 so the box positions in the IT have been re-adjusted to improve the detector acceptance. A new system based on the ATLAS BCAM5 was installed to provide real-time monitoring of the movement of the IT stations. A BCAM consists of laser diodes that are used as the light source and a CCD (charge coupled device) camera that can reconstruct the position of reflective targets that were installed on each IT station. Two BCAMs are required per station and the BCAM analyses the 4 PVSS and WinCC-OA are both SCADA (supervisory control and data acquisition) systems. 5 Brandeis (or Boston) CCD Angle Monitor [8].
relative position of the light spot that is reflected onto the CCD. The intersection of the camera lines of sight allows an absolute measurement of the position of the IT station with a precision of the order of tenths of millimetres. The position of the stations is continuously monitored and movements of up to 1 cm have been observed during changes in the polarity of the dipole magnet.
5. Radiation damage studies The forward geometry of the LHCb detector means that the flux of particles through the various sub-detectors is non-uniform. The fluence varies by up to three orders of magnitude between the inner and outer regions of the TT. The highest 1-MeV-neutron equivalent fluence, neq , expected is estimated from a FLUKA
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The expected change in the leakage current, ΔI , can be calculated using ΔI = αϕV where V is the volume of the sensor, ϕ is the fluence and α is the current related damage rate which depends on the temperature and the annealing [12]. The predicted leakage current is also shown in Fig. 9 and is found to agree well with the measured values. 5.2. Depletion voltage
Fig. 9. Leakage current measured as a function of the delivered luminosity for the two-sensor sectors in the TT stereo layers (U and V).
simulation [9,10] of the LHCb detector to be 4·1012 cm−2 in TT and 2.5·1012 cm−2 in IT per fb 1 of delivered luminosity at a centre-ofmass energy of 8 TeV. A total luminosity of 3.5 fb 1 was delivered during Run 1 where the majority of data was collected at a centreof-mass energy of either 7 or 8 TeV. An additional 5 fb 1 of proton–proton collisions is expected to be delivered during Run 2 at a centre-of-mass energy of 13 TeV (or higher). Both TT and IT will be replaced by new detectors at the end of Run 2 [11]. Radiation damage in the silicon leads in changes in the leakage current and the depletion voltage of the sensors. The methods used to monitor these two quantities are described in the following sub-sections. 5.1. Leakage current The change in the leakage current is directly proportional to the delivered luminosity and, therefore, provides an excellent way to measure the radiation damage. The current in the sensors is monitored continuously and the maximum current measured during each data taking period (or fill) is stored. These values are then plotted as function of the total delivered luminosity as shown in Fig. 9 for the TT. The measured current is normalised to the volume and temperature of the silicon sensors. The operating temperature is 8° in both sub-detectors.
The depletion voltage of the sensors was measured directly using capacitance–voltage (C − V ) scans in the laboratory. The sensors used in the construction of the detector had initial depletion voltages in the range (160–280)V for TT and (60–140) V for IT. It is impossible to measure the depletion voltage using this method once the modules have been constructed and installed in the detector. Therefore, regular charge collection efficiency (CCE) scans are made using collision data. The CCE scans consist of three parts: firstly, the voltage applied to the sensors is changed; secondly, the signal sampling time is scanned for each voltage step to account for changes in the charge collection time when operating at lower voltages; and finally, the depletion voltage is extracted from the distribution of the collected charge as a function of the applied voltage. The scan is performed in different layers of the detector. Reconstructed tracks are extrapolated to the layer under study and the charge in the strips around extrapolated track position is summed up to give the total charge. The resulting signal charge distribution is shown in Fig. 10 for two voltage steps in the TT scan. The most probable value is extracted from a fit of the sum of two Landau distributions (signal hits) convolved with the sum of two Gaussian distributions (noise hits) to the data. This is repeated for all of the timing steps at each voltage. The resulting distribution of the charge as a function of the sampling time is shown in Fig. 11 for the same two voltage steps in the TT scan. The charge collected at each voltage step is extracted from fits of a half-Gaussian that describes the expected pulse-shape to the data. The maximum value of this function gives the charge collected at each voltage. The charge collected in one sensor is shown as a function of the applied voltage in Fig. 12 for scans taken in July 2011 and January 2013. The delivered luminosity at the time of each scan was 0.48 fb 1 and 3.47 fb 1, respectively. The depletion voltage is taken to be the value at (95 ± 2)% of the plateau charge. This point was chosen such that the depletion voltage extracted from the earliest CCE scan matched that measured using the C − V scans during the module production. The difference between the depletion voltage extracted in the CCE scans compared to that
Fig. 10. Charge distribution for two voltages in the TT scan. The most probable value is extracted from the fit of the sum of two Landau distributions (signal hits, red dashed curve) convolved with a double Gaussian distribution (noise, blue curve) to the data. The second Landau distribution (red dotted curve) is used to describre photon conversions in the material before the ST and has an MPV that is fixed to twice the value of first Landau distribution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. Pulse-shape from the timing scan for two different voltages in the TT scan. The black points are the measured MPVs for each timing setting and the red curve is the functional form of the pulse-shape that was fitted to the data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Charge collected as a function of the bias voltage, Vbias, for a TT sector in July 2011 (a) and January 2013 (b). The grey band is the uncertainty on the measured value of the depletion voltage. The nominal operating voltage during Run 1 was Vbias = 300 V .
function of the 1-MeV neutron-equivalent fluence where the fluence was obtained using FLUKA simulations and the actual running conditions. The predicted depletion voltage was calculated using the model described in Ref. [13] and is also shown in Fig. 13. The expected depletion voltage agrees well with the measured values, and the expected evolution of the depletion voltage shows that there will be no type inversion in the silicon before the end of Run 2 even for the innermost regions of the TT.
6. Conclusions
Fig. 13. Depletion voltage measured in the TT as a function of the 1-MeV neutron equivalent fluence. The red circles and blue triangles show the measurements for read-out sectors where the initial depletion voltage of the sensors, measured using C − V scans during the module production, was in the range (160–220) V and (220– 280) V, respectively. The predicted depletion voltage was calculated using the Hamburg model [12] and is shown as a solid black line. The dashed lines indicate the uncertainty on the predicted values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
measured in the C − V scans can be related to a ballistic deficit from the shaping of the signal in the front-end electronics. The measured depletion voltage is shown in Fig. 13 as a
The LHCb Silicon Tracker performed extremely well during Run 1 of the LHC. A number of improvements were made to the system during LS1, including the installation of a new cooling plant for both sub-detectors, and an alignment system to monitor the positions of the IT stations. The radiation damage to the sensors is monitored using measurements of the leakage current and depletion voltage, and is found to be in good agreement with the expection from simulation. The radiation damage is not expected to result in problems for the operation of the detectors up until the end of Run 2 when both detectors will be replaced by a new tracking system.
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References [1] [2] [3] [4] [5] [6]
M. Tobin et al., CERN-LHCb-PROC-2016-011, 2016. A.A. Alves Jr., et al., JINST: J. Instrum. 3 (2008) S08005. R. Aaij, et al., Int. J. Mod. Phys. A30 (2015) 1530022. N. van Bakel et al., CERN/LHCC 2001–046, 2001. G. Haefeli, et al., Nucl. Instrum. Methods A560 (2006) 494. W. Hulsbergen, Nucl. Instrum. Methods A600 (2009) 471–477.
[7] [8] [9] [10]
J. Amoraal, et al., Nucl. Instrum. Methods A712 (2013) 48–55. K. Hashemi et al., CERN-ATL-MUON-2000-024, 2000. T.T. Bohlen, et al., Nucl. Data Sheets 120 (2014) 211–214. A. Ferrari et al., FLUKA: a multi-particle transport code, CERN-2005-10, INFN/ TC_05/11, SLAC-R-773, 2005. [11] LHCb Tracker Upgrade Technical Design Report, CERN-LHCC-2014-001, 2014. [12] M. Moll, et al., Nucl. Instrum. Methods A426 (1999) 87. [13] M. Moll, Radiation Damage in Silicon Particle Detectors, DESY-THESIS-1999-040.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The LHCb Silicon Tracker Mark Tobin, On behalf of the LHCb Silicon Tracker group1 Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 November 2015 Received in revised form 25 May 2016 Accepted 27 May 2016 Available online 10 June 2016
The LHCb experiment is dedicated to the study of heavy flavour physics at the Large Hadron Collider (LHC). The primary goal of the experiment is to search for indirect evidence of new physics via measurements of CP violation and rare decays of beauty and charm hadrons. The LHCb detector has a largearea silicon micro-strip detector located upstream of a dipole magnet, and three tracking stations with silicon micro-strip detectors in the innermost region downstream of the magnet. These two sub-detectors form the LHCb Silicon Tracker (ST). This paper gives an overview of the performance and operation of the ST during LHC Run 1. Measurements of the observed radiation damage are shown and compared to the expectation from simulation. & 2016 Published by Elsevier B.V.
Keywords: Tracking detectors Silicon micro-strips Radiation damage
1. Introduction The LHCb detector [2] is a single-arm forward spectrometer designed to study the heavy flavour sector using proton–proton collisions at the LHC. The experiment makes measurements of CPviolation using decays of b- and c-hadrons, as well as studying rare decays of these hadrons, to search indirectly for New Physics. The detector covers a large pseudorapidity range, 2 < η < 5, in order to exploit the fact that b- and c-hadrons are typically produced in the forward (or backward) direction close the LHC beampipe. The detector consists of: a high precision tracking system with a silicon micro-strip detector around the proton–proton interaction region; a large area silicon micro-strip detector (TT) before a cold dipole magnet; and three tracking stations downstream of the magnet (T1, T2 and T3) with silicon micro-strips in the region closest to the beam-pipe and straw tubes outside. In addition, there are two ring-imaging Cherenkov detectors, a calorimeter system and muon detectors that are used to distinguish various types of particle. Events are selected using a trigger that consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. The layout of the LHCb detector is shown in Fig. 1 and the performance of the detector during LHC Run 1 is described in Ref. [3]. The experiment recorded an integrated luminosity of around 3 fb 1 during Run 1. The majority of the data was taken during 2011 and 2012 when the centre-of-mass energy was 7 TeV and 8 TeV, respectively. The LHC provided collisions with instantaneous luminosities up to E-mail address: Mark.Tobin@epfl.ch M. Tobin et al., CERN-LHCb-PROC-2016-011, 2016 [1].
1
http://dx.doi.org/10.1016/j.nima.2016.05.115 0168-9002/& 2016 Published by Elsevier B.V.
4 × 1032 cm−2 s−1 and a bunch spacing of 50 ns. The resulting average number of visible interactions per bunch crossing, μvis, was approximately 1.7.
2. Silicon Tracker The Silicon Tracker is composed of two sub-detectors: the Tracker Turicensis (TT) and the Inner Tracker (IT). The TT covers the full acceptance before the magnet while the IT covers a crossshaped region in the centre of the three tracking stations downstream of the magnet. Both detectors use silicon micro-strip detectors with p þ -on-n sensors from Hamamatsu.2 The total active area of the TT and IT is 8 m2 and 4.2 m2, respectively. The TT consists of four detection planes arranged in an (x − u − v − x ) geometry where the strips in each plane are orientated at (0°, þ5°, 5°, 0°) with respect to the vertical direction.3 Each sensor is 500 μm thick and has 512 strips with a pitch of 183 μm. The TT modules have read-out hybrids located outside of the acceptance at the top and bottom of the station. Each read-out sector has one, two, three or four sensors that are bonded together. The sectors with a single sensor are located in the central region close to the beam-pipe and are connected to the read-out hybrid via a flex cable. The two- and three-sensor sectors are placed outside in regions of decreasing occupancy. They are also connected to the read-out hybrid via flex cables. The four2 Hamamatsu Photonics K.K., 325–6, Sunayama-cho, Naka-ku, Hamamatsu City, Shizuoka Pref., 430–8587, Japan. 3 LHCb follows a right-handed co-ordinate system with z along the beam-axis, y vertical and x horizontal.
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Fig. 1. The LHCb detector [2].
Fig. 3. Schematic of one Inner Tracker station [2].
boards use FPGAs to perform pedestal subtraction, common mode noise subtraction and zero suppression of the data. Both detectors are cooled using C6F14 and the cooling plant operates at 0°. The temperature of the sensors is around 8° when the electronics are powered. Fig. 2. Schematic of the third detection layer in TT [2]. The different read-out sectors are indicated by the different shadings.
sensor sectors are located furthest away from the beam-line and are connected directly to the hybrid. The read-out strips vary in length up to 37 cm. There are 280 read-out sectors and 143360 read-out channels. A picture of one layer in the TT is shown in Fig. 2. The Inner Tracker consists of three stations each containing four boxes arranged in a 120 cm by 40 cm cross-shaped region around the beam-pipe. The modules in the boxes either side of the beam-pipe have two sensors bonded together while those above and below have a single sensor. This is shown schematically for one station in Fig. 3. The single sensor modules are 320 μm thick while the two sensor sectors use sensors with a thickness of 410 μm. Both sensor types have 384 strips with a strip pitch of 198 μm. There are four detection layers in each box with seven modules in each layer, and the strips in each layer are orientated at (0°, + 5°, − 5°, 0°) with respect to the vertical direction. In total, there are 336 read-out sectors and 129024 read-out channels. The read-out of both detectors uses the Beetle ASIC [4] to amplify the analogue signals from the strips. These signals are then transmitted via copper cables to digitiser boards contained in service boxes that are located outside the detector acceptance. The signals are digitised and VCSEL diodes are used to transfer the data to the TELL1 [5] read-out boards in the counting house. The TELL1
3. Performance in run 1 3.1. Detector operation The detector operated with a high data taking efficiency during Run 1. The fraction of working channels varied as a function of time and the luminosity-weighted average of the fraction of working channels was 99.7% and 98.6% for the TT and IT, respectively. These numbers differ slightly as repairs can be made to the TT read-out during short technical stops whereas problems with the IT read-out can only be fixed during the LHC shutdowns at the end of each year. Three full read-out sectors were disabled in the IT during 2011 due to problems with the configuration of the readout while two were disabled in 2012. One other major cause of inefficiency was the failure of the VCSEL diodes that are used to transmit optical data. In total, 47 diodes had to be replaced during Run 1. The total number of failures is shown as a function of time in Fig. 4. No correlation with the delivered luminosity was observed and no environmental change was found that could be used to explain the failure of the diodes. The cause of death for these VCSEL diodes is still unknown. 3.2. Signal-to-noise ratio The signal-to-noise ratio (S/N) was measured using the clusters
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31/12 2009
02/07 2010
31/12 2010
02/07 2011
01/01 2012
01/07 2012
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0
Fig. 4. Cumulative number of dead VCSELs as a function of time. The delivered luminosity is also shown.
on tracks that were reconstructed with a momentum greater than 5 GeV/c. It is defined as the ratio between the Most Probable Value of the cluster charge distribution and the rms-noise of the central strip of the cluster. The S/N measured in the TT is shown in Fig. 5 as a function of the four different strip capacitances and it was found to be in the range 12–15. The measured S/N is shown for all IT sectors in Fig. 6. The two- and one-sensor modules in IT have a S/N of 16.5 and 17.5, respectively. A third group of S/N values with a peak around 23 can be seen because some single-sensor modules were constructed using 410 μm thick sensors by mistake. The mean size of a cluster is around 1.9 in TT and 1.6 in IT, and the cluster size increases with the angle of the track. 3.3. Spatial alignment and hit resolution The spatial alignment of the detector was performed using a global χ2 minimisation based on Kalman track residuals [6] with additional requirements on the decay vertices and invariant mass of the particles used [7]. The hit resolution is calculated from the distance between the measured hit position and the extrapolated track position after the measured hit has been removed from the track fit. The resolution is given by the spread of this distribution after correcting for the uncertainty in the track parameters. The hit resolution measured using the 2011 data is 52.6 μm and 50.3 μm for the TT and IT, respectively. The resolution measured in the IT is shown as a function of the sector number in Fig. 7 where the sector number corresponds roughly to the x-direction. The
Fig. 6. Signal-to-noise ratio measured for all read-out sectors in IT. Long and short ladders correspond to one- and two-sensor modules, respectively.
resolution is slightly worse in the central regions closest to the beam-pipe where the track angles are smallest and, consequently, where there is the least amount of charge sharing between strips. The strip pitch is 183 μm in TT and 198 μm in IT, and the measured resolution is slightly better than the resolution that would be obtained from a binary system. 3.4. Hit efficiency The hit efficiency of the sensors was measured. Reconstructed tracks are used to probe whether or not the hits that are expected to be used to construct the track were found. Particle candidates from J/ψ → μ+ μ− decays were used to ensure that a track sample with a high purity was used. The tracks were required to have momentum greater than 10 GeV/c to reduce the effect of multiple scattering and additional cuts were placed on the track quality to minimise the effect of fake tracks on the efficiency measurement. A search is made for hits in a window around the intersection point between the track and each sensor where a hit was expected. The efficiency is defined as the ratio between the number of hits found and the number of hits expected for a given sector. The overall hit efficiency is determined to be greater than 99.7% and 99.8% for TT and IT, respectively. The hit efficiency in each IT sector measured using 2012 data is shown in Fig. 8. There are two modules which are known to be affected by large common mode noise and these sectors have a much lower efficiency than the rest of the detector.
4. Long shutdown 1
Fig. 5. Signal-to-noise ratio measured in TT as a function of the strip capacitance.
The temperature of the detectors was kept at 0° during the LHC Long Shutdown 1 (LS1) which ran from 2013 until 2015. They were only warmed up for maintenance of the cooling system. Regular operation periods were scheduled to monitor the status of the detectors. Several improvements were made to the system to improve its reliability during Run 2. The C6F14 cooling system regularly lost cooling power and required daily interventions by the end of Run 1. A new cooling system was installed to improve the reliability of the system and has been running without problems since April 2014. The old cooling plant remains as a back-up. The control software has been re-written resulting in a 43% reduction in the configuration time of the electronics. The full system was migrated from PVSS to WinCC-OA after Siemens
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Fig. 7. Hit resolution measured using 2011 data as a function of the IT sector number. The sector number corresponds roughly to the x-coordinate. The labels X1, U, V and X2 denote the four detection planes in each detector box. The resolution in the stereo layers (U and V) is worse than that measured in the x-layers ( X1 and X2). This effect was not seen in simulation so is most likely caused by a small remaining misalignment in y. The x-layers are not sensitive to misalignments in y so their measured resolution is better than that in the stereo layers.
Fig. 8. Hit efficiency measured using 2011 data as a function of the IT sector number. The sector number corresponds roughly to the x-coordinate. The labels X1, U, V and X2 denote the four detection planes in each detector box. Two modules have a lower efficiency than the rest of the detector.
acquired the company that supplied the PVSS software.4 Both IT and TT were opened at the start of LS1 while a new beam-pipe was installed. The previous beam-pipe was not at its nominal design position during Run 1 so the box positions in the IT have been re-adjusted to improve the detector acceptance. A new system based on the ATLAS BCAM5 was installed to provide real-time monitoring of the movement of the IT stations. A BCAM consists of laser diodes that are used as the light source and a CCD (charge coupled device) camera that can reconstruct the position of reflective targets that were installed on each IT station. Two BCAMs are required per station and the BCAM analyses the 4 PVSS and WinCC-OA are both SCADA (supervisory control and data acquisition) systems. 5 Brandeis (or Boston) CCD Angle Monitor [8].
relative position of the light spot that is reflected onto the CCD. The intersection of the camera lines of sight allows an absolute measurement of the position of the IT station with a precision of the order of tenths of millimetres. The position of the stations is continuously monitored and movements of up to 1 cm have been observed during changes in the polarity of the dipole magnet.
5. Radiation damage studies The forward geometry of the LHCb detector means that the flux of particles through the various sub-detectors is non-uniform. The fluence varies by up to three orders of magnitude between the inner and outer regions of the TT. The highest 1-MeV-neutron equivalent fluence, neq , expected is estimated from a FLUKA
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The expected change in the leakage current, ΔI , can be calculated using ΔI = αϕV where V is the volume of the sensor, ϕ is the fluence and α is the current related damage rate which depends on the temperature and the annealing [12]. The predicted leakage current is also shown in Fig. 9 and is found to agree well with the measured values. 5.2. Depletion voltage
Fig. 9. Leakage current measured as a function of the delivered luminosity for the two-sensor sectors in the TT stereo layers (U and V).
simulation [9,10] of the LHCb detector to be 4·1012 cm−2 in TT and 2.5·1012 cm−2 in IT per fb 1 of delivered luminosity at a centre-ofmass energy of 8 TeV. A total luminosity of 3.5 fb 1 was delivered during Run 1 where the majority of data was collected at a centreof-mass energy of either 7 or 8 TeV. An additional 5 fb 1 of proton–proton collisions is expected to be delivered during Run 2 at a centre-of-mass energy of 13 TeV (or higher). Both TT and IT will be replaced by new detectors at the end of Run 2 [11]. Radiation damage in the silicon leads in changes in the leakage current and the depletion voltage of the sensors. The methods used to monitor these two quantities are described in the following sub-sections. 5.1. Leakage current The change in the leakage current is directly proportional to the delivered luminosity and, therefore, provides an excellent way to measure the radiation damage. The current in the sensors is monitored continuously and the maximum current measured during each data taking period (or fill) is stored. These values are then plotted as function of the total delivered luminosity as shown in Fig. 9 for the TT. The measured current is normalised to the volume and temperature of the silicon sensors. The operating temperature is 8° in both sub-detectors.
The depletion voltage of the sensors was measured directly using capacitance–voltage (C − V ) scans in the laboratory. The sensors used in the construction of the detector had initial depletion voltages in the range (160–280)V for TT and (60–140) V for IT. It is impossible to measure the depletion voltage using this method once the modules have been constructed and installed in the detector. Therefore, regular charge collection efficiency (CCE) scans are made using collision data. The CCE scans consist of three parts: firstly, the voltage applied to the sensors is changed; secondly, the signal sampling time is scanned for each voltage step to account for changes in the charge collection time when operating at lower voltages; and finally, the depletion voltage is extracted from the distribution of the collected charge as a function of the applied voltage. The scan is performed in different layers of the detector. Reconstructed tracks are extrapolated to the layer under study and the charge in the strips around extrapolated track position is summed up to give the total charge. The resulting signal charge distribution is shown in Fig. 10 for two voltage steps in the TT scan. The most probable value is extracted from a fit of the sum of two Landau distributions (signal hits) convolved with the sum of two Gaussian distributions (noise hits) to the data. This is repeated for all of the timing steps at each voltage. The resulting distribution of the charge as a function of the sampling time is shown in Fig. 11 for the same two voltage steps in the TT scan. The charge collected at each voltage step is extracted from fits of a half-Gaussian that describes the expected pulse-shape to the data. The maximum value of this function gives the charge collected at each voltage. The charge collected in one sensor is shown as a function of the applied voltage in Fig. 12 for scans taken in July 2011 and January 2013. The delivered luminosity at the time of each scan was 0.48 fb 1 and 3.47 fb 1, respectively. The depletion voltage is taken to be the value at (95 ± 2)% of the plateau charge. This point was chosen such that the depletion voltage extracted from the earliest CCE scan matched that measured using the C − V scans during the module production. The difference between the depletion voltage extracted in the CCE scans compared to that
Fig. 10. Charge distribution for two voltages in the TT scan. The most probable value is extracted from the fit of the sum of two Landau distributions (signal hits, red dashed curve) convolved with a double Gaussian distribution (noise, blue curve) to the data. The second Landau distribution (red dotted curve) is used to describre photon conversions in the material before the ST and has an MPV that is fixed to twice the value of first Landau distribution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. Pulse-shape from the timing scan for two different voltages in the TT scan. The black points are the measured MPVs for each timing setting and the red curve is the functional form of the pulse-shape that was fitted to the data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. Charge collected as a function of the bias voltage, Vbias, for a TT sector in July 2011 (a) and January 2013 (b). The grey band is the uncertainty on the measured value of the depletion voltage. The nominal operating voltage during Run 1 was Vbias = 300 V .
function of the 1-MeV neutron-equivalent fluence where the fluence was obtained using FLUKA simulations and the actual running conditions. The predicted depletion voltage was calculated using the model described in Ref. [13] and is also shown in Fig. 13. The expected depletion voltage agrees well with the measured values, and the expected evolution of the depletion voltage shows that there will be no type inversion in the silicon before the end of Run 2 even for the innermost regions of the TT.
6. Conclusions
Fig. 13. Depletion voltage measured in the TT as a function of the 1-MeV neutron equivalent fluence. The red circles and blue triangles show the measurements for read-out sectors where the initial depletion voltage of the sensors, measured using C − V scans during the module production, was in the range (160–220) V and (220– 280) V, respectively. The predicted depletion voltage was calculated using the Hamburg model [12] and is shown as a solid black line. The dashed lines indicate the uncertainty on the predicted values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
measured in the C − V scans can be related to a ballistic deficit from the shaping of the signal in the front-end electronics. The measured depletion voltage is shown in Fig. 13 as a
The LHCb Silicon Tracker performed extremely well during Run 1 of the LHC. A number of improvements were made to the system during LS1, including the installation of a new cooling plant for both sub-detectors, and an alignment system to monitor the positions of the IT stations. The radiation damage to the sensors is monitored using measurements of the leakage current and depletion voltage, and is found to be in good agreement with the expection from simulation. The radiation damage is not expected to result in problems for the operation of the detectors up until the end of Run 2 when both detectors will be replaced by a new tracking system.
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