Design and construction of the new BESIII endcap Time-of-Flight system with MRPC Technology

Journal Pre-proof Design and construction of the new BESIII endcap Time-of-Flight system with MRPC Technology P. Cao, H.F. Chen, M.M. Chen, H.L. Dai, Y.K. Heng, X.L. Ji, X.S. Jiang, C. Li, X. Li, S.B. Liu, Z. Liu, X.L. Luo, X.Y. Ma, M. Shao, S.S. Sun, Y.J. Sun, Z.B. Tang, X.Z. Wang, Z. Wu, M.H. Xu, R.X. Yang, M. Ye, J. Zhang, Y.H. Zhang, J.Z. Zhao

PII: DOI: Reference:

S0168-9002(19)31406-8 https://doi.org/10.1016/j.nima.2019.163053 NIMA 163053

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 2 April 2019 Revised date : 26 October 2019 Accepted date : 26 October 2019 Please cite this article as: P. Cao, H.F. Chen, M.M. Chen et al., Design and construction of the new BESIII endcap Time-of-Flight system with MRPC Technology, Nuclear Inst. and Methods in Physics Research, A (2019), doi: https://doi.org/10.1016/j.nima.2019.163053. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

*Manuscript Click here to view linked References

Journal Pre-proof Design and construction of the new BESIII endcap Time-of-Flight system with MRPC Technology P. Cao a,c, H.F. Chen a,c, M.M. Chen b,c, H.L. Dai b,c, Y.K. Heng b,c,d, X.L. Ji b,c, X.S. Jiang b,c, C. Lia,c, X. Li a,c, S.B. Liu a,c, Z. Liua,c, X.L. Luo b,c, X.Y. Ma b,c, M. Shao a,c, S.S. Sun b,d, Y.J. Sun a,c, Z.B. Tang a,c, X.Z. Wang a,b,c, Z. Wu b,c, M.H. Xu b,c, R.X.Yang a,c , M. Ye b,c, J. Zhang b,c, Y.H. Zhang b,c, J.Z. Zhao b,c

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a Department of Modern Physics, University of Science and Technology of China(USTC), Hefei 230026, China b Institute of High Energy Physics(IHEP), Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Particle Detection and Electronics, China d University of Chinese Academy of Sciences, Beijing 100049, China

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E-mail: [email protected], [email protected]

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Table of Contents

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Abstract ..................................................................................................................................... 1

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2. Design and production of MRPC .......................................................................................... 3

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2.1 Structure of upgraded ETOF ........................................................................................... 3

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2.2 Design of MRPC module ................................................................................................ 3

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2.3 Mass production and quality control ............................................................................... 5 3.

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Frontend electronics .......................................................................................................... 7 3.1Design and analysis .......................................................................................................... 7

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3.2 Performance and reliability tests ..................................................................................... 8

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4. Readout electronics ............................................................................................................. 10

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4.1 Electronics logic ............................................................................................................ 10

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4.2 Calibration-Threshold-Test-Power (CTTP) .................................................................. 11

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4.3 Trigger signal and power ............................................................................................... 12

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4.4 Time-to- Digital (TDIG) converter modules ................................................................. 13

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4.5 Fast control modules and CLOCK modules .................................................................. 15

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4.6 The trigger sub-system .................................................................................................. 18

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5. Test and installation of MRPC ............................................................................................ 22

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5.1 Cosmic ray test .............................................................................................................. 22

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5.2 Gas system ..................................................................................................................... 23

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5.3. Slow control ................................................................................................................. 24

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5.4 Installation ..................................................................................................................... 24

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6. ETOF PID performance ...................................................................................................... 25

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6.1 Event start time determination....................................................................................... 25

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6.2 Time-Over-Threshold .................................................................................................... 26

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6.3 Timing calibration and time resolution ......................................................................... 26

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6.4 Particle identification with upgraded ETOF .................................................................. 29

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In order to improve the particle identification capability, the Beijing Spectrometer (BESIII) collaboration has upgraded the End-cap Time-Of-Flight detector (ETOF) based on Multi-gap Resistive Plate Chamber (MRPC) technology. In this paper, the design and engineering development of each part of the project are reported. There are 72 MRPC modules, forming 2 rings. Adjacent modules are staggered placed to avoid dead regions. Each MRPC module contains 12-layer thin gaps to get fast signals with high efficiency and 12 strips to readout the induced signals from two ends, effectively reducing the timing uncertainties from the scattering and positioning. Also, the analog-digital conversion is done near the MRPC and only the digital signals are transferred through thin coax cables, ensuring good signal-to-noise ratio. The complex electromagnetic noises in the BESIII colliding area are well shielded to protect the tiny signals from the MRPC. After careful correction and calibration, the total time resolution of upgraded ETOF system is 65ps.

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1. Introduction

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Many important results on the light hadron spectroscopy, charmonium spectrum, charm meson decay property, QCD and tau physics have been achieved since the operation of Beijing Electron-Positron Collider II (BEPCII) and Beijing Electron Spectrometer III (BESIII) in 2008 [1, 2]. The Time-Of-Flight (TOF) is the key detector in BESIII for charged hadron identification. It is composed of two parts: barrel TOF (BTOF) and endcap TOF (ETOF). The BTOF consists of two-layer plastic scintillators and fine-mesh Photo-Multiplier Tubes (PMT) as readout. A time resolution of 80ps can be achieved for 1GeV/c muons. In comparison, the first version of ETOF has been a one-layer scintillation detector. The effect of multiple scattering and the uncertainty of positioning of particles are significant, leading to a poor time resolution of ~140ps, and the K/π separation (3σ) is limited to momentum range below 1GeV/c [3, 4]. The BESIII is designed for high precision measurement in the tau-charm energy region. According to the simulation, the momentum distribution of J/ψ (or Psi', Y4260 etc.) decayed hadrons can reach 1.5 GeV/c or even higher momenta. Although the proportion of hadrons above 1 GeV/c is small, high precision measurements for BES physics and rare decay events (e.g. CP violation and mixing parameters of neutral D mesons) requires accurate particle identification (PID) in the full momentum range. Therefore, the BESIII-TOF group has upgraded the ETOF based on high time resolution Multigap Resistive plate Chamber (MRPC) technology [5]. Simulation has shown that an MRPC-based ETOF system can effectively suppress the multiple scattering effects on the time resolution and uncertainty of particle hit positions. With a proposed time resolution of 80ps for the upgraded endcap TOF system, K/π separation can reach 1.4GeV/c at 95% confidence level [6, 7, 8]. With the successful operation experience of MRPC-TOF in RHIC-STAR and LHC-ALICE experiments [9, 10], the technical design of upgraded BESIII ETOF has been completed in 2013 [11-14]. The double-stack (26) gas gaps and dual-end readout structure are applied in the baseline MRPC design. The ETOF system, including east and west parts, is composed of 236 MRPC modules with 1728 readout channels, and high voltage (HV), gas and data acquisition (DAQ) system. In September 2016, the whole ETOF system has been successfully installed in BESIII and operated, collecting a large amount of experimental data. An offline calibration result shows that the system time resolution of ETOF is 70 ps for pions. Its average detection efficiency is 97.5% [15-17]. In the following sections, we will present the design and construction of the upgraded ETOF with MRPC technology. The relevant electronics and calibration method will also be discussed in detail.

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2. Design and production of MRPC

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2.1 Structure of upgraded ETOF

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The layout of the upgraded ETOF system is schematically shown in Fig.1. In total, there are 72 MRPC modules, 36 on each end, forming 2 rings. Adjacent modules are staggered placed to avoid dead regions. The effective area of the ETOF ring has an inner radius of 501 mm and an outer radius of 822 mm. The two MRPC endcaps are located outside the Main Drift Chamber (MDC) endcaps which are about ±1330 mm along the direction of the positron beam. Since each MRPC is readout by 12 strips, its granularity is increased by 12 fold in R direction, while in the previous ETOF each plastic scintillator module is readout by only one PMT from the inner end. This finer granularity greatly reduces the hit-position related timing jitter.

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Fig.1 BESIII detector and the schematic drawing of upgraded MRPC endcap TOF.

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2.2 Design of MRPC module

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In order to better cover the endcap region, the MRPC module is trapezoidal shaped. The readout strips, ranging from 9.1 cm to 14.1 cm, is read out from both ends (so-called dual-end readout), as shown in Fig.2 (left). The interval between adjacent strips are 3 mm. The structure of MRPC module from the side view can be found in Fig.2 (right). A series of resistive glass plates composes of 12 gas gaps, arranged in two stacks. In each stack, the thickness of the two external glass plates is 0.55 mm and that of the other internal glass plates is 0.4 mm. The gap between the glass plates is 0.22 mm wide, defined by the diameter of the nylon fishing line (used as spacer). All the glass plates are fixed in the right position with support pillars at four corners. These pillars fit into the space between the gas box and MRPC. On the outer surface of the external glass in each stack, a layer of resistive graphite is painted and works as the HV electrode. Three layers of printed circuit boards (PCB) are used to collect the induced signals. In-between the PCB and the HV electrode, Mylar films ensure the

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Journal Pre-proof insulation. At last, two honeycomb boards protect the entire structure from the outmost sides and the total thickness of the module is less than 20 mm.

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Fig.2 The layout of MRPC readout board (left), and the schematic drawing of the MRPC module cross-section (right). The adoption of such a readout pattern is based on the simulation analysis and experimental results. A GEANT4 [18, 19] Monte-Carlo model with standard GEANT4 electromagnetic (EM) physics process is developed to study the performance of ETOF at BESIII [7]. It’s found that the multi-hit rate (per readout channel) is 71.5% for the former scintillator-based ETOF and can be significantly reduced to 21.8% for the MRPC ETOF with dual-end readout design. This is mainly due to the much smaller readout cell size of MRPC than the scintillators. For the scintillator ETOF, the multi-hits include the contribution from the primary electron, secondary electron/position and gamma. Since gamma has a much smaller probability to induce signals in MRPC compared to others, this is also helpful in reducing the multi-hit rate. During the R&D phase, a single-end readout design was also considered, as shown in Fig.3. The simulated multi-hit rate is as low as 16.7% in this case and the timing performance should be better. But, in order to delegate the signal transmission time along the strip, precise tracking is needed for a position calibration; for the dual-end readout, this effort is negligible since the averaging time from both ends cancels the uncertainty of the hit position along the strip. This effect has been verified in experimental test, as discussed in details in reference [13]. Especially, taking into account the poor tracking precision in the forward end-cap region, the dual-end readout is finally selected.

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Fig.3 The layout of the single-end MRPC readout board. In beam tests, the performances of MRPC prototypes with different readout strip designs, under different high voltages, particle species and momenta have been

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Journal Pre-proof extensively studied [11-14]. As shown in Fig.4, when the applied HV is higher than ±7 kV, both efficiencies are greater than 98% for 800 MeV/c pions and protons and the time resolution is better than 40 ps for protons. Due to the low pion statistics of the beam component, a scan was only made at different momenta under a fixed voltage of ±7.3 kV, as shown in Fig.5. This result confirmed the time resolution is better than 50 ps for the pions (MIPs). During the test, the gas component used is 90% Freon + 5% SF6 + 5% iso-C4H10 with the pressure of 2.5 mbar under the room temperature.

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Fig.4 MRPC beam test results of the efficiency and time resolution at different HV.

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Fig.5 MRPC beam test results of the time resolution at different momenta.

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The MRPC mass production is carried out at the University of Science and Technology of China (USTC) [20]. The quality control process includes the raw material inspection such as geometry size, the glass surface quality and the bulk resistivity, the graphite painting quality and surface resistivity, the cleanness requirements, etc. To guarantee the same excellent performances for each MRPC prototype, a set of quality assurance tests has been developed. These tests are applied 5

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to both the components and the assembled detectors. Since the environment condition has a significant influence on the quality of MRPC production, all assembling and testing procedures are finished in class 100k clean room enclosed environment (the working desk reaches class 100). 2.3.1 Test on source materials First of all, the planarity of the rigid components (honeycomb boards, PCBs and the glasses) have to be checked to avoid construction faults. Besides, due to the space limitation of the BESIII ETOF system, the thickness has to be strictly controlled. The thickness is measured in eight different positions along the border edge. The edge and corner polishing check for glasses are required. The uniformity of the nylon fishing-line spacer is also important to form a uniform gas gap. The diameters are measured in ten points per roll. The details are shown in Table 1. Table 1: The thickness summary of main materials (unit: mm)

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The surface resistivity of the HV electrodes are measured by putting two electric bars (with the length same as their distance) on the surface of the graphite paint and reading the resistance from a Meg-ohmmeter. Three different points are measured along the central axis of each trapezoid-shaped glass to ensure a good uniformity. The acceptable surface resistivity of the graphite paint is about several M𝛺/□. All the components are checked and selected before the assembly of the single module. A detailed assembly process flow diagram is shown in Fig.6. Material procurement

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Fig.6 The diagram of the assembly process

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Journal Pre-proof 2.3.2 After the MRPC assembly The module is placed in a gas-tight aluminum box with total thickness of 2.5 mm. The gas leakage tests are designed to check the MRPC gas leakage and pressure standing after the assembly. The pressure variation over time is measured under an over-pressure of 10 mm H2O applied on the detector. Only MRPCs with the pressure loss less than 5 mm H2O in a 5 min period can pass the test. The qualified assembly modules are flowed with working gas for more than 24 h. The working gas is a standard gas mixture for MRPC (90% Freon + 5% SF6 + 5% iso-C4H10). A working HV (±7.3kV) is then applied, which is higher than the operation voltage (±7kV) , because the higher voltage is easier for the leak current of MRPC decreased rapidly to normal level. The dark current is required below 10nA after a few hours of stabilization.

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The front-end electronics digitizes output signals of MRPC, and then transfers them to the back-end electronics extracting the hit information. The average output charge of a MRPC signal is about 300 fC [21] and its leading time is less than 1ns [22].The design of the circuit must consider the amplification and shaping for the output signals. Therefore the NINO chip developed by the experimental group of ALICE / TOF using CMOS technology [23] is adopted. With differential signal processing, the output signal of NINO chip has ultra-fast leading time and adjustable threshold. The jitter of leading edge is below 10ps and make use of Time-Over-Threshold (TOT) method. Each NINO chip can amplify and shape eight signals and consume only 40mW for a single channel. The pulse width is exponentially related to the amount of charge carried by the input signal. Due to the measurement requirement of the readout electronics, the output pulse is stretched by 10 ns [24]. In order to reduce the jitter, the pre-amplifier chip is placed on the side of the

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2.3.3 On-site test with cosmic rays Each module is tested with cosmic ray in the laboratory at USTC to check its efficiency and time resolution. The testing setup consists of four MRPC modules. Two trapezoidal plastic scintillator detectors are placed at the top and bottom as a trigger system. All detectors are placed vertically on the steel bracket. Fig. 7 shows the block diagram of the testing system. The Front-End Electronics (FEE) is a 16-channel amplifier/discriminator card. The signals from the MRPCs are fed into a VME TDC (CAEN V1290A, 32 independent multi-hit /multi-event Time to Digital Converter) for digitization. The test results show that the average efficiency of each MRPC module is ~95%, and the time resolution per strip achieves 65 ps. All the quality control data have been recorded and saved.

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detector to minimize the distance between detector and electronics and reduce the input capacitance. A rigid-flexible circuit board design is applied to keep the signal impedance matching in the transmission path. From 4 daughter-boards to mother-board of FEE, using the flexible PCB board made of FR4 instead of the traditional cable connection significantly reduces not only the system noise, but also the complexity of the system and its cost. The SAMTEC's high-speed micro-coaxial shielded cables were selected as the signal transmission cable to shield the impact of the environment on the signal and power supply between front end electronics and back end electronics. The diagram of front-end electronics is shown in Fig.7. The MRPC signal interface board is the signal transition board between the detector and FEE. The entire interface board uses a fully differential processing to minimize the effects of noise. It also uses the discharge resistances as the current discharge protection, avoiding sparks from accumulating charges. The discharge resistors are two 20kΩ resistors in parallel to improve reliability. The Interface board is divided into two parts, connected to the left and right sides of the MRPC. For 72 MRPCs, there are 144 interface boards and each interface board transmits 12 differential signals. Each FEE consists one main module and four relatively independent daughter modules. The main function of each daughter module was ran by a NINO chip, including amplification and discrimination. Each daughter module processes 6 differential signals. FEE with rigid-flexible design maintain impedance matching in the signal transmission path. Each daughter board is connected to the main board through a flexible board. There is a total of 72 FEE for 72 MRPCs and each FEE processes 24 signals from a MRPC.

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Front-end electronics need to be installed inside the Beijing spectrometer and its reliability requirements are relatively high. Therefore, besides the performance test, more stringent tests of front-end electronics are processed, including large signal impact test, radiation resistance test and thermal test. Fig.8 shows the leading-edge time resolution of the front-end electronics output signals and the signals’ width as the input charge ranges from 40 fC to 1400 fC. When the input charge is above 100 fC, the time resolution is less than 20 ps. A large signal generator (Agilent 8114A, 100V / 2A) is used to output its maximum

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Because front-end electronics are housed inside the shield box in BESIII, heat radiation test is necessary. Two temperature sensors [25] were used to detect the temperature while the FEE was working. The test results are shown in Fig. 10: after sufficient heat ex-changing, the temperatures of Point A (air in the room) and B (the surface of NINO chip) can both keep below 30oC, as required by the BESIII experiment. During the testing time, the Calibration-Threshold-Test-Power (CTTP) current was stable, as shown in Fig.10, and the FEE system worked normally. In the future running environment, there will be dry air blowing over the surface of the detectors all the time (by air cooling system), so the working status of the FEE should be more reliable. A large-current beam test [25] was also carried out to examine the protection circuit

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of the FEE. The test was performed at the BEPC E2 line with a 2.5GeV incident electron beam. The FEE and MRPC module were put in the path of the beam and the high voltage of the MRPC was at ±7kV. The frequency of beam bunch is 12.5 Hz and the electrons number in each bunch ranges from 105 to 108 . As the beam intensity increased continuously, the high voltage of MRPC is tripped because its leak current exceeded the setting value. So we increased and decreased the beam intensity iteratively and kept the election beam passing trough MRPC continuously when the MRPC is kept at the working voltage. The whole test lasted about two hours and the FEE worked normally, indicating that the protection circuit of the FEE is reliable.

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Fig.10 Results of heat radiation test.

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The ETOF upgrade need to consider the compatibility between barrel electronics and new endcap electronics. In Fig.11 is reported the whole TOF electronics block diagram. It shows that the pulse signals from the front-end electronics are sent back and fed into the TDIG modules in the readout electronics for time measurement. The two VME crates on the left side of the figure are new upgrades with back-end electronics TDIG modules, clock modules, fast control modules and Trigger Data Pre-Processor modules (TDPP). The two NIM crates on the left side of the figure are new upgrades with two types of Calibration-Threshold-Test-Power (CTTP) modules inside to provide power, threshold and calibration signal to the front-end electronics. The CTTP modules also transfer hit signals to the front-end triggering modules from front-end electronics. The three crates on the right are the old VME crates, but the clock modules and the fast control modules for BTOF electronics are upgraded. There are new Signal Integration and Fan-out (SIF) module and End-cap TOF Trigger (ETOFT) module in trigger crate.

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The CTTP modules provide power, threshold and test signal to FEE and receive the hit signal from FEE, which is then sent to the trigger system as the basis of the first level judgment. The CTTP modules also send the power monitoring data to DAQ system and accept the control signals from the fast control module. There are two types of CTTP modules: eight CTTP-P modules and two CTTP-C modules. CTTP-P modules are main modules for processing, while CTTP-C modules are responsible for communication between four CTTP-P and fast control system. To FEE: Power, Test, Threshold; From FEE: OR signals

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Fig.11 Block diagram of ETOF electronics

Fig.12 shows the CTTP module working diagram. There are a total of 8 CTTP modules based on NIM standard, and each module is responsible for the 9 preamplifier circuit, transferring the signal and power supply. Each CTTP module is designed as the motherboard structure including 9 daughter boards and a read-out control board that communicates with the fast-control module. The calibration circuit (shown in Fig.13) uses a 12-bit DAC to output the calibration voltage: generates a step signal through the high-speed Single Pole Double Throw (SPDT) switch, and feed it into the cable by a differential amplifier.

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The trigger circuit mainly receives the hit signal (LVDS_2.5V level standard) from FEE. After the level is converted to LVPECL_3.3V, it is sent to the trigger system through the front panel socket and cable. Threshold circuit mainly provides discrimination threshold for FEE. FEE requires two thresholds, each set within the range of 0.5V ~ 2V. The actual threshold is the difference between these two thresholds. The front-end electronics is designed to provide a uniform threshold signal for 24 signals from each MRPC. The threshold voltage circuit sets a DAC on each daughter board to provide the threshold voltage directly to FEE. This voltage is programmable through the DAQ system. calibration FEE Hit signal Start Fast control module

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Fig.13 Calibration signal circuit diagram

The power supply mainly provides the necessary power for FEE, with monitoring and protection. FEE working environment is harsh, and it is possible to make module malfunction. In order to avoid the malfunction module affects other normal modules, there are four relatively independent power supplies for each FEE daughter board. In addition, there is real-time current monitoring for each independent power supply, transferring current data to the DAQ system. When some daughter board in FEE is malfunction, the power supply of the circuit can be automatically shut down once the monitored current reaches to the threshold value and the protected data is sent to the DAQ system. DAQ system will alarm and wait for the on-call person.

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Each TDIG module receives 72 channels of time signal from 3 MRPC detectors via 3 individual high speed and high-density shielded differential twisted pair cables respectively. The cable length is about 10m. Fig.14 shows the TDIG module structure [26]. To support up to 72 electrical channels, each TDIG consists of a total of 9 high performance time-to-digital converters (HPTDCs) [24]. The TDCs are divided into 3 groups, each is formed into a daisy chain for the configuration or status checking through the Joint Test Action Group (JTAG) interface. Configuration data sent from the DAQ is received via the VME bus and buffered into local memory in the Configure & Status Extract module. Once the configuration command arrives, modules of digital logic named with JTAG Ctrl in the FPGA receive these configuration data and feed them into the HPTDC through the JTAG ports. These 3 JTAG Ctrl modules operate synchronously, which means they are configured simultaneously. In addition to configuration, the JTAG Ctrl modules also collects TDC status information such as the FIFO status which indicates whether there is TDC overflow or not. Furthermore, the status information can be fed to the TDIG Status Monitor module to generate a fast control signal transmitted to the BESIII fast control system. Token Ring

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occurring (T0) and the trigger input time, caused by trigger latency, can be eliminated by a trigger matching mechanism, where the HPTDC searches hit information in the L1 buffer and commits as valid only if this hit information is within a pre-set time range called the matching window. Due to variations in processing, however, HPTDC has large dependencies on processing parameters and each TDC chip needs to be calibrated. In BESIII ETOF upgrade system, all of the 1728 HPTDC channels working in very high resolution mode need integral nonlinearity (INL) correction. To improve the correction efficiency, an improvement of nonlinearity correction [27] is introduced into ETOF upgrade. In this improved INL correction procedure, the INL data is automatically imported from a non-volatile storage of read-only memories (ROM) instead of from DAQ, and then fed into FPGA with correction algorithm online. Fig.15 shows the typical nonlinearity results for TDIG. In Fig.15, code in horizontal axis means the encoding data output from the HPTDC chip, it ranges from 0 to 1023 in a counting period.

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(a) (b) Fig.16 Typical timing resolution for TDIG without (a) and with (b) FEE 14

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For the purpose of interacting with the trigger system, each ETOF crate contains an extra fast control (FCTL) module besides the TDIGs. The fast control system in the TOF electronics fans out fast control signals, such as L1 triggers and system reset signals from the trigger system, to each VME TOF module including the BTOF FEE and ETOF TDIG. Meanwhile, the FCTL collects the status of the TOF readout modules and transfers this information back to the trigger system. The BESIII trigger system can only interact with 2 FCTL modules, so the FCTL module in BTOF crate-A has to be redesigned. It is set up as a router to transfer signals from FCTLs in the ETOF crates to the trigger system, as shown in Fig.17. The FCTL in BTOF crate-B is unchanged. Fast control signals can be classified into 3 groups: trigger, control and status signals. The upgraded TOF FCTL system consists of 4 VME modules installed in 4 BTOF and ETOF crates respectively. As well as the unchanged BTOF FCTL [29] module on the ground floor of the BESIII building, there are 3 newly developed FCTL modules, one is set up as master or router in BTOF crate-A (3rd floor), and the other two are set up as slaves in 2 ETOF crates respectively. To simplify the design and use of the FCTL, the master FCTL module has the same hardware structure as the slaves.

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modules. Fast control status from the BTOF is sent back to the trigger system merged with that of the ETOF. The existing BESIII fast control system uses TLK1501 [30] to distribute fast control signals to all sub-systems. The same technique is used in the ETOF module design for compatibility purposes. Unfortunately, each time when the SerDes (serializer or de-serializer) chip powers up, there is an uncertainty [31] in the phase of the recovered clock, which should be eliminated. Unlike the BTOF FCTL, in ETOF upgrade electronics, this latency uncertainty is determined by a Time-DigitalConverter embedded in a FPGA and eliminated by re-capturing at synchronous and determinate time. Compared with the previous method of BTOF, it has advantages of flexible structure, easy calibration and good adaptability. In Fig.18, the transmitted test signal (yellow line) is used as the trigger source of the oscilloscope, while the recovered signal (blue line) at the recovering side is monitored in infinite persistence mode, which means the uncertainty between the transmitted and recovered signals can be captured by the oscilloscope. Fig.18 shows that the latency between these two signals is fixed, and the fast control signal uncertainty is eliminated. Additionally, Fig.18 was captured after this test platform had run for 8 hours with SerDes reset repeatedly, which means this proposed method has good stability.

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Fig.18 Fast control signal uncertainty monitoring result (yellow line: transmitted signal, blue line: recovered signal, purple line: recovered clock; top half: global view, bottom half: enlarged view). The horizontal axis label is time (100ns/div), while the vertical axis label is voltage (1.0V/div for C1 and C2 signals, 2.0V/div for C3 signal). To ensure the time resolution of TOF electronics below 25ps, the clock jitter must be less than 20 ps (RMS) and the clock phase should be highly synchronized to the beam collision time [32]. As MRPC detectors are utilized in upgraded system, there is a significant increase in the number of electronic channels and two more VME64xP crates as well as another two clock modules will be installed in ETOF electronic system. Thus, the original clock system needs to be upgraded with more clock output

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Fig.19 Block diagram for BESIII TOF clock system with ETOF upgrade To test the stability of the TOF clock system, the phase between the system clock and a periodic signal at 1.25 MHz named the bunch synchronization signal (BSYNC) signal has been checked by an oscilloscope in infinite persistence mode. The BSYNC signal is derived from the beam revolution clock in the accelerator and transferred to the master module for synchronizing the TOF clock with beam bunches. Fig.20 shows the evaluation result which shows that there’s no uncertainty between these two signals.

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channels. On the other hand, each time the original TOF system is powered on, it requires a series of manual operations to configure the clock synchronization which is not very convenient [33]. To avoid this trouble and meet these requirement of TOF electronics, the upgraded clock system consists of two parts: one is the transmission of the RF signal, and the other is VME clock module, providing multi-channel high quality clocks as well as synchronization and phase-control among them. Fig.19 illustrates the block diagram of the whole TOF clock system with ETOF upgrade. The accelerator provides RF clock signal transmitted by a phase-stabilized optical fiber (PSOF). To match the structure of the upgraded TOF readout system, the new clock system is made up of four clock modules: one is master module and the others are slave modules. By selecting different clock sources of the clock fan-out chip, clock module will work in master or slave mode. In master mode, system clock is the 499.8MHz RF clock divided by 12 from the accelerator; in slave mode, it is a 41.67MHz optical signal output from the master clock module. Besides, every clock module has an 83.3MHz crystal oscillator onboard for clock generation by itself under the off-line mode.

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Fig.20 Waveform of synchronized clock and BSYNC in infinite persistence mode (blue curve: system clock, purple curve: BSYNC). The horizontal axis label is time (10ns/div), while the vertical axis label is voltage (500mV/div). 4.6 The trigger sub-system

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End-cap TOF trigger sub-system is to accurately search good events for ETOF detector. It generates three trigger conditions: hit number greater than or equal to 1 in ETOF(NETOF.GE.1), hit number greater than or equal to 2 in ETOF(NETOF.GE.2), and hits back-to-back in ETOF(NETOF.B2B). There are 12 MRPC strips in one MRPC. And strip signals are double-end readout by Front-End Electronic (FEE). Totally, 24 channels signals are readout from one MRPC by FEE board. In FEE board 6 neighboring channels on one side are sent together to ETOF trigger subsystem as a trigger logic signal, as shown in Fig.21. Therefore there are 4 signals send to trigger system from one MRPC. Totally, 288 hit signals are sent to ETOF trigger subsystem for trigger logic.

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Fig.21 MRPC FEE readout channels, channel 18, 16,20,14,22,12 are worked together as one OR output (CTTP0); channel 23, 13, 21, 15,19,17 are worked 18

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In BESIII there is a Super Conducting Magnet on the outside layer of TOF, which is used to generate Constant magnetic field with the intensity of 1 T. After the Electron and Positron collision in the center of BESIII detector, electron and positron moves following Lorentz force law. Assuming a Bhabha event and a MRPC position as a reference hit by electron, according to the momentum distribution of positron, Hit position distribution can be calculate on the other side MRPC. BESIII related parameters are as follows i. Magnetic field intensity B = 1 T; ii. Semi-major axis of BTOF a=1.33m; iii. Radius of ETOF: R0=0.454m, R1=0.649m, R2=0.844m. iv. Pt range: Pt > 660Mev/C; As the Lorentz force for positive and negative electrons which moving in the same direction in the magnetic field is opposite, the Back to Back matching area is central symmetry with the reference MRPC. Based on this method and BESIII parameters, ETOF Back to Back matching area is 7 MRPCs (3+1+3), including 1 MRPC central symmetry with the reference MRPC, 3 MRPCs on the right and 3 MRPCs on the left, as shown in Fig.22. And the simulated trigger efficiency for back-to-back events is 100% (efficiency of detector is not considered).

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together as one OR output (CTTP1); channel 6,4, 8,2,10,0 are worked together as one OR output (CTTP2); channel 11,1, 9,3,7,5 are worked together as one OR output (CTTP3). I. Trigger simulation

Fig.22 Bhabha Back to back event simulate position.

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II. Construct of trigger sub-system ETOF trigger sub-system is consisted of Trigger Data Pre-Processor board (TDPP), End-cap TOF Trigger board (ETOFT) and Signal Integration and Fan-out board version 2(SIF2), as shown in Fig.23. Hit signals are pre-processed near detector in BESIII hall by TDPP and then sent to ETOFT via 10 high speed fiber links at 2.5Gbps, which is difference from the line rate in old trigger system. Data alignment protocol is used to compatible new line rate with old trigger system and compensate different 19

Journal Pre-proof hardware delay in 10 channels. SIF2 plays an important role in the new system integration. It integrates sub-system trigger conditions and sends these information to Global Trigger Logic (GTL).

Fig.23 Block diagram of ETOF trigger sub-system A. Trigger Data Pre-Process

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The ETOF hit signal is about one system clock (41.65MHz) width. Hit signals in one event have the uncertain time about 30ns which caused by particle drift time [34]. After e+e- collision in the central point of the detector, particles drift to and hit on different positions of TOF with different flight times. The time difference of particles drifting to nearest and furthest position of TOF is about 30 ns. We call this uncertainty time. In trigger pre-processing, the time window of hit signals should be stretched above 30 ns to make sure all hit signals in one event can match together. It may cause up to 2 system clock skew when signal arrived at trigger system. Therefore, signals are stretched to 3 system clocks (41.65MHz) width before trigger logic to make sure signals in one event can be matched together. Asynchronous stretch circuit is designed to improve the trigger efficiency. B. ETOF trigger unit logic algorithm

In trigger logic, one MRPC shown in Fig.21 is used as one trigger unit. OR logic is used to improve trigger efficiency. For example, if one signal detected from either CTTP0 to CTTP3, one hit event can be confirmed on MRPC. III. ETOF trigger efficiency

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ETOF trigger subsystem has been installed on BESIII in Sept. 2015 and has already run stably for three years. ETOF trigger conditions efficiency is also studied. Ideally, the trigger efficiency would be determined from random trigger events, however this suffers from very low statistics as soon as a physics selection is applied. Instead, some special events are used. Bhabha events selected online by EMC detector are used to check efficiency of ETOF conditions. Considering the efficiency of EMC and TOF detector, the trigger efficiency of ETOF can’t be 100%. The results are shown in 20

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Table.2. Table 2. ETOF trigger conditions efficiency Trigger Conditions Efficiency NETOF.GE.1 (99.74±0.03)% NETOF.GE.2 (97.73±0.09)% NETOF.GE.BB (96.33±0.11)%

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The architecture of the former BESIII data acquisition (DAQ) system is shown in

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system. The readout system receives interrupts in each VME crate, readout data

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mode, and concatenates them crate by crate in parallel [36]. And then the data is

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Flight). As shown in Fig.25, there are two VME crates which host the ETOF

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When the mass production has been finished, a long-time cosmic ray test is performed in IHEP as a final step of the QA procedure. All 72 MRPC modules are stacked on four semicircle platforms, 18 modules each layer. In order to trigger on the cosmic-rays, 18 pairs of fan-shaped scintillator counters, with active areas slightly smaller than the MRPC active areas, are placed directly above and below the MRPC

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Journal Pre-proof module separately. In this large test stand, the gas and HV system, the readout electronics and the data acquisition system for the upgraded ETOF are used and tested along with the MRPCs. During the cosmic ray test, all the hardware runs smoothly. The average time resolution of MRPC is measured to be 65ps and the average detection efficiency is above 96% [17]. For cosmic ray test, there is no tracking system, particle with different incident angle and different momentum can’t be separated, so corresponding corrections can’t be done during the cosmic ray data analysis. More accurate results about the performance will be given by the collision data.

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ETOF gas detector has 72 MRPC modules mounted on the endcap of Electro-Magnetic Calorimeter , the east and west sides in half . Each half will be 36 MRPC modules with 18 gas in and out pipe. The basic design is two MRPC module will connect in series and share a gas pipe. The normal gas flow rate is 1.5 x 10-4 m3/min. We put the 5 times larger gas flow rate as the upper limit of the gas flow controller. The pressure of detector is 2.5 mbar.

The purpose of the ETOF gas system is to provide with high precision the working gas mixtures, 90% Freon+5% iso-butane +5%SF6 , by using MKS mass–flow controllers to the ETOF at the correct temperature and pressure. The gas mixture is stored in a 3500 l tank and then supplied to the detector at a rate of approximately 0.6 l/min and an over-pressure <5 mbar . The system operates nominally as an open loop gas system and the exhausted mixture gas is vented out of the gas control room to the air. The gas delivery is conducted by using brass and PTFE pipes, which limit the access of oxygen, water vapor and other gases into the MRPC detector. Two main impurity gas contents are specifically monitored, one is Oxygen and the other is water vapor.

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5.2.3 Gas slow control system

The gas slow control system, which is composed of supervisor PC and two frontal PLCs, collects and logs the gas system operating parameters. It can also realize the long distance control of the mass flow meters switch. The detailed history of the gas flow, pressure and other gas parameters are available via the online slow-control system. In addition, the system can provide automatic alarm to undesired system conditions.

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The high voltage system of BESIII ETOF supplies positive and negative voltage (7kV) to the electrode independently during operation. In the HV system, a CAEN NIM8304 crate with nine modules (Mod. N1470 with four channels) is employed to supply 36 channels HV to the distributor, in which one HV input channel is divided into four output channels, to feed all the 72 MRPC modules. The HV control and monitoring system is incorporated into the BESIII slow control system. In this subsystem, the CAEN NIM8304 mainframe with a remote monitoring and control via TCP/IP is adopted, and the communication program is developed on the basis of power supply control drivers provided by CAEN Company. In the VME monitoring and control system, the Wiener VME crate provides a way for remote access via CAN bus. An OPC server for the Wiener VME crate has been developed by the IT-CO-FE group at CERN for computer communication.

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According to the project design, each MRPC module together with FEE is mounted on the endcap of Electro-Magnetic Calorimeter (endcap EMC). The total 36 MRPCs are arranged in two lapped layers, as shown in Fig.26. To ensure the installation position of each MRPC match the detector’s design precisely, marking on the endcap EMC and pre-assembly of each MRPC are implemented firstly. For the final installation of MRPCs, both double-faced adhesive tape and screws are used. The Kapton film is inserted between the MRPC based ETOF and the endcap EMC for insulation, and all the connection screws are made of Teflon. To fit the new design of ETOF, 24 pieces of old shielding block are modified and increased to 36 pieces, as shown in Fig.27. These blocks is installed between collision points and endcap EMC, they are used to shield lost beam to protect the crystals. The slots are designed for cable routing inside the shielding blocks. When the MRPCs are ensured to be installed at the proper location, the gas connection tubes are installed between two adjacent MRPCs. At the same time, 36 pieces of modified shielding block are installed. The installed MRPCs are tested with random triggers and the noise ratio of each channel should be less than 0.1%.

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The new ETOF has been commissioned into the BESIII experiment since January 2016, and all the hardware run smoothly. Physics data-taking begins after several days debugging of ETOF trigger sub-system. The noise rate has been measured to be 9.0 Hz/cm2, which reflects the low noise level of upgraded ETOF [15].

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In the case of 𝑒 + 𝑒 − collisions, BEPCII is operated in the double-ring, multi-bunch colliding mode. Bunches with spacing of 8 ns or 6 ns in time, correspondingly 2.4 m or 1.8 m in space, are filled in each storage ring. The collision time, which is defined as event start time t0, is essential for sub-detector reconstruction and particle identification. A combinatorial algorithm has been developed for the determination of t0 according to the time information of final-state particles [39]. The TOF signals associated with charged tracks are always of high priority, thus the signals of upgraded ETOF also contribute to the calculation of t0, especially benefiting from its higher accuracy of measured time [40]. The specific multiplicity of finial-state particles in 𝜏 − 𝑐 energy region is not large enough to improve the precision of the event start time. The results of t0 are set as discrete values with integral multiple of 8 or 6 ns durations between different collisions, which means the spatial and temporal distributions of electron or positron bunches are neglected in the algorithm. The uncertainties caused by the bunch shape and collision time are propagated to the over-all time resolution of TOF system.

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The signal width is determined as the time difference between leading and trailing edges of the analog signal discriminated with a pre-defined threshold value. This “Time-Over-Threshold” (TOT) method, instead of signal amplitude, is used in time walk correction to remove signal propagation delay introduced by the fixed threshold. Fig.28 shows a typical distribution of TOT from Bhabha events. A clear multi-peak structure is observed caused by the mismatching of impedance between the strip and the twisted pair cable. The hit position dependence of the TOT distribution is shown in Fig.29, depicting a clear ringing effect at the trailing edge due to improper termination. Two patterns, horizontal and inclined bands, are observed corresponding to the signal reflections at the same readout end and the opposite end of the strip respectively. The time interval between any adjacent bands in each pattern is a constant, which is equal to twice of the signal propagation time over the entire length of the readout strip. This feature has been involved in the ETOF timing calibration.

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Electron and positron samples in Bhabha events, selected with online event filtering algorithm, are used for the timing calibration of ETOF. The charged track extrapolation from MDC to ETOF is based on a reliable algorithm which calculates the position and momentum of a track for a specific particle hypothesis at any given hit point in outer sub-detectors with the magnetic deflection, ionization loss of the particle and multiple scattering effect taken into account. The hit position along the direction of readout strip with the strip center as the reference point (local coordinate system) of electron or positron in Bhabha events is derived from the extrapolated hit points in the laboratory system [40].

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𝑡ℎ𝑖𝑡−𝑐𝑜𝑟𝑟 = 𝑡𝑙𝑒𝑎𝑑𝑖𝑛𝑔 − 𝑡0 − ∑3𝑖=0 𝑎𝑖 ⋅ 𝑧 𝑖 ,

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where the coefficients 𝑎𝑖 are determined through fitting and 𝑧 is the hit position along the strip. The scattering plot of time difference between the measured raw time with hit position correction and the expected time 𝑡𝑒𝑥𝑝 versus TOT is shown in

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Fig.30. The expect time is calculated as 𝑡𝑒𝑥𝑝 = 𝑐 , where L is the path length, c is

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the velocity of light in vacuum,  = √𝑝2 + 𝑚2 is the velocity of charged particle,

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𝑝 is the MDC measured momentum, 𝑚 is the mass of the particle hypothesis i (electron or positron in TOF calibration). Since the signals are leading-edge discriminated at a fixed threshold, the plot shows a clear TOT dependence of signal propagation delay. Based on our analysis of the correlations between the leading time, TOT and hit position, an empirical formula is constructed for the timing calibration,

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𝑡𝑐𝑜𝑟𝑟 = 𝑝0 +

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𝑝10 ⋅ 𝑧 + 𝑝11 ⋅ 𝑧 2 + 𝑝12 ⋅ 𝑧 3 , (2) where 𝑞 is the TOT of the signal, 𝑝𝑖 (𝑖 = 0,1, … … ,12) are the calibration parameters. 𝑝0 term represents the delay introduced by the cable lengths and by the electronics. A 3-order polynomial, 𝑝10 − 𝑝12 terms, which has almost same function form except the constant item as Equation 1, is used to correct the propagation time of the inductive signal in the strip. The time walk effect correction is applied using the 𝑝1 − 𝑝9 terms which is constructed to describe the dependence of hit position corrected time on the time-over-threshold as shown in Fig.30, the main contribution is

793

𝑝1⁄√𝑞 + 𝑝3 ⁄𝑞 , 𝑝2 and 𝑝4 terms are used to describe the correlation between the

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TOT and hit position.

(1)

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Fig.29 Measured hit position dependence of the MRPC TOT distribution. The measured raw time, 𝑡𝑙𝑒𝑎𝑑𝑖𝑛𝑔 , depends on the hit position because of the finite signal propagation speed along the strip. The hit position correction is expressed as

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𝐿

𝑝1 +𝑝2 ⋅𝑧

+

𝑝3 +𝑝4 ⋅𝑧 𝑞

+ 𝑝5 ⋅ 𝑞 ⋅ 𝑧 + 𝑝6 ⋅ 𝑞 + 𝑝7 ⋅ 𝑞 2 + 𝑝8 ⋅ 𝑞 3 + 𝑝9 ⋅ 𝑞 4 +

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Fig.30 Scattering plot of time difference with hit position correction versus TOT distribution. 2 A 𝜒 minimization method is applied by defining a set of

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𝑟𝑒𝑎𝑑𝑜𝑢𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝜒 2 (𝑟𝑒𝑎𝑑𝑜𝑢𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙) = ∑𝑡𝑟𝑎𝑐𝑘(𝑡𝑚𝑒𝑎 − 𝑡𝑒𝑥𝑝 ) / 𝜎 2

800 801

for each electronics readout channel to extract the calibration parameters. The measured time from a single end of the strip 𝑡𝑚𝑒𝑎 is defined as

802

𝑟𝑒𝑎𝑑𝑜𝑢𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑡𝑚𝑒𝑎 = 𝑡𝑙𝑒𝑎𝑑𝑖𝑛𝑔 − 𝑡0 − 𝑡𝑐𝑜𝑟𝑟 .

803 804

The calibration parameters are obtained for each readout channel using the calibration data sample by setting the derivative of 𝜒 2 with respect to 𝑝𝑖 to zero. The final

805

measured time of the strip 𝑡𝑚𝑒𝑎 is an average of the measured times from the left

806

and right ends of the same strip 𝑡𝑚𝑒𝑎 ,

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𝑠𝑡𝑟𝑖𝑝 𝑡𝑚𝑒𝑎 =

808 809 810

The time differences between the measured times and the expected time for the left and right ends and for the strip are shown in Fig.31. The time resolution of the strip for electron and positron in Bhabha events is about 58 ps.

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2

(3)

(4)

𝐿/𝑅

𝐿 𝑅 𝑡𝑚𝑒𝑎 +𝑡𝑚𝑒𝑎

2

.

(5)

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𝑠𝑡𝑟𝑖𝑝

pro

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Fig.31 The distributions of time differences between the measured time and expected time for left end (a), right end (b) and for the strip (c). A control sample of pion is used for the performance check of ETOF using the calibration constants obtained from electron and positron in Bhabha events. Several processes such as 𝑒 + 𝑒 − → 𝜋 + 𝜋 − 𝜋 + 𝜋 − , 𝑒 + 𝑒 − → 𝜋 + 𝜋 − 𝜋 + 𝜋 − 𝜋 0 , 𝑒 + 𝑒 − → 28

Journal Pre-proof 𝐾 + 𝐾 − 𝜋 + 𝜋 − and so on are used for the event selection of pion sample, and the momentum dependence of time resolution for overall ETOF system is shown in Fig.32. The time resolution in the momentum range 0.75 − 0.85𝐺𝑒𝑉/𝑐 is about 65 ps.

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Fig.32 Momentum dependence of the ETOF time resolution for pion samples.

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6.4 Particle identification with upgraded ETOF

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Particle identification with TOF is applied by comparing the measured time of flight to the expected one from a given particle species hypothesis. A χ is defined in the following way:

827

𝜒=

828

𝑖 where 𝑡𝑒𝑥𝑝 is the expected flight time for a particle species 𝑖, 𝜎(𝑝, 𝑠𝑡𝑟𝑖𝑝) is the

829 830 831 832 833 834

time resolution for the given momentum p and the readout strip. In the endcap region, the number of MDC hits decreases significantly as the value of 𝑐𝑜𝑠𝜃 increasing, thus the extrapolation uncertainty of hit positions introduces the strip number dependence of time resolution. Several effects would contribute to the time resolution for different momenta, such as the ionization, the incident angle, the precision of hit position and expected time.

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𝑖 𝑡𝑚𝑒𝑎 −𝑡𝑒𝑥𝑝

,

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𝜎(𝑝,𝑠𝑡𝑟𝑖𝑝)

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Fig.33 𝐾/𝜋 separation capability for ETOF region. 29

Journal Pre-proof In the 𝜏 − 𝑐 energy region, particle identification, especially 𝐾/𝜋 separation, plays an important role. Fig.33 shows the 𝐾/𝜋 separation capability at the endcap region of BESIII, with the 2σ separation requirement as the red dash line. With an overall time resolution around 65 ps, a lower limit of 1.4 GeV/c for 𝐾/𝜋 2σ separation is achieved in the full ETOF coverage.

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7. Summary

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The Time-of-Flight (TOF) is the key detector in BESIII for charged hadron identification. It is composed of two parts: barrel TOF (BTOF) and endcap TOF (ETOF). The barrel TOF consists of two-layer plastic scintillators readout by fine-mesh Photo-Multiplier Tubes (PMT). A time resolution of 70 ps can be achieved at 1GeV/c for muons even after the barrel TOF has run for about 10 years. In comparison, the old ETOF uses only one-layer scintillation detector and readout by PMT in one end, which leads to a poor time resolution of about 140ps because of the scattering and positioning uncertainty of incident particles. During 2013 to 2015, 72 MRPCs have been successfully produced to build the new ETOF. All the bare MRPCs were carefully selected with strict production quality control. The subsystems of new ETOF, including the HV system and the gas control system, the readout and the trigger electronics, the VME based data acquisition system were successfully integrated into the BESIII with a stable operation after the installation. To achieve excellent time resolution, the upgrade has applied the following technologies and methods: 1) the MRPC with thin gas gaps producing fast signals and 12 layers ensuring the high efficiency; 2) small pads with dual-end readout, whose timing resolution is independent on the uncertainty of positioning; 3) the analog-digital conversion is placed beside the readout pad as close as possible; 4) the electromagnetic noise are shielded well. The total ETOF time resolution, including the uncertainties from colliding time, the tracking and the position uncertainty, has reached 58 ps for Bhabha events and 65ps for pions. The K/π separation capability with 2σ separation has reached a lower momentum range of 1.4 GeV/c.

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Acknowledgements The authors are gratefully acknowledged for the suggestions and helps of Naiyan Wang, Huanqiao Zhang, Yifang Wang, Weiguo Li, Kejun Zhu, Xiaoyan Shen, Zhenan Liu, Kanglin Liu, Huaimin Liu, et al. This work is supported by the National Natural Science Foundation of China (No.10979003, 11675172) and the Repairing and Upgrading Fund for Huge Devices from the Chinese Academy of Sciences. References 1. BESIII Collaboration，Measurement of branching fractions for ψ(3686) → γη′ , γη and γπ0，Phys. Rev. D 96, 052003， 2017.9 2. M. Ablikim et al., Design and construction of the BESIII detector, Nucl. Instr. 30

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: