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Time of flight technology based on multi-gap resistive plate chamber

Wang Yi Zhang Qiu-Nan Han Dong Li Yuan-Jing

Time of flight technology based on multi-gap resistive plate chamber

Wang Yi, Zhang Qiu-Nan, Han Dong, Li Yuan-Jing
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  • Particle identification is very important in nuclear and particle physics experiments. Time of flight system (TOF) plays an important role in particle identification such as the separation of pion, kaon and proton. Multi-gap resistive plate chamber (MRPC) is a new kind of avalanche gas detector and it has excellent time resolution power. The intrinsic time resolution of narrow gap MRPC is less than 10 ps. So the MRPC technology TOF system is widely used in modern physics experiments for particle identification. With the increase of accelerator energy and luminosity, the TOF system is required to indentify definite particles precisely under high rate environment. The MRPC technology TOF system can be defined as three generations according to the timing and rate requirement. The first-generation TOF is based on the float glass MRPC and its time resolution is around 80 ps, but the rate is relatively low (typically lower than 100 Hz/cm2). The typical systems are TOF of RHIC-STAR, LHC-ALICE and BES III endcap. For the second-generation TOF, its time resolution has the same order as that for the first generation, but the rate capability is much higher. Its rate capability can reach 30 kHz/cm2. The typical experiment with this high rate TOF is FAIR-CBM. The biggest challenge is in the third-generation TOF. For example, the momentum upper limit of $ {\rm{K}}/{\text{π}}$ separation is around 7 GeV/c for JLab-SoLID TOF system under high particle rate as high as 20 kHz/cm2, and the time requirement is around 20 ps. The readout electronics of first two generations is based on time over threshold method, and pulse shape sampling technology will be used in the third-generation TOF. In the same time, the machine learning technology LSTM network is also used to analyze the time performance. As a very successful sample, MRPC barrel TOF has been used in RHIC-STAR for more than ten years and many important physics results have been obtained. A prominent result is the observation of antimatter helium-4 nucleus. This discovery proves the existence of antimatter in the early universe. In this paper, we will describe the evolution of MRPC TOF technology and key technology of each generation of TOFs including MRPC detector and related electronics. The industrial and medical usage of MRPC are also introduced in the work finally.
      Corresponding author: Wang Yi, yiwang@mail.tsinghua.edu.cn
    [1]

    Acosta D, Ahn M, Anikeev K, et al. 2004 Nucl. Instrum. Methods Phys. Res. Sect. A 492 605

    [2]

    王景波 2013 博士学位论文 (北京: 清华大学)

    Wang J B 2013 Ph. D. Dissertation (Beijing: Tsinghua University) (in Chinese)

    [3]

    Wang Y, Wang J B, Cheng J P, et al. 2010 Nucl. Instrum. Methods Phys. Res. Sect. A 613 200

    [4]

    Wu J, Bonner B, Chen H F, et al. 2005 Nucl. Instrum. Methods Phys. Res. Sect. A 538 243

    [5]

    Akindinov A, Anselmo F, Basile M, et al. 2000 Nucl. Instrum. Methods Phys. Res. Sect. A 456 16

    [6]

    Williams M C S 1998 Nucl. Phys. B 61B 250

    [7]

    Shao M, Ruan L J, Chen H F, et al. 2002 Nucl. Instrum. Methods Phys. Res. Sect. A 492 344

    [8]

    Anghinolfi F, Jarron P, Krummenacher F, Usenko E, Williams M C S 2004 IEEE Trans. Nucl. Sci. 5 1974

    [9]

    http://tdc.web.cern.ch/TDC/hptdc/docs/hptdc_manual_ver2.2.pdf/ [2018-12-13]

    [10]

    Agakishiev H, Aggarwal M M, Ahammed Z, et al. 2011 Nature 473 353

    [11]

    Boine-Frankenheim O 2010 Proceedings of IPAC’10 Kyoto, Japan, May 23-28, 2010 p2430

    [12]

    Höhne C 2016 PoS 272

    [13]

    Abbrescia M, Peskov V, Fonte P 2018 Resistive Gaseous Detectors: Designs, Performance, and Perspectives (Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA) pp234-235

    [14]

    Wang J B, Wang Y, Zhu X L, et al. 2010 Nucl. Instrum. Methods Phys. Res. Sect. A 621 151

    [15]

    Wang J B, Wang Y, Gonzalez-Diaz D, et al. 2013 Nucl. Instrum. Methods Phys. Res. Sect. A 713 40

    [16]

    Deppner I, Herrmann N, Akindinov A, et al. 2014 JINST 9 C10014

    [17]

    Wang Y, Lyu P F, Huang X, et al. 2016 JINST 11 C08007

    [18]

    Ciobanu M, Herrmann N, Hildenbrand K D, et al. 2008 IEEE Nucl. Sci. Symp. Conf. Rec. Dresden, Germany, October 19—25, 2008 p2018

    [19]

    The GSI Event Driven TDC ASIC GET4 V1.23, Flemming H, Deppe H http://dx.doi.org/10.15120/GR-2014-1-FG-CS-11/ [2018-12-13]

    [20]

    Cebra D, Geurts F, Depper I, et al. 2016 arXiv: 1609.05102 [nucl-ex]

    [21]

    Gao H, Gamberg L, Chen J P, et al. 2011 Eur. Phys. J. Plus 126 2

    [22]

    Wang F Y, Han D, Wang Y, et al. 2018 arXiv: 1812.02912v2 [physics.ins-det]

    [23]

    Wang F Y, Han D, Wang Y, et al. 2018 arXiv: 1805.02833 [physics.ins-det]

    [24]

    Ritt S, 2008 IEEE Nucl. Sci. Symp. Conf. Rec Dresden, Germany, October 19-25 2008, p1512

    [25]

    Guida R, Mandelli B, Rigoletti G 2019 The 15th Vienna Conference on Instrumentation Vienna, 21 February.

    [26]

    Couceiroa M, Blancoa A, Ferreira Nuno C, et al. 2007 Nucl. Instrum. Methods Phys. Res. Sect. A 580 915

    [27]

    Wang J, Wang Y, Wang X, et al. 2016 JINST 11 C11008

    [28]

    Eric O, Jean F G, Herv G, et al. 2013 arXiv: 1309.4397v1 [physics.ins-det]

    [29]

    Wang J H, Liu S B, Zhao L, et al. 2011 IEEE Trans. Nucl. Sci. 58 2011

  • 图 1  几种不同时间分辨飞行时间谱仪系统的$ {\text{π}}/{\rm{K}}$鉴别能力, 飞行距离L = 8 m

    Figure 1.  $ {\text{π}}/{\rm{K}}$ separation power of TOF system with different time resolution, flight distance L = 8 m.

    图 2  MRPC探测器结构示意图

    Figure 2.  The structure diagram of MRPC.

    图 3  STAR-TOF MRPC结构及照片

    Figure 3.  Structure and picture of STAR-TOF MRPC.

    图 4  STAR-TOF tray集成照片

    Figure 4.  Picture of STAR-TOF tray.

    图 5  时间游走原理图

    Figure 5.  Schematic of time slewing.

    图 6  对MRPC的时间幅度信号进行校正, 以修正定时误差

    Figure 6.  Slewing correction of MRPC to improve time precision.

    图 7  STAR-TOF的粒子鉴别图

    Figure 7.  The PID of STAR-TOF.

    图 8  上图和中图是通过STAR-TOF测得的带电粒子质量和能量损失的二维图; 下图是带电粒子质量的一维图, 反氦4核的质量等于3.73 GeV/c2. 利用飞行时间谱仪, 在10亿次碰撞产生的5000亿条径迹中清晰地分辨出18个反氦4物质

    Figure 8.  The top two panels show the dE/dx of charged particles as a function of mass measured by the TOF system; The bottom panel shows the mass distribution of charge particles. The mass of antimatter helium-4 nucleus is 3.73 GeV/c2. 18 antimatter helium-4 nucleus are discriminated from around 500 billion tracks generated by one billion collisions.

    图 9  测试得到的MRPC探测效率和时间分别随粒子计数率的变化[15]

    Figure 9.  Measured efficiency and time resolution of MRPC change with particle rate.

    图 10  CBM-TOF结构

    Figure 10.  The structure of CBM-TOF.

    图 11  MRPC3a探测器照片

    Figure 11.  Picture of MRPC3a.

    图 12  由5个MRPC和相应电子学组成的飞行时间探测器模块

    Figure 12.  CBM-TOF module is consisted of 5 MRPC counters and related electronics.

    图 13  不同PADI阈值下, MRPC3a探测器的时间分辨, 探测效率和簇大小

    Figure 13.  Time resolution, efficiency and cluster size of MRPC3a at different threshold of PADI.

    图 14  同方威视公司密云生产车间正在进行高计数率MRPC的批量生产

    Figure 14.  High rate MRPC were produced at Miyun manufacture base of NUCTECH Ltd.

    图 15  STAR-eTOF的粒子鉴别

    Figure 15.  The PID of STAR-eTOF.

    图 16  高时间分辨MRPC及读出电子学

    Figure 16.  High resolution MRPC and read out electronics.

    图 17  粒子到达MRPC的时间点${t_a}$可以由信号波形前沿得到

    Figure 17.  The time point ${t_a}$ of particle arriving at MRPC can be obtained from pulse shape.

    图 18  用于MRPC时间重建的LSTM网络架构

    Figure 18.  The structure diagram of LSTM network used for time reconstruction of MRPC.

    图 19  模拟得到MRPC探测效率和时间分辨随气隙场强的变化, 可以看出, 采用LSTM网络法重建出的时间分辨比时幅校正得到结果要好

    Figure 19.  Simulated efficiency and time resolution of MRPC change with electric field in the gas gap. It can be seen the time resolution reconstructed with LSTM network is better than with slewing correction.

    图 20  采用LSTM网络方法分析得到MRPC的测试时间谱

    Figure 20.  Time spectrum of MRPC in cosmic test analyzed with LSTM network.

    表 1  三代MRPC飞行时间谱仪性能列表

    Table 1.  Performance of three generation MRPC TOF.

    TOF系统时间分辨
    /ps
    计数率
    /kHz·cm–2
    电极电阻率
    /Ω·cm
    电子学分析方法典型实验
    第一代80< 0.1~1012NINO + HPTDCTOT slewing correctionRHIC-STAR
    LHC-ALICE
    第二代80>20 ~1010PADIX + GET4TOT slewing correctionFAIR-CBM
    第三代20>20 ~1010Fast amplifier + SCATOT slewing correction
    Deep learning
    JLab-SoLID
    DownLoad: CSV

    表 2  低电阻玻璃性能

    Table 2.  The performance of low resistive glass.

    性能参数典型值
    标准尺寸33 cm × 27.6 cm
    体电阻率/Ω·cm~1010
    标准厚度/mm0.7, 1.1
    厚度均匀性/μm20
    表面粗糙度/nm< 10
    介电常数7.5–9.5
    DC测试累积电荷达1 C/cm2
    DownLoad: CSV
  • [1]

    Acosta D, Ahn M, Anikeev K, et al. 2004 Nucl. Instrum. Methods Phys. Res. Sect. A 492 605

    [2]

    王景波 2013 博士学位论文 (北京: 清华大学)

    Wang J B 2013 Ph. D. Dissertation (Beijing: Tsinghua University) (in Chinese)

    [3]

    Wang Y, Wang J B, Cheng J P, et al. 2010 Nucl. Instrum. Methods Phys. Res. Sect. A 613 200

    [4]

    Wu J, Bonner B, Chen H F, et al. 2005 Nucl. Instrum. Methods Phys. Res. Sect. A 538 243

    [5]

    Akindinov A, Anselmo F, Basile M, et al. 2000 Nucl. Instrum. Methods Phys. Res. Sect. A 456 16

    [6]

    Williams M C S 1998 Nucl. Phys. B 61B 250

    [7]

    Shao M, Ruan L J, Chen H F, et al. 2002 Nucl. Instrum. Methods Phys. Res. Sect. A 492 344

    [8]

    Anghinolfi F, Jarron P, Krummenacher F, Usenko E, Williams M C S 2004 IEEE Trans. Nucl. Sci. 5 1974

    [9]

    http://tdc.web.cern.ch/TDC/hptdc/docs/hptdc_manual_ver2.2.pdf/ [2018-12-13]

    [10]

    Agakishiev H, Aggarwal M M, Ahammed Z, et al. 2011 Nature 473 353

    [11]

    Boine-Frankenheim O 2010 Proceedings of IPAC’10 Kyoto, Japan, May 23-28, 2010 p2430

    [12]

    Höhne C 2016 PoS 272

    [13]

    Abbrescia M, Peskov V, Fonte P 2018 Resistive Gaseous Detectors: Designs, Performance, and Perspectives (Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA) pp234-235

    [14]

    Wang J B, Wang Y, Zhu X L, et al. 2010 Nucl. Instrum. Methods Phys. Res. Sect. A 621 151

    [15]

    Wang J B, Wang Y, Gonzalez-Diaz D, et al. 2013 Nucl. Instrum. Methods Phys. Res. Sect. A 713 40

    [16]

    Deppner I, Herrmann N, Akindinov A, et al. 2014 JINST 9 C10014

    [17]

    Wang Y, Lyu P F, Huang X, et al. 2016 JINST 11 C08007

    [18]

    Ciobanu M, Herrmann N, Hildenbrand K D, et al. 2008 IEEE Nucl. Sci. Symp. Conf. Rec. Dresden, Germany, October 19—25, 2008 p2018

    [19]

    The GSI Event Driven TDC ASIC GET4 V1.23, Flemming H, Deppe H http://dx.doi.org/10.15120/GR-2014-1-FG-CS-11/ [2018-12-13]

    [20]

    Cebra D, Geurts F, Depper I, et al. 2016 arXiv: 1609.05102 [nucl-ex]

    [21]

    Gao H, Gamberg L, Chen J P, et al. 2011 Eur. Phys. J. Plus 126 2

    [22]

    Wang F Y, Han D, Wang Y, et al. 2018 arXiv: 1812.02912v2 [physics.ins-det]

    [23]

    Wang F Y, Han D, Wang Y, et al. 2018 arXiv: 1805.02833 [physics.ins-det]

    [24]

    Ritt S, 2008 IEEE Nucl. Sci. Symp. Conf. Rec Dresden, Germany, October 19-25 2008, p1512

    [25]

    Guida R, Mandelli B, Rigoletti G 2019 The 15th Vienna Conference on Instrumentation Vienna, 21 February.

    [26]

    Couceiroa M, Blancoa A, Ferreira Nuno C, et al. 2007 Nucl. Instrum. Methods Phys. Res. Sect. A 580 915

    [27]

    Wang J, Wang Y, Wang X, et al. 2016 JINST 11 C11008

    [28]

    Eric O, Jean F G, Herv G, et al. 2013 arXiv: 1309.4397v1 [physics.ins-det]

    [29]

    Wang J H, Liu S B, Zhao L, et al. 2011 IEEE Trans. Nucl. Sci. 58 2011

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  • Received Date:  13 December 2018
  • Accepted Date:  25 March 2019
  • Available Online:  01 May 2019
  • Published Online:  20 May 2019

Time of flight technology based on multi-gap resistive plate chamber

    Corresponding author: Wang Yi, yiwang@mail.tsinghua.edu.cn
  • Key Laboratory of Particle and Radiation Imaging of Ministry of Education, Department of Engineering Physics, Tsinghua University, Beijing 100084, China

Abstract: Particle identification is very important in nuclear and particle physics experiments. Time of flight system (TOF) plays an important role in particle identification such as the separation of pion, kaon and proton. Multi-gap resistive plate chamber (MRPC) is a new kind of avalanche gas detector and it has excellent time resolution power. The intrinsic time resolution of narrow gap MRPC is less than 10 ps. So the MRPC technology TOF system is widely used in modern physics experiments for particle identification. With the increase of accelerator energy and luminosity, the TOF system is required to indentify definite particles precisely under high rate environment. The MRPC technology TOF system can be defined as three generations according to the timing and rate requirement. The first-generation TOF is based on the float glass MRPC and its time resolution is around 80 ps, but the rate is relatively low (typically lower than 100 Hz/cm2). The typical systems are TOF of RHIC-STAR, LHC-ALICE and BES III endcap. For the second-generation TOF, its time resolution has the same order as that for the first generation, but the rate capability is much higher. Its rate capability can reach 30 kHz/cm2. The typical experiment with this high rate TOF is FAIR-CBM. The biggest challenge is in the third-generation TOF. For example, the momentum upper limit of $ {\rm{K}}/{\text{π}}$ separation is around 7 GeV/c for JLab-SoLID TOF system under high particle rate as high as 20 kHz/cm2, and the time requirement is around 20 ps. The readout electronics of first two generations is based on time over threshold method, and pulse shape sampling technology will be used in the third-generation TOF. In the same time, the machine learning technology LSTM network is also used to analyze the time performance. As a very successful sample, MRPC barrel TOF has been used in RHIC-STAR for more than ten years and many important physics results have been obtained. A prominent result is the observation of antimatter helium-4 nucleus. This discovery proves the existence of antimatter in the early universe. In this paper, we will describe the evolution of MRPC TOF technology and key technology of each generation of TOFs including MRPC detector and related electronics. The industrial and medical usage of MRPC are also introduced in the work finally.

    • 现代物理实验中粒子鉴别是非常重要的, 其中飞行时间谱仪(TOF)在质子, K介子, $ {\text{π}}$介子以及电子的鉴别中发挥了重要作用. 飞行时间谱仪通过测量带电粒子的飞行速度以达到测量粒子质量的目的. 我们知道:

      其中m是粒子质量, L是飞行距离, t是飞行时间. 可以看出, 通过测量粒子的飞行时间, 可以得到粒子质量. 对于具有相同动量p, 质量分别为m1, m2的两种粒子, 其飞行时间差可表示为

      由于$pc = \beta E$, 其中E是粒子能量, 则有:

      由于$p \gg mc$, 则${\left( {{{mc}}/{p}} \right)^4} \to 0$, 因此可以得到下列近似:

      将(4)式代入(3)式, 可以将粒子鉴别能力${n_{{\sigma _{{\rm{TOF}}}}}}$表示为

      其中${\sigma _{{\rm{TOF}}}}$为飞行谱仪系统的时间分辨. 图1显示了不同时间分辨的飞行时间谱仪系统的粒子鉴别能力. 可以得到, 系统时间分辨越高, 粒子鉴别能力越强. 在飞行距离为8 m时, 对于60 ps 的时间分辨, 对动量为4 GeV/c的$ {\text{π}}/{\rm{K}}$鉴别能力可达3σ, 如果要求对于7 GeV/c的$ {\text{π}}/{\rm{K}}$鉴别能力达到3σ, 则时间分辨需达到20 ps.

      Figure 1.  $ {\text{π}}/{\rm{K}}$ separation power of TOF system with different time resolution, flight distance L = 8 m.

      早期的飞行时间谱仪一般采用快闪烁探测器技术[1], 由于存在强磁场, 信号读出通常采用抗磁场的光电倍增管, 虽然系统时间分辨能够达到100 ps量级, 但系统造价较高, 而且闪烁体存在辐照损伤. 20世纪90年代后期, 一种新型的气体探测器—多气隙电阻板室(MRPC)以其优异的时间分辨和相对便宜的造价, 在物理实验的粒子鉴别中得到了广泛应用. 图2是MRPC探测器的典型结构图. MRPC的显著特征有以下几点[2]: 1)采用阻性电极, 电极体电阻率达1010―1012 Ω·cm; 2)采用阻性高压层, 面电阻率达 kΩ―MΩ/$\text{}$; 3)中间极板通过静电感应获得相应电位; 4)工作气体为氟里昂(C2F4H2)等强电负性气体; 5)气隙宽度为100―300 μm, 气隙中场强达105 V/cm量级以上; 6)感应读出, 读出可以为块状或条状.

      Figure 2.  The structure diagram of MRPC.

      MRPC探测器一般工作于雪崩模式, 其时间分辨小于100 ps, 探测效率高于95%, 耐辐照, 性能稳定, 已被众多物理实验(如美国RHIC-STAR, LHC-ALICE等[3-5])采用, 用来建造飞行时间谱仪.

      最初的MRPC采用浮法玻璃研制[6], 由于玻璃电阻率高(约为1012 Ω·cm量级), 适用于粒子计数率较低的环境, 随着加速器能量和实验亮度的提高, 物理实验对MRPC飞行时间谱仪的粒子计数率和时间分辨要求也随之提高, 研制MRPC的电极材料, 读出电子学及时间分析方法都进行了改进. 根据这些方面的不同, 到目前为止, MRPC飞行时间谱仪技术可以归纳为三代, 它们的性能特征及差别如表1所示.

      TOF系统时间分辨
      /ps
      计数率
      /kHz·cm–2
      电极电阻率
      /Ω·cm
      电子学分析方法典型实验
      第一代80< 0.1~1012NINO + HPTDCTOT slewing correctionRHIC-STAR
      LHC-ALICE
      第二代80>20 ~1010PADIX + GET4TOT slewing correctionFAIR-CBM
      第三代20>20 ~1010Fast amplifier + SCATOT slewing correction
      Deep learning
      JLab-SoLID

      Table 1.  Performance of three generation MRPC TOF.

      自2000年以来, 我国的清华大学、中国科学技术大学、中国科学院上海应用物理研究所、华中师范大学以及山东大学等单位先后加入了RHIC-STAR, FAIR-CBM及JLab-SoLID等国际合作组, 在这些合作组相应物理实验的飞行时间谱仪的设计建造、运行刻度及数据分析中, 中国合作组均做出了巨大贡献. 尤其是清华大学和中国科学技术大学在MRPC探测器研制、批量建造、质量控制等方面, 都取得了举世瞩目的成绩, 得到国际同行的认可. 下面以典型物理实验来介绍三代谱仪的组成结构及性能.

    2.   第一代MRPC飞行时间谱仪
    • 典型的第一代飞行时间谱仪是为RHIC-STAR和LHC-ALICE实验所建造的. 以美国布鲁克海文国家实验室的相对论重离子对撞机(RHIC)上的螺旋形径迹探测器(STAR)为例, 其飞行时间谱仪位于中心探测器(时间投影室)的外围, 总面积约60 m2. MRPC的结构及照片如图3所示. MRPC由厚度为0.7 mm的普通浮法玻璃制成, 包含六个气隙, 气隙宽度为0.22 mm. 探测器包含6个读出块, 读出块面积为3.1 × 6.0 cm2. 工作气体为95%的氟利昂和5%的异丁烷的混合气体, 工作场强为106 kV/cm. 束流测试表明, 其时间分辨可达60 ps[7]. 整个TOF由120个tray组成, 每个tray里面有32个MRPC, 因此整个TOF含有3840个MRPC, 电子学通道数为23040. 图4为集成好的tray的照片. Tray一端包括气体, 高压等接头, 顶板上集成有电子学.

      Figure 3.  Structure and picture of STAR-TOF MRPC.

      Figure 4.  Picture of STAR-TOF tray.

      MRPC输出的差分信号经过50 cm的扁平电缆与基于NINOs芯片的差分快前放电路连接, 这样可以有效降低噪声, 提高时间精度. NINO差分放大器是欧洲核子研究中心(CERN)的研究小组采用ASIC技术开发的[8], 已被多个实验所采用, 其主要特性有: 差分输入, 上升时间小于1 ns, 输出为LVDS信号, 信号宽度TOT代表信号电荷大小, 低功耗(45 mW/channel). TDC也是由CERN研制的基于ASIC技术的HPTDC[9], 也已被很多实验采用, 其每道的时间精度达到25 ps.

      系统的定时采用过阈定时方法, 而信号过阈时间与信号上升时间或信号幅度有很大关系. 一般采用时间幅度校正(slewing correction)的方法来修正这个定时误差, 图5显示了由于信号上升时间或幅度不同所造成的时间游走的原理. 时间游走造成了图6所示的时间幅度关系, 可以看出, 幅度越小, 时间游走越大. 一般采用多项式进行slewing correction. STAR飞行时间谱仪从2009年建成以来, 一直稳定运行, 在实验中发挥了重要作用, 取得了多项物理成果. 图7显示了STAR-TOF的粒子鉴别能力, 可以看出, 对$ {\rm{p}}/\left( {{\rm{K}},{\text{π}}} \right)$的鉴别能力达到3 GeV/c. 图8表示STAR合作组于2011年在金金对撞中捕获到的反氦核的信号[10]. 该发现具有重要意义, 证明了宇宙早期反物质的存在.

      Figure 5.  Schematic of time slewing.

      Figure 6.  Slewing correction of MRPC to improve time precision.

      Figure 7.  The PID of STAR-TOF.

      Figure 8.  The top two panels show the dE/dx of charged particles as a function of mass measured by the TOF system; The bottom panel shows the mass distribution of charge particles. The mass of antimatter helium-4 nucleus is 3.73 GeV/c2. 18 antimatter helium-4 nucleus are discriminated from around 500 billion tracks generated by one billion collisions.

    3.   第二代飞行时间谱仪
    • 典型的第二代飞行时间谱仪是德国FAIR[11]上的CBM-TOF[12]. 与RHIC和LHC不同, FAIR的重离子碰撞是固定靶实验, 能量每核子可达40 GeV. CBM (compressed baryonic matter) 实验的研究目标包括高重子数密度区间的QCD相结构、相变线上连接连续相变和一级相变的临界点位置、致密物质中媒质效应对强子性质的影响、寻找理论预言的新相——夸克素物质和物质的奇特态. 根据模拟, CBM飞行时间谱仪中心区域粒子计数率高达20 kHz/cm2. 第一代飞行时间谱仪无法满足CBM-TOF要求. 这对MRPC飞行时间谱仪是一个巨大挑战. 我们知道, MRPC探测器的计数率能力与气隙中的压降${{\bar V}_{{\rm{drop}}}}$有直接关系[13]

      式中${V_{{\rm{ap}}}}$为外加高压, ${V_{{\rm{gap}}}}$ 为气隙中的有效电压, ϕ为粒子计数率, q为雪崩电荷量, ρ为电极体电阻率, d为电极厚度. 可以看出, ${{\bar V}_{{\rm{drop}}}}$与粒子计数率、雪崩电荷量、电极体电阻率和电极厚度均有关系. 但大幅度提高计数率的最有效途径是降低电极体电阻率ρ[13]. 从2008年开始, 我们一直致力于研制性能优良的低电阻玻璃. 通过改进玻璃材料成份, 研究制作工艺, 经过多次实验, 终于研制成功TUYK-LRG10型低电阻玻璃, 玻璃性能如表2所列[14].

      性能参数典型值
      标准尺寸33 cm × 27.6 cm
      体电阻率/Ω·cm~1010
      标准厚度/mm0.7, 1.1
      厚度均匀性/μm20
      表面粗糙度/nm< 10
      介电常数7.5–9.5
      DC测试累积电荷达1 C/cm2

      Table 2.  The performance of low resistive glass.

      由于MRPC电极间的气隙窄, 气隙中的场强高, 因此对电极材料的厚度均匀性、表面光滑度均有很高要求, 我们研制的低电阻玻璃这些主要性能与浮法玻璃接近, 实验证明可以用作MRPC的电极材料. 另外玻璃高压测试累积电荷达1 C/cm2, 这相当于CBM-TOF最高计数率区域工作五年的累积电荷, 能够保证探测器的长期稳定工作. 采用此低电阻玻璃, 我们研制了读出块和读出条的高计数率MRPC原型, 并赴德国德累斯顿Helmholtz-ZentrumDresden-Rossendorf(HZDR)采用其强流电子束流测试了探测器在强束流下的性能, 结果如图9所示. 可以看出, 探测效率和时间分辨均受计数率的影响, 即使计数率达到70 kHz/cm2, MRPC探测器效率仍高于90%, 时间分辨优于80 ps. 原型探测器的性能大大超过了CBM-TOF的要求.

      Figure 9.  Measured efficiency and time resolution of MRPC change with particle rate.

      CBM合作组已采用我们的技术建造飞行时间谱仪系统. 图10为CBM-TOF探测器结构图[16], 其中MRPC1, MRPC2和MRPC3a均采用低电阻玻璃制造, MRPC3b和MRPC4采用超薄浮法玻璃制造, 总面积约120 m2, 电子学道数达10万道. 前端电子学采用PADIX, 时间数字化采用GET4, 电子学的时间抖动小于30 ps.

      Figure 10.  The structure of CBM-TOF.

      清华大学负责了高计数率MRPC3a的设计与制造, 中国科学技术大学负责MRPC3b和MRPC4的设计建造. 这三种探测器的结构类似, 只是电极材料不同. 图11所示为MRPC3a的照片. 该探测器采用两层结构, 每层4个气隙, 共8气隙, 气隙宽度为0.25 mm. 探测器共有32个信号读出条, 读出条尺寸为27 cm × 1 cm. CBM飞行时间探测器模块由五个MRPC组成, 为了减小噪声, 前放PADIX也放置在气盒中, 如图12所示.

      Figure 11.  Picture of MRPC3a.

      Figure 12.  CBM-TOF module is consisted of 5 MRPC counters and related electronics.

      采用束流测试MRPC3a探测器的性能, 结果如图13所示[17]. 可以看出, 探测器时间分辨达50 ps, 效率达97%, 簇大小为1.6. 这些性能均达到或超过CBM-TOF的要求, 可以用于建造CBM-TOF系统.

      Figure 13.  Time resolution, efficiency and cluster size of MRPC3a at different threshold of PADI.

      CBM-TOF的电子学由德国GSI实验室研发, ASIC放大器为PADIX[18], TDC为GET4[19]. 二者组成系统的时间抖动小于30 ps, 保证了CBM-TOF系统的高分辨时间性能. 目前我们已经开始高计数率MRPC的批量生产, 图14显示了在同方威视公司密云生产基地批量生产的照片.

      Figure 14.  High rate MRPC were produced at Miyun manufacture base of NUCTECH Ltd.

      由于CBM实验2025才开始运行, 我们将MRPC首先用于美国RHIC-STAR实验的端部飞行时间谱仪(STAR-eTOF[20])上. STAR-eTOF由36个模块组成. 每模块包含3个MRPC, 总共由108个MRPC组成. 这样一方面可以检验MRPC的性能, 另一方面可以进行STAR的二期能量扫描实验, 取得相应的物理结果. 图15表示2018年STAR实验28 GeV金金对撞的粒子鉴别结果, 可以看出系统对$ {\rm{P}}/\left( {{\rm{K}},{\text{π}}} \right)$的鉴别能力可达3 GeV/c. 至2018年底, STAR-eTOF全部建成, 将在STAR二期能量扫描中发挥重要作用.

      Figure 15.  The PID of STAR-eTOF.

    4.   第三代飞行时间谱仪
    • 第三代飞行时间谱仪的典型要求是在高本底下达到优秀的时间性能. 如美国JLab实验室A实验大厅将要建造的高亮度大接收度谱仪(SoLID)[21]采用11 GeV的高能电子打靶来研究核子结构, 其飞行时间谱仪要求对$ {\rm{K}}/{\text{π}}$的分辨能力达到7 GeV/c, 因此时间分辨要求达到20 ps, 并且本底粒子计数率达到20 kHz/cm2. 这等于是在第二代高计数率飞行时间谱仪的基础上, 进一步提高时间分辨. 我们知道, 整个TOF系统的时间抖动包括MRPC和电子学的时间抖动:

      要使 σTOF小于20 ps,则MRPC的时间抖动σMRPC和电子学系统的时间抖动σelectronics都必须小于 14 ps. 我们知道, 窄气隙MRPC的本征时间分辨可达10 ps, 但是第一\二代飞行时间谱仪所用的电子学 NINO (PADIX) + HPTDC (GET4)的时间抖动一般大于20 ps. 这样必须采用高速波形采样技术如高速开关电容阵列SCA或者高速FADC. 这种技术路线如图16所示. MRPC包含32个气隙, 气隙宽度为104 μm. 高速电流放大器需采用差分输入, 带宽大于350 MHz, 高速波形采样可以采用DRS4-V5芯片, 其采样率可达5 GHz.

      Figure 16.  High resolution MRPC and read out electronics.

      一般地, 根据得到的输出波形, 可以采用常规的过阈定时和时幅校正技术分析MRPC的时间性能. 由于上述方法只利用了波形的过阈时间点和波形积分信息, 忽略了波形上升沿、达峰时间点等关键信息, 因此常规分析方法存在一定局限性. 因此可以采用先进的深度学习方法来得到入射粒子到达MRPC的时间点, 准确地说是入射粒子在MRPC中发生初始电离的时间点. 如图17所示, 可以采用机器学习方法, 从信号波形得到粒子到达MRPC的精确时间点ta. 通过构建深度神经网络, 搭建完整的MRPC蒙特卡罗模拟系统, 为神经网络提供训练样本, 得到粒子入射到MRPC的精确时间点[22,23].

      Figure 17.  The time point ${t_a}$ of particle arriving at MRPC can be obtained from pulse shape.

      为此, 我们建立一套完整的MRPC模拟系统, 从模拟工作气体参数开始, 综合考虑初级电离能量沉积、电离位置分布、电离的雪崩倍增、信号感应与成型以及电子学响应等过程, 模拟得到MRPC探测器的输出信号, 以此作为深度学习的样本.

      采用长短期记忆网络(LSTM)进行学习, 如图18所示. 其输入为信号前沿各时间点, 输出为粒子到达时间点${t_a}$.

      Figure 18.  The structure diagram of LSTM network used for time reconstruction of MRPC.

      图16中提出的32气隙MRPC进行模拟分析, 结果如图19所示. 分别使用时幅校正和神经网络LSTM进行探测效率和时间分辨率的分析, 可以看出, 当使用LSTM方法时, 效率一样, 但时间分辨较好. 两种分析方法得到的效率坪区的时间分辨均优于20 ps. 这证明了设计方案的可行性.

      Figure 19.  Simulated efficiency and time resolution of MRPC change with electric field in the gas gap. It can be seen the time resolution reconstructed with LSTM network is better than with slewing correction.

      同时也进行了实验验证. 研制出图16所示两个结构相同的MRPC探测器, 电子学采用高速前放和基于DRS4[24]的波形采样电路, 采用宇宙射线进行了测试. 工作气体为90%的氟利昂, 5%的异丁烷和5% SF6的混合气体. 气隙中工作场强为150 kV/cm, 达到了效率坪场强. 分别采用时幅校正和LSTM网络方法分析MRPC的时间分辨. 采用时幅校正得到的时间分辨是19.8 ps. 图20是采用神经网络分析的结果, 可以看出, 神经网络分析结果较好, 达到16.7 ps.

      Figure 20.  Time spectrum of MRPC in cosmic test analyzed with LSTM network.

    5.   总 结
    • 本文着重介绍了MRPC飞行时间谱仪的主要技术特点及演变过程. 从MRPC发明之初的百ps时间分辨, 几百Hz/cm2的粒子计数率能力, 发展到现在16 ps的时间分辨, 70 kHz/cm2的高计数率能力, 我们在MRPC探测器的物理机理、电极材料、读出电子学和时间重建方法等方面都进行了深入研究, 取得了丰硕的成果. 随着加速器能量、流强和物理实验要求的提高, 技术上对飞行时间谱仪的要求会越来越苛刻. 除了研究更高计数率、更高时间精度的技术外, 当今国际上对该类阻性电极探测器的研究还有以下几方面热点:

      1)新型环保工作气体研究. MRPC的标准工作气体F134a和SF6的温室效应都比较显著, 寻找替代气体刻不容缓. 目前欧洲核子研究中心的ATLAS和CMS实验组均在进行相关研究[25], 也取得了一定的进展;

      2)在工业及医学方面的应用. 应用于科学的探测器只有得到工业及医学的广泛应用才能获得强大的生命力和技术资金支持. MRPC以其优异的时间性能和位置分辨能力, 在医学正电子湮灭断层成像技术[26]、宇宙射线缪子无损检测技术[27]等方面具有很好的应用前景.

      3)高速波形采样技术. 高速波形采样电子学是充分发挥MRPC优秀时间性能的关键. 一般采用开关电容阵列(SCA)[28]技术和FPGA技术[29]实现高时间精度MRPC的波形数字化. 只有开发出更高采样频率, 更高幅度精度的电子学, 才能尽可能发挥出MRPC本征时间分辨的优势, 提高飞行时间谱仪的粒子鉴别能力.

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