Plastic scintillator detector with the readout based on an array of large-area SiPMs for the ND280/T2K upgrade and SHiP experiments
A. Korzenev, C. Betancourt, A. Blondel, D. Breton, A. Datwyler, D. Gascon, S. Gomez, M. Khabibullin, Y. Kudenko, J. Maalmi, P. Mermod, E. Noah, N. Serra, D. Sgalaberna, B. Storaci
PPlastic scintillator detector with the readout based onan array of large-area SiPMs for the ND280 / T2Kupgrade and SHiP experiments
A. Korzenev , C. Betancourt , A. Blondel , D. Breton , A. Datwyler , D. Gascon ,S. Gomez , M. Khabibullin , Y. Kudenko , , , J. Maalmi , P. Mermod , E. Noah , N. Serra ,D. Sgalaberna , B. Storaci DPNC, University of Geneva, Geneva, Switzerland Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland Laboratoire de L’acc´el´erateur Lin´eaire from CNRS / IN2P3, Orsay, France Institut de Ci`encies del Cosmos, Universitat de Barcelona, Barcelona, Spain Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia Moscow Institute of Physics and Technology, Moscow Region, Russia Moscow Engineering Physics Institute (MEPhI), Moscow, Russia European Organization for Nuclear Research (CERN), Geneva, SwitzerlandE-mail: [email protected]
Plastic scintillator detectors have been extensively used in particle physics experiments for decades. Alarge-scale detector is typically arranged as an array of staggered long bars which provide a fast triggersignal and / or particle identification via time-of-flight measurements. Scintillation light is collectedby photosensors coupled to both ends of every bar. In this article, we present our study on a directreplacement of commonly used vacuum photomultiplier tubes (PMTs) by arrays of large-area siliconphotomultipliers (SiPMs). An SiPM array which is directly coupled to the scintillator bulk, has aclear advantage with respect to a PMT: compactness, mechanical robustness, high PDE, low operationvoltage, insensitivity to magnetic field, low material budget, possibility to omit light-guides. In thisstudy, arrays of eight 6 × area SiPMs were coupled to the ends of plastic scintillator barswith 1.68 m and 2.3 m lengths. An 8 channel SiPM anode readout ASIC (eMUSIC) was used forthe readout, amplification and summation of signals of individual SiPMs. Timing characteristics of alarge-scale detector prototype were studied in test-beams at the CERN PS. This technology is proposedfor the ToF system of the ND280 / T2K II upgrade at J-PARC and the timing detector of the SHiPexperiment at the CERN SPS.
KEYWORDS:
MPPC, SiPM array, Plastic scintillator, Timing detector, ToF, SHiP, ND280, T2K
1. Introduction
A study of timing properties of a detector assembled from long plastic scintillator bars which areread out by silicon photomultiplier (SiPM) arrays is presented. SiPMs are widely employed in highenergy physics detectors. The fast evolution of the SiPM market opens new possibilities. Large-areasensors are nowadays available in the market at a reasonable price. When assembled in a 2D arraythey can cover a sizeable area, therefore they can be considered as a promising direct replacementfor traditional photomultiplier tubes (PMTs). In particular, the sensors can be coupled directly to ascintillator bulk to provide a time resolution on a sub 100 ps level [1]. An obvious advantage of anSiPM array is that it can take a form of the bar cross-section, thus avoiding complex shape adiabaticlight-guides. 1 a r X i v : . [ phy s i c s . i n s - d e t ] J a n AQ module: 64ch SAMPICReadout by SiPM-arrays c m c m c m c m c m c m c m c m Fig. 1.
A front view of the prototype detector which was assembled in summer 2018. Two insertson the left-hand side represent a zoomed view of the eMUSIC readout board ( top ) and the 64 channelSAMPIC data acquisition module ( bottom ).A large SiPM capacitance increases the rise time and width of a signal and thus worsens the timeresolution. In this regard, a large monolithic sensor or many smaller sensors with common cathode andanode cannot be employed. An improvement can be achieved by using an independent sensor readoutto isolate the sensor capacitances from each other. Signals can be amplified and summed afterwardseither by a discrete circuit [2] or by an ASIC without the drawback of adding the sensor capacitancesat the input. The eMUSIC ASIC [3] was adopted as an input stage of the acquisition system.A detector configuration where SiPM arrays are coupled to long plastic scintillator bars, signalsof SiPMs are read out by the eMUSIC chip and, finally, analog signals are digitized by a Waveformand TDC converter (WTDC) is proposed for the time-of-flight (ToF) system of the ND280 / T2K IIupgrade [4] at J-PARC and the timing detector of the SHiP experiment [5] at the CERN SPS.
2. Large-scale prototype
A 22 bar prototype, shown in Fig. 1, was assembled and exposed for one month to test beamsof the CERN PS in summer 2018. In addition to the test of operating performance of the prototypeitself it was also used as a time-of-flight detector for the identification of particles with momenta up to6 GeV / c . The particles were identified via measurements of the time di ff erence between the prototypeand a beam counter, S1, installed 10.9 m upstream.The choice of scintillator material was driven by the need to achieve precision timing by detectingas many photons as possible for interactions occurring all along the full length of a bar. The scintillatorEJ-200 provides an optimal combination of a high light output, suitable optical attenuation length ofabout 4 m, and fast timing (rise time of 0.9 ns).An array of 8 surface-mount devices S13360-6050PE (area 6 × , pixel pitch 50 µ m) fromHamamatsu has been mounted to a custom-made PCB, as shown in Fig. 2 (left). A charge produced inthe array was sent via a high-density connector to a general purpose mini-board which was based on2 MUSIC evaluation board
MUSICASIC micro-controller
DAQ moduleSAMPIC 64ch analog SE-signal SP I bu s U S B i n t e r f ace SP I bu s S i P M c onn ec t o r S i P M c onn ec t o r Laptop P CB w i t h S i P M s LV + HVPowersupply
USB toTTLadapterSPI bussplitter eMUSIC boardsUART bus USB2 interface L V r e gu l a t o r P o w e r c onn ec t o r c onn ec t o r c onn ec t o r c onn ec t o r c onn ec t o r c onn - ec t o r c onn - ec t o r Fig. 2.
A block diagram of the eMUSIC mini-board profiles ( left-hand side ) and a schematic represen-tation of a data flow ( right-hand side ).the eMUSIC chip [3] providing either an individual SiPM readout or an analog sum of all eight SiPMchannels. The outputs of the chip could be easily monitored via coaxial RF connectors as a di ff erentialor single-ended signals. The eMUSIC chip has several configuration parameters accessible via an SPIprotocol, i.e. a tunable pole-zero cancellation providing output signals with less than 10 ns FWHM;two di ff erent gain options; a bias voltage of every SiPM could be controlled using an internal DACwith 1 V dynamic range; any of channels could be powered-o ff .A block diagram of the eMUSIC mini-board is presented in Fig. 2. There are two ways to modifyand upload parameters to the ASIC.1. A limited programmability profile (UART). In this case, the eMUSIC board has to be connectedto a PC via an external UART-to-USB adapter. An in-board microcontroller is used as a bridgeto send configurations to a non-volatile memory of EEPROM. The microcontroller dumps thisconfiguration to the eMUSIC chip as soon as the board has been switched on.2. A full programmability profile (SPI). In this mode, the embedded microcontroller is bypassedand the external master takes control of the SPI bus. This profile is suitable for applicationswhere a frequent reconfiguration of the eMUSIC chip is required. In this mode, the control istaken by an FPGA of the DAQ module. An SPI bus from multiple eMUSIC mini-boards ispropagated to the DAQ module using SPI-splitter boards.A 64 channel SAMPIC module [6] was used to digitize the signals (see Fig. 1). SAMPIC is a16 channel ASIC implementing a novel type of digitizing electronics which performs both the func-tion of a TDC and a waveform sampler based on a switched capacitor array. Having the waveformsrecorded, one can extract various kinds of information such as baseline, amplitude, charge and time.The SAMPIC circular bu ff er contains 64 cells which makes possible to cover a 20 ns time window atthe sampling frequency of 3.2 GS / s. In addition to the TDC, the ASIC contains one on-chip ADC percell which makes the charge digitization particularly fast. Each SAMPIC channel integrates a discrim-inator which can trigger itself independently of other channels. It is an important feature for neutrinoexperiments which do not have triggers induced by incoming particles.The time resolution of bars with dimensions 230 × × and 168 × × is shown inFig. 3. The technique of the measurements is described in Ref. [1]. The resolution evolves from 80 psfor the crossing point located near the sensor, to 310 ps (230 cm bar) and 180 ps (168 cm bar) for thecrossing point located at the opposite end of the bar to that of the sensor, due to light propagation alongthe full length of the bar. The resolution, calculated as a weighted mean between SiPM-arrays located3
50 100 150 200 x [cm] T i m e r es o l u t i on [ n s ] x [cm] T i m e r es o l u t i on [ n s ] Fig. 3.
Time resolution as measured by 8-SiPM arrays attached at both ends of a 230 × × bar of ToF detector of ND280 ( left ) and a 168 × × bar of timing detector of SHiP ( right ) as afunction of the interaction point along the bar. Arrays of eight SiPMs fixed inside of light-tight casesare shown on top of corresponding figures. The sensitive area of each SiPM is 6 × .at two ends of each bar, makes the distribution more constant, i.e. 130 ps and 85 ps on average for twobars, respectively.Some results obtained with the 22 bar prototype are presented in Fig. 4. The primary goal was toidentify particles by calculating the time di ff erence detected by the S1 counter and the prototype. Alonger time was required by heavier particles to traverse this distance which could serve to identify thespecies. As an example, protons arrive at the prototype about 20 ns later as compared to positrons at0.8 GeV / c . This interval shrinks to 3.8 ns at 2 GeV / c .An ionization produced by charged particles and, in turn, an amount of optical photons generatedin de-excitation processes depends on particle masses and is quite di ff erent at low momenta. Thee ff ect can be observed in the amplitude of recorded signals as shown in Fig. 4 (left). Amplitudes ofthe deuteron and proton signals ranges around 300 – 400 mV whereas signals of e, µ and π are spreadfrom 50 to 400 mV. This information can also be used for the particle identification.The technology described in this article has been proposed for two time measuring detectors whichare presented in the following sections. Both detectors are located in zones of strong magnetic field.
3. Time-of-flight detector for the ND280 / T2K II upgrade
The goal of the T2K experiment is to study neutrino flavor oscillations employing an o ff -axis neutrinobeam from the J-PARC accelerator facility. The near detector of T2K, ND280, is used to study neutrinointeractions aiming for neutrino cross-section measurements and reduction of systematic uncertaintiesin neutrino oscillation analyses.A layout of the ND280 detector proposed for the upgrade of T2K [4] and the ToF detector itselfare shown in Fig. 5. The ToF system will consist of 6 planes surrounding the active target [7] and twoTPCs. Each plane will have about 5 m surface area. The ToF aims at precise measurements of thecrossing time of charged particles as they exit or enter the TPCs. A time resolution of 500 ps or better isrequired for an unambiguous determination of the flight direction of charged particles. An additionalgoal is to improve particle identification, requiring the time resolution of 100 – 200 ps. The wholesystem will be located inside the UA1 magnet which will create a field of 0.2 T. The presence of themagnetic field and a very limited space for the detector makes the use of the SiPM readout particularly4 [ns] S1 -t ToF t - - - A m p li t ude [ V ] Deuteron proton p , m e, p = 0.8 GeV/c [ns] S1 -t ToF t - - - E v en t s p = 0.8 GeV/c proton 19.7nsDeuteron 56.3nspion 0.5ns p = 0.8 GeV/c [ns] S1 -t ToF t - - - E v en t s p = 2.0 GeV/c proton 3.8nsDeuteron 13.5nsKaon 1.1ns p = 2.0 GeV/c Fig. 4.
Left : a correlation plot presenting a signal amplitude versus time required by 0.8 GeV / c particlesto traverse a 10.9 m distance between the beam counter S1 and the ToF prototype. Right : a timerequired by 0.8 GeV / c and 2 GeV / c particles to traverse the distance between S1 and the ToF prototype.advantageous. The detector will be assembled from 118 bars with a length of 2 to 2.3 m which willprovide a time resolution of approximately 150 ps along the bar as shown in Fig. 3 (left). In total, thenumber of DAQ channels is 236 and the number of SiPMs is 1888.
4. Timing detector of SHiP
SHiP is a new general-purpose experiment [5] proposed for installation at a beam dump facility ofthe CERN SPS to search for hidden particles as predicted by a very large number of recently elaboratedmodels of Hidden Sectors which are capable of accommodating dark matter, neutrino oscillations, andthe origin of the full baryon asymmetry in the Universe.The SHiP spectrometer and the timing detector are shown in Fig. 6. The timing detector willbe positioned downstream of the vacuum decay vessel and will cover a 5 ×
10 m area. The mainpurpose of the detector is a reduction of the combinatorial background (vertices made by a randommuon crossing) by tagging particles belonging to a single event. The time resolution is required to bebetter than 100 ps. The detector can also be used for identification of few GeV particles. The 100 psresolution constraint limits the possible bar length to approximately 2 m. Therefore an array of 3columns and 182 rows assembled from 167 cm long bars is considered as a base option for the presentdesign of the detector. Altogether, it results in 546 bars, 1092 channels and 8736 SiPMs. The test-beammeasurements for a single bar resulted in about 85 ps time resolution as shown in Fig. 3 (right).
5. Conclusions
A study on a direct replacement of vacuum photomultiplier tubes by arrays of large-area silicon pho-tomultipliers has been presented. Arrays of SiPMs were coupled to the ends of long plastic scintillatorbars. An 8 channel chip, eMUSIC, was used for the readout, amplification and summation of signals ofindividual SiPMs. A 64 channel SAMPIC module was used for the data acquisition. Results obtainedin test-beams with a 22 bar prototype detector have been presented. This technology is proposed for theToF system of the ND280 / T2K II upgrade at J-PARC and the timing detector of the SHiP experimentat the CERN SPS. 5 . m T P C T P C T o F basketmagnet t a r g e t Fig. 5.
Layout of the ND280 detector proposed for the T2K II upgrade, with magnets opened such asto see the inner basket ( right ). The part of the basket to be upgraded is shown on the left . It includesthe active target and two TPCs, all surrounded by 6 ToF planes.
Beam dump (target) Active muon filter spectrometer Hidden Sector spectrometerVacuum vessel ~ m Timing Detector
Fig. 6.
Left : the SHiP experimental facility.
Right : schematic view to the SHiP timing detector whichwill be located downstream of the vacuum vessel, in front of an electromagnetic calorimeter.
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Application of large area SiPMs for the readout of a plastic scintillator based timingdetector , JINST (2017) P11023, [ ].[2] W. Jung et al., Development of TPC Trigger Hodoscope for J-PARC E42 / E45 hadron experiment , in
International Workshop on New Photon Detectors (PD18): Tokyo, Japan, November 27-29, 2018 , 2018.[3] S. Gomez et al.,
MUSIC: An 8 channel readout ASIC for SiPM arrays , in
Proceedings, Optical Sensing andDetection IV, SPIE Photonics Europe, 2016, Brussels, Belgium , vol. 9899, 2016, DOI.[4] K. Abe et al.,
T2K ND280 Upgrade - Technical Design Report , .[5] M. Anelli et al., A facility to Search for Hidden Particles (SHiP) at the CERN SPS , .[6] E. Delagnes et al., The SAMPIC Waveform and Time to Digital Converter , in / MIC) , (Seattle, US), Nov., 2014, http: // hal.in2p3.fr / in2p3-01082061.[7] O. Mineev et al., Beam test results of 3D fine-grained scintillator detector prototype for a T2K ND280neutrino active target , ..