fast detectors for the mpd/nica time-of-ﬂight system

Bulgarian Chemical Communications, Volume 47, Special Issue B (pp. 215–221) 2015

Fast detectors for the MPD/NICA time-of-flight systemV. Babkin, S. Bazylev, O. Batenkov, P. Dulov∗, V. Golovatyuk, S. Lobastov, V. Petrov,

M. Rumyantsev, A. Schipunov, A. Shutov, I. Slepnyov, V. Slepnyov, A. Veschikov, S. Volgin,V. Yurevich

Laboratory of High Energy Physics, Joint Institute for Nuclear Research6 Joliot-Qurie Str., 141980 Dubna, Russia

The Multi-Purpose Detector (MPD) [1] is being developed to study properties of hot and dense baryonic matter in collisions oflight and heavy ions on the collider NICA which is under construction at JINR, Dubna [2]. Identification of charged particles in therange 0.1–2 GeV/c is carried out with Time Projection Chamber (TPC) in solenoid magnetic field and time-of-flight (TOF) system. Ithas been decided to use MRPCs with strip and pad readouts as a basic element of the TOF detector. Beam tests results of full scalemRPC prototypes with readout electronics are described in this article. As a start detector for the time-of-flight system we used detectorbased on microchannel plate photomultipliers with the time resolution of ∼ 37 ps.

Key words: MPD, MRPC, particle identification, ToF, FFD, fast electronics, transmition line

INTRODUCTION

The MultiPurpose Detector (MPD) is proposed fora study of hot and dense baryonic matter in collisionsof heavy ions over the atomic mass range A = 1 – 197at a centre-of-mass energy up to

√sNN = 11 GeV (for

Au+78) and luminosity L = 1027m−2c−1. The MPDexperiment is foreseen to be carried out at acceleratorcomplex facility for heavy ions – the Nuclotron-basedIon Collider fAcility (NICA) which under construc-tion now at JINR, Dubna.

The MPD consits of a barrel part and two end-caps located inside the magnetic field. The barrelpart is a shell-like set of various detector systems sur-rounding the interaction point and aimed to recon-struct and identify both charged and neutral particlesin the pseudorapidity region of | η |< 1.2. The end-caps are aimed for precise tracking and particle iden-tification over pseudorapidity range (1.2 <| η |< 2).The principal tracker is the time projection chamber(TPC) supplemented by the inner tracker (IT) sur-rounding the interaction region. Both subdetectors(IT and TPC) has to provide precise track finding, mo-mentum determination, vertex reconstruction and pat-tern recognition. The TPC and IT together with timeof flight system have to provide charged particles mo-mentum measurement.

Due to the fact that in most recent experimentsin high energy physics such as Alice(LHC) andSTAR(RICH) time-of-flight systems are based on

∗ To whom all correspondence should be sent:[email protected]

multi-gap resistive plate chambers (MRPC) [3] weresuccessfully used in the TOF systems, we decidedalso to use MRPCs on the base of flat glass forthe time-of-flight system of the MPD. This articlepresents the results of the tests of MRPCs on thedeuteron beam of Nuclotron at JINR of different de-sign and readout electrode geometry [4].

DESIGN OF MRPC PROTOTYPES

Two types of MRPCs were considered: with thepad signal readout and with the strip readout. Bothoptions have their special traits in terms of assem-bling and operation. But at low multiplicity of events,it makes sense to use a strip electrode for readouts. Itreduces the number of channels making the systemmore cost-effective. Therefore, strip MRPCs havebeen chosen for the barrel part of the MPD. At theend cup parts of the detector, there will be combinedpad and strip readouts.

The pad prototype (Fig. 1) consists of two stackshaving 5 gaps each. A 210 µm gas gap is formedby a ordinary fishing line of appropriate thick-ness. The readout electrodes are fiberglass boards of140×120 mm2 with etched rectangles of 16×35 cm2.They make up in total two rows of 8 pads in each.In order to supply high voltage to glasses closest tothe readout electrodes, they are coated with a graphiteconductive layer with a surface resistance of 2 –10 MOhm per 1 square. Signals from the pads aretransferred via the twisted pair cable to the amplifier.The detector is situated in a leak-tight box in whicha gas mixture (C2H2F4 – 90%/iC4H10 – 5%/SF6 –5%) is injected.

© 2015 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria 215

V. Babkin et al.: Fast detectors for the MPD/NICA time-of-flight system

Fig. 1. Schematic view of a pad MRPC.

Full-scale strip MRPC prototypes were developedand produced for the barrel part of the time-of-flightsystem. The readout electrodes of these prototypeswere designed as thin narrow strips. The sectionalview of this prototype is shown in Fig. 2. The activeregion area of the MRPC was determined by the sizeof the glass and amounts 590×290 mm2. The designalmost completely duplicated the one of pad detec-tors. Three different strip MRPCs were assembled forthe first test run. A single-stack MRPC prototype wasassembled for the first test run at the Nuclotron beam

having 5 gas gaps with the thickness of 250 µm and24 strips with the size of 600×10 mm2. For the sec-ond run the design of the strip detector was changedand two prototypes were assembled: a single-stackprototype having 6 gas gaps of 220 µm and 48 stripsof 600× 5 mm2 and a double-stack MRPC with 12gas gaps of 220 µm (6 per a stack) and the same stripsthat the single-stack prototype had. Such a designmodification was fulfilled based on the first experi-ment’s results.

Fig. 2. Sectional view of the full-scale strip MRPC prototype.

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FFD START DETECTOR

The signal from the Fast Forward Detector (FFD)will be used as a start signal for the time-of-flight sys-tem [5]. The Fast Forward Detector is a double-armCherenkov detector for high energy photons. Eacharm of the detector consists of 12 modules laid in theshape of square around the beam pipe. The arms areplaced 75 cm off the expected interaction point onboth sides along the beam axis. The main functionsof the detector are the following:• generation of the fast high-precision start signal

for time-of-flight measurements;• reconstruction of the interaction point along the

beam axis with the accuracy of ±1.5 cm;• production of signal for the L0-trigger system

with selection of nuclei collisions occurringclose to IP.

Each module of the FFD is a sequential assem-bly of the following elements: a 7–10-mm thick leadplate; a 15-mm thick quartz radiator; a micro-channelXP85012Q photomultiplier by Photonis; front-endelectronics and a high voltage divider. High energyphotons are converted into relativistic electrons in thelead plate. These electrons entering the quartz radi-ator emit Cherenkov light which is registered by thephotomultiplier. The active area of the photocathodeof the XP85012Q photomultiplier, which is sensitiveto the visible and ultraviolet regions of wave lengths(170–670 nm), is 53×53 mm2 and covers 81% of thefront area of the photomultiplier. The photomultiplierhas multi-anode structure of 8×8 anodes; its gain ofthe order of ∼ 105–106 depends on the applied highvoltage value, good linearity of the output signal withthe leading edge of 0.6 ns and the transit time spredof σ ∼ 37 ps [6].

Each module has 4 quartz radiators of 3×3 cm2

which are light-isolated from each other. There are4× 4 anodes of the photomultiplier per each quartzradiator; signals from them are added together andtransferred to the front-end electronics. The front-endelectronics produce analog and digital LVDS signalsand its width informs of the pulse height. Thus, eachmodule has 4 independent channels.

READOUT ELECTRONICS

In order to increase the differential signal acceptedup from the MRPC’s pads and strips, a 24-channelamplifier on the base of a NINO chip was used [7],which was specially designed for the time-of-flight

system of ALICE and now is the most common am-plifier for time-of-flight MRPCs. This amplifier hasa highly rapid output signal (<1 ns) and small jitterof the output signal (< 10 ps), which is provided bythe low noise level of the amplifier. Another charac-teristic feature of the NINO chip is the possibility tochange input resistance in order to match it with thereadout electrodes. A discriminator with the minimaltriggering threshold of about 25 fC is imbedded intothe NINO chip. The discrimination threshold was setat 75 fC for all the conducted experiments. The out-put signal from the discriminator is LVDS-standarddifferential signal. The leading edge of the pulse cor-responds to the time of its passing through the thresh-old, and its width corresponds to the amplitude of theinput pulse, which allows one to make time-amplitudecorrection with the use of only one TDC.

32-channel time-to-digital converters (TDC32VL)based on HPTDC chip were developed and producedfor digitization of LVDS signals and data acquisi-tion [8].

Experiments on the MRPC time resolution eval-uation conducted in March of 2011 showed that in-tegral nonlinearity occurs with TDC operating in themode of 24.4 ps per bin as a result of cross talk tochannels of the signals from clock speed generatorsin the HPTDC chip. This nonlinearity causes strongdegradation of time distribution. The contribution ofthis nonlinearity and the method to eliminate it has al-ready been shown in the HPTDC Manual [9] as wellas in other publications [10, 11].

The method of uniform filling the time gap withrandom events (code density test) was used forcalibration (consideration of nonlinearity) of VMETDC32VL module. Minor differential (Fig. 3 (a))and strong integral (Fig. 3 (b) nonlinearity in terms oftime were observed distracting the measured value for8 bins (∼ 200 ps) from the real time, which deterio-rated significantly the time resolution of the electron-ics. Calibration matrices were developed for furtherdata processing for each channel of the TDC allowingfor elimination of the integral nonlinearity contribu-tion. Native time resolution of a TDC32VL channelmade up about 25 ps after applying the calibrations.

TESTING THE PROTOTYPES ATTHE ACCELERATOR BEAM

Tests of the prototypes were carried out at the Nu-clotron deuteron beam at LHEP JINR in December2012 and March 2013. The deuteron beam with the

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(a) (b)

Fig. 3. Differential (a) and integral (b) nonlinearity of HPTDC.

energy of 1 GeV/n and spill duration of 2 s was in-jected into Building 205 and with the use of mag-net optics was sent to the experimental setup (Fig. 4).A scintillation telescope made up of C1-C5 counterswas used for trigger signal obtaining. At that, C5counter corresponded in size to the FFD prototype’seffective surface and was placed immediately beforeFFD-1. The following MRPC prototypes were tested:two identical double-stack pad prototypes (PRPC)and three full-sized strip prototypes (SRPC).

Two identical FFD prototypes were studied in theexperiments. The detectors were placed one after an-other along the beam line with the minimal gap be-tween them. The signal from one of the detectors wasconsidered as “start”, and the signal from another – as“stop”.

Fig. 4. Layout of the experimental setup. S1–S5 – a scintil-lation counters; FFD-1, FFD-2 – start detectors (FFD pro-totypes); PRPC – pad prototypes; SRPC – strip prototypes,VME – a crate with the readout electronics.

The detectors FFD1 and FFD2 was used as a startdetector T0 in all the experiments and had the timeresolution of 37 ps with TDC32VL, as it was mea-sured early [5]. This value was used later on for cal-culation of the time resolution of MRPCs.

The dependencies of efficiency and time resolu-tion on voltage for pad chambers and a single-stackchamber with a 10 mm-wide strip were measuredin the experiment in December 2012. Time differ-ence between the T0 detector and PRPC was mea-sured without any corrections. Time distribution forPRPC is worse than 120 ps in this case. The LVDSpulse width corresponds to the height of the signal.Therefore the time-over-threshold method of correc-tion was used. Approximation of the distribution oftime depending on the width of the signal allows oneto obtain the ratio for correction. When the time-over-threshold dependence was taken into consideration,time resolution improved significantly. The same pro-cedure was done for the SRPC. The best time resolu-tion came to 60 ps for PRPC and 80 ps for the SRPCin December 2012 experiment was achived.

Fig. 5 demonstrate the dependencies of the ef-ficiency and time resolution of the first prototypesversus voltage. The detector operated in the optimalmode as the maximal resolution could be achievedat the efficiency of about 98%. In contrast to the padMRPC, the efficiency of the first prototype of the stripMRPC made up 80% at the best time resolution andit didn’t exceed 93% even at the voltage increasedup to 16 kV. Such degradation of the time resolutionat the low efficiency was associated with the contri-

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(a) (b)

Fig. 5. Time resolution and efficiency for PRPC (a) and SRPC (b) in first experiment.

bution of the signal reflection from the edge of thestrip. The reflection was caused by bad matching ofthe strip (which impedance is about 75 Ohm, accord-ing to the calculations and measurements) with cableand amplifier. An 8-cm long ribbon twisted pair ca-ble with the impedance of 110 Ohm was used. Forthis reason splitting was observed of the width spec-trum of the output LVDS signal. As a consequence,the information on the actual pulse height was dis-torted. Such splitting may be caused by interferenceof the reflected signal with the trailing edge of themain pulse. It is supported by the fact that the posi-tion of the second width peak and its value dependson the coordinate of the particle passing the detectoralong the strip (Fig. 6).

A similar problem arose during the work withthe MRPC with a long strip [12]. In order to avoidsplitting of the width spectrum, the strip is matched

with the amplifier by adjusting the input impedanceof the amplifier. According to the manual, the inputimpedance of the amplifier can be changed within therange of 20 – 100 Ohm. In our case, such a methodhardly influenced the width spectrum and it was de-cided to change the design of the detector and the wayof accepting the signal because the main reflectiontake place in the point of strip-cable.

Two new MRPC prototypes with strip readoutswere assembled for the experiment at the Nuclotronbeam in March 2013. The first prototype representeda single-stack six-gap chamber with the gap width of220 µm and a 5-mm wide and 600-mm long read-out strip. The active area of the detector remained ofthe same size of 290× 590 mm2, however the num-ber of channels per one detector increased twice. Aspulse generator tests demonstrated, this constructionwas well matched with the signal transmission line.

(a) (b) (c)

Fig. 6. Splitting of the width spectrum of the SRPC (10 mm) LVDS signal as a function of the coordinate. (a) shifted by7.5 cm from the strip center; (b) in the center of the strip; (c) shifted by +7.5 cm from the center.

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(a) (b) (c)

Fig. 7. Spectra of the single-stack MRPC (5 mm) prototype with the strip readout and a matched strip for particles passedthrough different points of the strip: (a) shifted by 7 cm from the strip center; (b) in the center strip; (c) shifted by +7 cmfrom the strip center.

It was discovered in the course of the experiment thatthe width spectrum splitting was almost absent withsuch a configuration (Fig. 7).

Calculations predicted the efficiency of the single-stack chamber will not exceed 90%. Therefore, adouble-stack MRPC was assembled with the samestacks as the described single-stack one. The resultson the efficiency and time resolution of the single- anddouble-stack strip MRPC prototypes obtained fromMarch’s run are shown in Fig. 8 (a).

As it was expected, the efficiency of the single-stack prototype made up 92%. In this case the besttime resolution was in the limit of 70 ps. The effi-ciency of 99.9% and time resolution of about 65 pswere achieved for the double-stack version. An im-portant feature of the two-ends strip readout is possi-

bility to position quite precisely the particle passingalong the strip. Using the time difference between ar-rivals of signals from the left and the right ends ofthe strip, it is possible to reconstruct the point of theparticle transit. The position resolution in this case isabout 5 mm.

It was suggested that due to non-symmetric loca-tion of strips in the double-stack SRPC, degradationof the pulse front edge might occur and, as a result,the time resolution might decrease. In order to provethis effect, the dependence was measured of the timeresolution on the point of the particle transit throughthe strip (Fig. 8(b)). It is shown that the time reso-lution remains almost the same with the coordinatebeing changed. I.e. dispersion has little impact on thesignal.

(a) (b)

Fig. 8. (a) test results for the MRPC prototype with a 5-mm wide strip. (b) time resolution of the double-stack SRPC as afunction of the coordinate of a particle’s flight.

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CONCLUSIONS

The optimization of the full-sized MRPC proto-type design was fulfilled for the time-of-flight systemof the MPD. The two experiments at the Nuclotronbeams allowed us to reveal imperfections of the de-tectors’ design and to develop the MRPCs optimal forthese conditions.

The results obtained for the double-stack MRPCwith the strip readout and for the double-stack MRPCwith the pad readout meet the requirements to thetime-of-flight system of the MPD. Further investiga-tions will be aimed at the optimization of the readoutand front-end electronics. The matching will be per-formed for strips of various configurations, which willallow for reduction of the total number of the elec-tronics channels.

REFERENCES

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[10] Daniel McDonald, Calibration of the STAR Time-of-Flight Detector for Particle Identication, MS degreethesis, Houston, Texas, December 2010.

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ВРЕМЕ-ПРЕЛИТНА СИСТЕМА (TOF) НА ДЕТЕКТОРА MPD (NICA)

П. Дулов1, В. Бабкин1, М. Румянцев1, В. Галватюк1, В. Чолаков2

1 Лаборатория по високи енергии, Обединен институт за ядрени изследвания,ул. “Академик Балдин”№ 4, 141980 Дубна, Московски регион, Русия

2Физически факултет, Пловдивски университет “Паисий Хилендарски”,ул. “Цар Асен”№24, 4000 Пловдив, България

(Резюме)

Концептуалният дизайн на многоцелевия детектор MPD (Multi-Porpose Detector) е създаден за изучаването на горещата иплътна барионна материя от сблъсъците на тежки йони с атомна маса в диапазона A = 1–197 с максимална енергия 11 GeV заAu79+.

MPD експеримента ще се извърши в Обединения институт за ядрени изследвания, Дубна в новостроящия се колайдер NICA(Nuclotron-based Ion Collider fAcility) при средна яркост L = 1027 cm−2s−1.

Идентификацията на заредените адрони с междинни импулси (0,1-2) GeV се постига с измерване на времето за прелитанеот TOF детектора. Основен елемент на TOF е mRPC (Multi-gap Resistive Plate Chambers) – многопроцепна

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fast detectors for the mpd/nica time-of-ﬂight system

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