Performance of compact plastic scintillator strips with WLS-ﬁber and PMTSiPM readout Min Li1 2Zhimin Wang1yCaimei Liu1 2Peizhi Lu3Guang Luo3Yuen-Keung Hor3Jinchang Liu1and Changgen Yang1 1Institute of High Energy Physics Beijing 100049 China

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Performance of compact plastic scintillator strips with WLS-ﬁber and PMT/SiPM readout∗

Min Li,1, 2 Zhimin Wang,1, †Caimei Liu,1, 2 Peizhi Lu,3Guang Luo,3Yuen-Keung Hor,3Jinchang Liu,1and Changgen Yang1

1Institute of High Energy Physics, Beijing 100049, China

2University of Chinese Academy of Sciences, Beijing 100049, China

3Sun Yat-sen University, Guangzhou 510275, China

This work presents the design and performance study of compact strips of plastic scintillator with WLS-ﬁber

readout in a dimension of 0.1×0.02 ×2m3, which evaluates as a candidate for cosmic-ray muon detector

for JUNO-TAO. The strips coupling with 3-inch PMTs are measured and compared between the single-end and

double-end readout options ﬁrst, and the strip of double-end option coupling with SiPM is further measured and

compared with the results of that with the PMTs. The performance of the strips determined by a detailed survey

along their length with cosmic-ray muon after a detailed characterization of the used 3-inch PMTs and SiPMs.

The proposed compact strip of plastic scintillator with WLS-ﬁber coupling with SiPM provides a good choice

for cosmic-ray muon veto detector for limited detector dimension in particular.

Keywords: PMT, SiPM, plastic scintillator, WLS-ﬁber, muon detection, efﬁciency

I. INTRODUCTION

Muon ﬂux reaching the surface of the Earth makes the

most abundant cosmic-ray–induced radiation at sea level[1].

It has studied and utilized after the discovery by Anderson and

Neddermeyer at Caltech in 1936[2]. To tag and veto cosmic

muons with highly efﬁciency, veto systems are crucial for low

background experiments such as searching for neutrino[3,4],

Dark Matter[5,6] and Double Beta decay[7]. Generally, we

can accomplish the discrimination of muon using two meth-

ods: the expected energy deposition with a simple energy

threshold and a coincidence measurement. Requirements for

such a muon tag and veto system are high efﬁciency for muon

identiﬁcation, immunity from the ambient gamma-ray back-

ground, size in a limited detector dimension, and low cost

per mass unit[8]. It is important for experiments with limited

overburden or even deep underground experiments.

The Taishan Antineutrino Observatory (JUNO-TAO)[9] is

a satellite experiment of the JUNO experiment[10], a ton-

level liquid scintillator (LS) detector placed at ∼30 meters

from a reactor core of the Taishan Nuclear Power Plant in

Guangdong, China. The main purposes of TAO are to provide

a reference antineutrino spectrum for JUNO to remove model

dependencies in the determination of the neutrino mass order-

ing, and to provide a benchmark measurement to test nuclear

databases. A compact and high-efﬁciency muon detector is

needed to suppress the muon-related background, where the

location of the TAO detector near the reactor only has limited

space, limited overburden, and a higher muon ﬂux. Follow-

ing the proposal of the JUNO-TAO detector[9], a multi-layer

detector of the plastic scintillator is proposed as a muon tag

and veto detector to cover around 4 m×4 m on the top of TAO

detector.

Many kinds of detectors use for detecting muons. High-

efﬁciency organic plastic scintillation (PS) detectors are

widely applied as a proven technology for their excellent opti-

∗Supported by NSFC (No. 11875282 and 11475205)

†Corresponding author,wangzhm@ihep.ac.cn.

cal transmission properties, simple production, low cost, sta-

bility, fast response time, sensitivity to all kinds of radia-

tion, and an excellent capacity to handle high-radiation back-

ground environments. In lots of high-energy physics projects,

plastic scintillator strips served as an anti-coincidence de-

tector to provide a trigger signal and are applied as sen-

sitive elements for tracking detector, such as OPERA[11],

MINOS [12], the K2K SciBar detector[13], Minerva[14],

TAE[15], AugerPrime[16], µCosmics[17], YBJ-HA[18],

LHAASO [19,20], muon tomography[21–23] and many

other applications[24,25].

In general, the light yield (LY)[26,27] of a scintillator, and

the detection efﬁciency, are the key criterion to describe the

quality of the detection set-up[16]. Excellent uniformity and

relatively high light collection are required to achieve high

efﬁciency for muons while maintaining good discrimination

from gammas. Wavelength shifting (WLS) ﬁbers coupled

with a photomultiplier (PMT) or multi-pixel silicon-based

avalanche photo-diodes operated in Geiger mode (SiPM) are

commonly used to avoid bulky light guides and read out the

light from scintillators[28–30]. The PS can be much more

mechanically robust and offer great ﬂexibility in detector size

and shape, better tolerance to magnetic ﬁelds, higher photon

detection efﬁciency, compactness, and low cost with SiPM in

particular[1,31,32]. Normally, the WLS ﬁbers are placed

into grooves or holes along the strip, and the detection efﬁ-

ciency can be signiﬁcantly increased by improving the opti-

cal contact between the scintillator and the ﬁber by adding an

optical ﬁller into the groove/hole with an optical glue having

high transparency and a refractive index close to the refractive

index of the strip base material (usual polystyrene), leading to

a light yield increase of up to 50% in comparison to the strips

without a ﬁller[33,34].

In this study, we proposed a basic design on compact strips

of PS with WLS-ﬁber in the dimension of 0.1×0.02 ×2m3,

aiming for a compact muon tagging detector with good efﬁ-

ciency and identiﬁcation of gammas. Two strips with 1 mm

WLS-ﬁber as prototype are fabricated and tested. The com-

parisons among the readout options of single-end or double-

end, and sensor options of PMT or SiPM are done in detail by

cosmic-ray muon survey. Sec. II will introduce the design of

arXiv:2210.03912v1 [physics.ins-det] 8 Oct 2022

the strips and the testing system. Sec. III will show the results

in detail. Finally, a summary will be provided in Sec. IV.

II. SYSTEM SETUP

Following the studies and strategies discussed in Sec. I, two

kinds of strips of PS with WLS-ﬁber as a prototype are de-

signed and produced for R&D. In this section, the strip design

and the testing system will be introduced. The used photon

sensors of 3-inch PMT or SiPM also will be characterized.

A. PS strip with WLS-ﬁber

The design of the compact PS strips with WLS-ﬁber is pro-

posed with two readout options of single-end or double-end

shown in Fig. 1(a), where the key features are the ﬁlled gaps

between the scintillator and the ﬁber, and the ﬂat surface of

the ﬁbers to the PS at its end (Fig. 1(b)). The dimension of

the PS strip is 0.1×0.02×2m3(Width×Thickness×Length),

with four 1 mm WLS-ﬁbers along its width direction (around

2 cm spacing between neighbor ﬁbers) which are inserted and

glued by a ﬁller into the grooves on the surface of the PS. The

four ﬁbers of the double-end option will be gathered into two

groups at each of the ends (four groups in total) and coupled

to the photon sensors. There will be only two groups at the

output end of the single-end option (no ﬁber cut at another

end) aims to reduce the sensor and electronics channels.

The prototypes of the designed PS strips are ﬁnalized and

fabricated by Beijing Hoton Nuclear Technology Co., Ltd.

[35]. The two strips are made with extruded plastic scin-

tillator type of SP101 polymerized with liquid polystyrene

added by P-triphenyl and POPOP. The groove is designed

to 6×6mm2in an optical window of 1×4cm2as shown

in Fig. 1(b), where the WLS-ﬁber BCF92[36] with diameter

1 mm is used and its end surface is ﬂat. The PS strip is cov-

ered with 0.08 mm Al ﬁlm ﬁrstly, then another 0.8 mm PVC

layer, and ﬁnally packaged by a black adhesive tape layer.

The scintillation photons will be collected through the WLS-

ﬁber and read out by photon sensors such as SiPM or PMT,

which will be measured and compared in detail and discussed

in the following sections.

B. Electronics and DAQ

A general schema of the testing system can be found in

Fig. 2. Two small PS modules (mini-module, PS1 and PS2)

locate above and below the PS strip, respectively (Fig. 3(b)),

which will use as muon tagging. The signals of the 2-inch

PMTs of the PS mini-modules send into a FIFO module (OC-

TAL LINEAR model 748) and then discriminated by a low

threshold discriminator (CERN N845), the coincidence of

which (CAEN logic module N455) used to tag a muon as a

trigger of the data taking system. The mini-modules will sur-

vey along the length of the PS strip for uniformity checking,

which will use for all the following tests single-end, double-

end, PMT and SiPM. The two 3-inch PMTs (SPMT) or four

SiPMs use to collect the photons, where the SiPMs (SPMTs)

are coupled to the PS strip through the air directly (Fig. 3(a)).

Please note that the two SPMTs are used for both single-end

and double-end options: one SPMT for one end (two ﬁber

groups) of double-end, and one SPMT for one ﬁber group of

single-end (two SPMTs used in total, Fig. 2(b)). Each of the

SiPMs (four in total) covers one ﬁber group of the double-

end option (Fig. 2(b)). All the signals of the sensors are sent

into the FIFO ﬁrst and then recorded in waveforms by an

FADC (CAEN DT5751, 1 GS/s, 1 V p-p) triggered by the

mini-modules. Each SiPM (SPMT) will have its own individ-

ual HV power supply, and an additional ampliﬁer is applied

to SiPM4 to improve its signal-to-noise ratio discussed later.

C. Mini-modules for muon trigger

The two mini-modules (PS1, 25 ×4×30 cm3and PS2,

15 ×1×22 cm3), equipped with 2-inch PMT, are used to

tag muons and calibrate the performance of the designed PS

strips. The measured spectra of the mini-modules in ampli-

tude and charge are shown in Fig. 4, where the 2-inch PMTs

set to -2200 V (PS1) and -1800 V (PS2) for a similar operat-

ing gain, respectively. The threshold of the mini-modules in

the amplitude sets to around 10 mV (PS1) and 2 mV (PS2)

(Fig. 4(a) to select muons in the following measurements.

The value difference of the threshold is mainly from their

light yield related to their thickness, which introduces differ-

ent valley locations on their charge spectra shown in Fig. 4(b).

The coincidence rate of the two mini-modules is around 6-

7 Hz.

D. SPMT and SiPM

The PS strips will be coupled with SiPMs and SPMT to

collect the photons around room temperature (25◦C). Four

pieces of SiPMs will use in total here, including three of HPK

S14160 in the dimension of 3×3mm3[37] (SiPM1, SiPM2,

SiPM3), which claims a higher PDE (50% at λp) and lower

operation voltage, and another from S12572 (SiPM4). The

3-inch PMTs (SPMT) from HZC[38] is used as a reference

to compare with the SiPMs, for single-end and double-end

options. The characterization of SiPM is done ﬁrstly with a

similar procedure as in [39] but mainly focuses on its gain

versus over-voltage (OV) by LED, cross-talk (CT), and dark

count rate (DCR).

The measurements of SiPMs are shown in Fig.5, where the

typical charge spectra measured with SiPM3 and SiPM4 are

shown in Fig. 5(a), and the working gain of the SiPMs can

calculate according to the peaks. The relationship of the gain

versus OV is plotted in Fig. 5(b), where the breakdown volt-

age (Vbd) is estimated by a ﬁtting to the curve of gain ver-

sus applied voltage, where it is around 41 V for SiPM1-3 and

71 V for SiPM4. The OV of 3 V sets to all the SiPMs. The

SiPM uses another fast ×10 ampliﬁer because of its low gain.

6.00

2000.00

100.00

20.00

6.00

SiPM

4.00

20.00

2.00

2000.00

100.00

groove

(a) Design of PS with WLS-ﬁber (b) Gathered ﬁbers and optical window

Fig. 1. Comparison of light yield and muon efﬁciency of PS strips with SiPMs and PMTs. green: double-end (SiPM), orange: double-end

(SPMT), blue: single-end (SPMT)

(a) Double-end setup with SPMTs (b) Single-end SPMTs and double-end SiPMs

Fig. 2. Schema of system setup. The mini-modules (PS1 and PS2) will survey nine locations of the PS strip with 0.2 m step between -0.8 m

(minimum -1 m, left end, side A) and 0.8 m (maximum 1 m, right end, side B)

The typical plot of DCR versus amplitude threshold of SiPM

is shown in Fig. 5(c), where the DCR is decreasing in steps by

increasing the threshold as expected. It is around 500 kHz at

a half p.e. equivalent threshold 55 kHz/mm 2). The measured

cross-talk (CT) ratio is around 12% for SiPM1-3 and 46%

for SiPM4. The very high cross-talk ratio of SiPM shown

in Fig. 5(d) will affect its charge measurement. A decreas-

ing factor of 1/8 was applied to estimate the DCR for every

other p.e. threshold increasing. The DCR and CT will also

affect the threshold setting, muon efﬁciency, and random co-

incidence expectation of the PS strip.

The gain of the two used SPMTs is calibrated and tuned

to 3×106with positive HV 1150 V and 1180 V respectively.

The QE of SPMTs is around 23%, and DCR is around 400-

700 Hz at a single p.e. threshold (∼2.5 mV/p.e.), which is

much smaller than the SiPMs. The measured charge of sin-

gle photo-electron (SPE) and the DCR versus threshold are

shown in Fig.6.

III. RESULTS AND DISCUSSION

With the introduced system in Sec. II, the measurements

and comparisons will be implemented for coupling with

SiPM and PMT, and single-end and double-end options of the

PS strips, respectively. The muon efﬁciency of the PS strips

will be measured and calculated at nine different locations

along its longitudinal direction. According to the measured

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PerformanceofcompactplasticscintillatorstripswithWLS-berandPMT/SiPMreadoutMinLi,1,2ZhiminWang,1,yCaimeiLiu,1,2PeizhiLu,3GuangLuo,3Yuen-KeungHor,3JinchangLiu,1andChanggenYang11InstituteofHighEnergyPhysics,Beijing100049,China2UniversityofChineseAcademyofSciences,Beijing100049,China3SunYat-senUnivers...

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Performance of compact plastic scintillator strips with WLS-ﬁber and PMTSiPM readout Min Li1 2Zhimin Wang1yCaimei Liu1 2Peizhi Lu3Guang Luo3Yuen-Keung Hor3Jinchang Liu1and Changgen Yang1 1Institute of High Energy Physics Beijing 100049 China.pdf

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Performance of compact plastic scintillator strips with WLS-ﬁber and PMTSiPM readout Min Li1 2Zhimin Wang1yCaimei Liu1 2Peizhi Lu3Guang Luo3Yuen-Keung Hor3Jinchang Liu1and Changgen Yang1 1Institute of High Energy Physics Beijing 100049 China

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