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Nuclear Instruments and Methods in Physics Research A 555 (2005) 142–147
www.elsevier.com/locate/nima
The timing properties of a plastic time-of-flight scintillator
from a beam test$
Chong Wua,b,, Yuekun Hengb, Yuda Zhaob,c, Xiaojian Zhaob, Zhijia Sunb, Jinjie Wub,
Zhenghua Anb, Li Zhaob,d, Linli Jiangb,d, Fengmei Wangb,e, Shengtian Xueb, Yifang Wangb
a
School of Physics and Materials Science, Anhui University, Hefei 230039, China
b
Institute of High Energy Physics, CAS, Beijing 100049, China
c
Department of Physics, Nanjing University, Nanjing 210093, China
d
Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
e
Institute of engineering physics, Zhengzhou University, Zhengzhou 450052, China
Received 26 June 2005; accepted 19 September 2005
Available online 11 October 2005
Abstract
The properties of a time-of-flight scintillator bar, EJ-200, wrapped with different reflective materials, were studied using 800 MeV/c
electron beam at the test beam of IHEP in China. It is 230 cm long with a cross-section of 5 6 cm2, viewed from both ends by R5924
photomultiplier tubes. The results show that the time resolution and the attenuation length depend on the reflective materials.
r 2005 Elsevier B.V. All rights reserved.
Keywords: Scintillator; Photomultiplier tube (PMT); Time resolution; Time-of-flight (TOF)
1. Introduction
After a successful operation for 13 years, the BESII
detector, with an upgrade in 1996, is dismantled and a new
detector, called BESIII, is under-construction for the
Beijing electron-positron collider (BEPCII) at a center of
mass energy of 1–2 GeV with a designed luminosity of
1 1033 cm2 s1. Precision time-of-flight (TOF) counters
made of plastic scintillator bars are used for particle
identification of the BESIII detector.
The scintillator, made of EJ-200 [1], is 2.3 m long with a
cross-section of 5 6 cm2. Photomultiplier tubes for high
magnetic environments, R5924 [2], are attached to each
end. R5924 has good time performance: anode rise time is
2.5 ns, transit time is 9.5 ns, and transit time spread
(FWHM) is 0.44 ns. The total time resolution of the
BESIII barrel TOF is designed to be 110 ps with an
$
Supported by National Science Foundation of China (10491305,
10225524).
Corresponding author. Tel.: +86 10 88236760; fax: +86 10 88233083.
E-mail address: [email protected] (C. Wu).
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2005.09.029
intrinsic time resolution of 90 ps, corresponding to the K/p
separation up to 800 MeV/c at 2s level [3].
In this paper, we report a beam test results for a
prototype scintillator bar made of EJ-200 type, wrapped
with different reflective materials.
2. Experimental set up
The measurements were performed on a test beam set up
at the Institute of High Energy Physics, Beijing, China.
Electron beam with a momentum of 800 MeV/c is selected.
The experimental set up and electronics are shown in Figs.
1 and 2. The origin of x coordinate is defined as the center
of the bar. A threshold Cherenkov counter together with
two scintillation counters, S1 and S2 (3 mm thick), are used
to select electrons. The coincidence signal, in addition to be
a trigger, is also used for an ADC (LeCroy 2249A) gate and
a TDC (CAEN C414) common start. Three multiwire
proportional chambers (MWPCs) with a spatial resolution
of 250 mm can reconstruct the trajectories of electrons, and
also eliminate multi-electron events. Two plastics scintillator bars (BC404) with a cross-section of 2 6 cm2 and a
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C. Wu et al. / Nuclear Instruments and Methods in Physics Research A 555 (2005) 142–147
Cherenkov
MWPC2
Y
0
Pb wall
T02
X
X
-
e beam
Z
west
S1
S2
MWPC1
MWPC3
T01
EJ-200
Fig. 1. Experimental set up for TOF on test beam.
Signal
from the
ends of
the bar
T01
Attenuator
Delay
cable
splitter
ADC
Gate
143
coming from the time walk due to the variation of pulse
amplitudes. Fig. 3(a) shows the correlation between the
TDC channels and amplitudes. The time walk can be
corrected with an equation:
pffiffiffiffi
(1)
T ¼ T 0 þ a þ b= Q
where T0 and T are measured and corrected time, while a
and b are parameters. Fig. 3(b) shows the TDC channels
versus amplitudes after correction.
The other one is a position correction. It is from the
position uncertainty of incoming electrons on the bar. The
LED
0
TDC
Delay
cable
CFD
Stop
-20
T02
S1 C S2
Start
Fig. 2. Schematics of the readout system.
thickness of 0.5 cm, T01 and T02, were coupled with H6533
PMTs with silicon grease. Their outputs were fed into
constant fraction discriminators (CFDs, CAEN 583) which
generate the reference start times to the TDC. The
reference time resolution was measured to be 58 ps.
The EJ-200 scintillator bar was purchased from Eljen
Corporation. It has a decay time of 2.1 ns, a bulk
attenuation length of 4 m, an index of refraction of 1.58,
and a peak in the emission spectrum at 425 nm. The six
surfaces were polished finely by Eljen Corporation. It can
minimize losses due to surface imperfections. The bar was
wrapped with different reflective materials and put
perpendicular to the electron beam. It was coupled with
R5924 PMT at each end without silicon grease. The output
of each R5924 was split into two: one went to TDC via a
leading edge discriminator (LeCroy 623b). The other was
fed into ADC through an attenuator (CAEN N109). Using
a pulse generator, the time resolution of the electronics was
measured to be 20 ps (r.m.s.).
3. Experimental results
-40
-60
-80
-100
150
175
200
225 250 275 300 325
amplitude (0.25pC/channel)
350
375
400
175
200
225 250 275 300 325
amplitude (0.25pC/channel)
350
375
400
(a)
60
40
TDC (24.7ps/channel)
Discriminator
TDC (24.7ps/channel)
Logic unit
S1, C, S2
Measurements were performed with the 5 cm wide
surface perpendicular to the electron beam. Five different
wrapping materials, Al film (used by BESII), Tyvek (two
layers), Teflon, Millipore and ESR [4] (VikuitiTM Enhanced Specular Reflector), were used for the experiment.
20
0
-20
-40
-60
150
3.1. Correction to amplitudes and positions
(b)
There are two important correction factors with
substantial influence on the intrinsic time resolution of
the scintillator bar: one is the pulse amplitude correction
Fig. 3. TDC channels vs. amplitudes wrapped with Al film (read by west
PMT for beam spot at the center of the bar). (a) TDC and amplitude
relation. (b) TDC and amplitude relation after correction (The fit
parameters a and b in Eq. (1) were subtracted).
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144
following corrected equation was applied to eliminate the
effect:
T 0 ¼ T þ c þ dx
(2)
0
where T is the time after correction and c and d are
parameters. Fig. 4(a) demonstrates TDC channels versus
the positions when the beam spot is at the center of the bar.
The slope of the fit line leads to an effective speed of light
along the x-axis of 15.070.3 cm/ns. Fig. 4(b) exhibits the
relation between average TDC channels and the positions
of incoming electrons on the bar. The cross point in the
figure is not zero because of the difference of the delay
80
cable. In this case, the effective speed of light is
14.870.2 cm/ns. Table 1 lists the effective speeds of light
in the bar wrapped with different reflective materials.
3.2. Time resolution
The time resolutions of the scintillator bar wrapped with
Al film is shown in Fig. 5. Both the position and the
amplitude correction as discussed above were applied. The
contribution to the time resolution of the reference time
was subtracted. The open symbols show the time resolution
using a single PMT. The dashed lines is the fit to the data
with
ss ðxÞ ¼ s0 ex=lt
(3)
where lt is time degradation length. It shows the time
resolution values worsen with the distance from the PMTs.
60
TDC (24.7ps/channel)
40
Table 1
Properties of the scintillator bar wrapped with different reflection
materials
20
0
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-2
-1.5
-1
-0.5
(a)
0
0.5
x (cm)
1
1.5
Reflection
materials
Time resolution
in center of bar
(ps)
Speed of light
(cm/ns)
Attenuation
length (cm)
Al film
ESR
Millipore
Teflon
Tyvek
9073
10073
9974
9473
10373
14.870.2
15.070.3
14.870.2
15.070.3
14.570.3
24674
32177
30576
29775
25675
2
200
600
west readout
east readout
σ (ps)
TDC (24.7ps/channel)
400
200
100
0
90
80
-200
-100
-400
-100
(b)
-75
-50
-25
0
25
x (cm)
50
75
-75
-50
-25
0
25
x (cm)
50
75
100
100
Fig. 4. Measured TDC time vs. distance wrapped with Al film. (a) relation
between TDC channels and the position at the center of the bar (x ¼ 0).
(b) relation between TDC channels and x positions at the bar.
Fig. 5. The time resolution wrapping with Al film. Open symbols are the
measured time resolution from PMTs at each end, filled symbols show the
combined resolution. The dashed lines show the fit to the data with Eq.
(3). (lt ¼ 192 11 cm) The solid line is fit curve with Eq. (6).
(sb ¼ 75:2 6:5 ps, lb ¼ 106 19 cm).
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C. Wu et al. / Nuclear Instruments and Methods in Physics Research A 555 (2005) 142–147
The average time of two PMTs at each end of the
scintillator, Tav, is calculated using the following relation:
120
ESR
where Tw (Te) and sw (se) are the time resolution and
variance of the scintillator from the PMT in the west (east)
side. The combined time resolution of the scintillator from
PMTs at both ends, can be obtained by the relation:
1
1
1
¼
þ .
s2 ðT av Þ s2w s2e
100
90
(5)
80
It can be seen from Fig. 5 that the combined time
resolution as indicated by filled symbols, is the worst at the
center of the bar. This is due to the less light collected by
PMTs.
The time resolutions can be fitted by [5]
(6)
The fit curve is shown in Fig. 5 with solid line.
The time resolutions of the scintillator wrapped with
different materials are shown in Figs. 6(a),(b), and Table 1.
The time resolution wrapped with Al film or Teflon is
better than that wrapped with other materials. Similar
results using Al foil and Tyvek are observed in [6].
Discuss amplitude (Fig. 7) first and then time resolution
(Fig. 6).
Figs. 7(a) and (b) show the amplitude spectra of the
scintillator wrapped with different reflective materials. Fig.
7(c) illustrates the reflectivity spectra of the materials,
which are measured at the National Institute of Metrology
of China with a Varian Cary 5E spectrophotometer.
Although the reflectivity of ESR, Teflon, Tyvek and
Millipore are similar, the signal amplitude of the scintillator wrapped with ESR is the highest. It is obvious that the
PMT can collect more photons when the bar is wrapped
with a specular reflective material.
From Figs. 6 and 7, it can be seen that the time
resolution wrapped with Al film, a specular reflective
material, is the best although its reflectivity and the
amplitude are small compared to that of the others.
Another specular reflective material, ESR, yields a time
resolution not very good although its reflectivity and
amplitude are large. A reasonable explanation is that the
high reflective materials can increase photon collection,
including both reflective and refractive photons. These
photons will change the rise time of the pulse, hence
increase the time walk effect and worsen the time
resolution. A Monte Carlo simulation of photon collection
process approves this hypothesis.
The attenuation length of the scintillator can be obtained
by a fit to the signal amplitude at various locations along
the scintillator bar using a modified Landau distribution
[7]:
BðCxÞ
Þ
þ D,
(7)
70
60
-150
-100
-50
(a)
0
x (cm)
50
100
150
50
100
150
120
Teflon
Tyvek
110
Millipore
100
σ (ps)
eðL=4lb Þ
sðxÞ ¼ sb pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi .
2 cosh ðx=lb Þ
ADC ¼ Ae1=2ðBðCxÞþe
Al foil
110
(4)
σ (ps)
T av
T w =s2w þ T e =s2e
¼
1=s2w þ 1=s2e
145
90
80
70
60
-150
(b)
-100
-50
0
x (cm)
Fig. 6. The time resolution with different reflection wrapping: (a) specular
reflection and (b) diffusion reflection.
where A, B are fitting parameters, C is the fitting
peak position of the ADC spectrum and D is pedestal
of ADC. The fit curves are shown in Fig. 7(a). Fig. 8
shows the relationship between ADC peaks and the
positions of incoming electrons on the bar. The lines are
exponent fit by which the effective attenuation lengths
can be obtained. They are listed in the fourth column in
Table 1. The effective attenuation length depends on the
reflectivity of the wrapping material and also on
the reflection type, specular or diffusion. The attenuation
length is longer if the reflectivity is higher or if the reflection
is the specular type.
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C. Wu et al. / Nuclear Instruments and Methods in Physics Research A 555 (2005) 142–147
146
250
500
Al film
Teflon
Millipore
ESR
Tyvek
Al film
tyvek
200
400
ADC peak
counts
150
100
300
50
200
0
100
200
(a)
300
400
500
600
amplitude (0.25pC/channel)
700
200
-100 -80
-60
-40
-20
teflon
ESR
40
60
80
100
Fig. 8. Average ADC peaks versus x positions.
millipore
150
counts
0
20
x (cm)
e- beam
100
-115cm
TOF module
115cm
0
50
R=81cm
θ
0
100
200
(b)
300
400
500
600
amplitude (0.25pC/channel)
700
x
Fig. 9. The set up for oblique incidence.
100
reflectivity or transmission (%)
3.3. Dependence on incident angle
80
The barrel TOF of the BESIII detector will be installed
surrounding the main drift chamber (MDC). The radius of
barrel TOF is 81 cm. Particles produced by e+e collisions
at the interaction vertex will flight through detectors,
probably with some incidence angle. For particle identifications, it is very important to study the time resolution
dependence on incident angle. The experimental set up for
oblique incidence is shown in Fig. 9. Fig. 10 shows the
results. It can be seen that in the range of the errors, the
time resolutions of oblique incidence are the same with that
of perpendicular incidence.
60
ESR
teflon
Millipore
40
tyvek
Al film
20
transmission
0
360
(c)
380
400
420
440
460
wavelength (nm)
480
500
520
Fig. 7. (a), (b) Amplitudes of the bar wrapped with different materials at
x ¼ 0; (c) reflectivity of the materials and transmission spectrum of EJ-200
(cylinder with a radius of 1.5 cm and a height of 6 cm).
4. Conclusions
The properties of a time-of-flight scintillator bar, EJ-200
wrapped with different reflective materials, were studied on
an 800 MeV/c electron beam at the test beam of IHEP in
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(2) If the reflectivity of specular reflective materials and
diffusion reflective materials is the same, the attenuation lengths of the bar wrapped with specular reflective
materials are longer than that with diffusion reflective
materials. The amplitude spectra of the bar wrapped
with specular reflective materials are bigger than that
with diffusion reflection materials.
(3) The effective speeds of light of the bar wrapped with
different reflective materials are almost the same.
(4) Within the range of error, the time resolutions with
incident angles are the same as that with perpendicular
angle.
incoming perpendicularly
incoming with θ = 23°
incoming with θ = 41°
120
σ (ps)
147
100
80
Acknowledgements
60
-100
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-60
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x (cm)
-20
0
20
The authors would like to acknowledge Prof. J.G. Lu,
J.C Li, X.Z. Cui and Y.M. Wu for their valuable
contribution and a great number of colleagues for their
continuous help and assistance during the experiments.
Fig. 10. The time resolutions with oblique and perpendicular incidence
(wrapped with ESR).
References
China. Our conclusions from the experiment are the
following:
(1) The time resolution is the best at both ends and the
worst at the center of the bar. It also depends on the
wrapping materials. The time resolution wrapped with
Al film is better than 90 ps.
[1]
[2]
[3]
[4]
http://www.eljentechnology.com/ej-200.html.
http://usa.hamamatsu.com/assets/pdf/catsandguides/PMT_Fine_Mesh.pdf.
Preliminary design report of the BESIII detector.
http://www.3m.com/product/v_index/Vikuiti(TM)_Enhanced_Specular_
Reflector_(DA)_00.jhtml.
[5] S. Denisov, et al., Nucl. Inst. & Meth. A 494 (2002) 495.
[6] S. Denisov, et al., Nucl. Inst. & Meth. A 478 (2002) 440.
[7] www.estec.esa.nl/wmwww/WMA/ProjectSupport/XMM/Documents/
tos-em-02-067.pdf.