Development of a scintillating-fibre detector with position-sensitive photomultipliers for high-rate experiments

Abstract

An extensive study was performed on the development of fast and precise scintillating-fibre detectors with position-sensitive photomultipliers (PSPM) for application in high-rate experiments. Several detector prototypes with Kuraray multi-cladding fibres of 0:5 mm diameter and Hamamatsu 16-channel H6568 PSPMs were constructed and tested under different beam conditions at the CERN PS and SPS beam lines. High time resolution of the order of 300 ps (r.m.s.) was obtained with spatial resolution of about 125 mm (r.m.s.) and with detection efficiency in excess of 98%. The detector prototype equipped with a 3-m-long light guide was also tested and showed a time resolution of about 540 ps (r.m.s.). Results of tests using a high-intensity muon beam show excellent stability of the detector performances in time and spatial resolutions as well as in detection efficiency under beam fluxes of up to 1:4 Â 10 8 muons per 2.4-second spill.

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Nuclear Instruments and Methods in Physics Research A 516 (2004) 34–49
Development of a scintillating-fibre detector with
position-sensitive photomultipliers for high-rate experiments
S. Horikawaa,d,*, I. Daitob, A. Gorinc, T. Hasegawad, N. Horikawab, T. Iwataa,1,
K. Kurodae, I. Manuilovc, T. Matsudad, Y. Miyachia,2, A. Riazantsevc,
A. Sidorovc, N. Takabayashia, T. Toedaa
aDepartment of Physics, School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
bCenter for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan
cIHEP, Protvino 142284, Russia
dFaculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan
eJINR, Dubna 141980, Russia
Received 25 November 2002; received in revised form 19 June 2003; accepted 28 July 2003
Abstract
An extensive study was performed on the development of fast and precise scintillating-fibre detectors with position-
sensitive photomultipliers (PSPM) for application in high-rate experiments. Several detector prototypes with Kuraray
multi-cladding fibres of 0:5 mm diameter and Hamamatsu 16-channel H6568 PSPMs were constructed and tested under
different beam conditions at the CERN PS and SPS beam lines. High time resolution of the order of 300 ps (r.m.s.) was
obtained with spatial resolution of about 125 mm (r.m.s.) and with detection efficiency in excess of 98%. The detector
prototype equipped with a 3-m-long light guide was also tested and showed a time resolution of about 540 ps (r.m.s.).
Results of tests using a high-intensity muon beam show excellent stability of the detector performances in time and
spatial resolutions as well as in detection efficiency under beam fluxes of up to 1:4 ? 108muons per 2.4-second spill.
r 2003 Elsevier B.V. All rights reserved.
PACS: 29.40.Gx; 29.40.Mc; 85.60.Ha
Keywords: Scintillating fibre; Position-sensitive photomultiplier; High-rate detector; Time resolution
1. Introduction
Scintillating-fibre (SciFi) detectors, developed
over the last 10 years, have come into use for the
tracking of charged particles in recent experiments
[1–7].3They have a wide range of specifications,
ARTICLE IN PRESS
*Corresponding author.
E-mail address: Sosuke.Horikawa@cern.ch (S. Horikawa).
1Present address: Department of Physics, Yamagata Uni-
versity, Yamagata 990-8560, Japan.
2Present address: Department of Physics, Tokyo Institute of
Technology, Tokyo 152-8551, Japan.
3Among many works on SciFi detectors, Refs. [1–7] provide
a partial list of recent papers about particle tracking.
0168-9002/$-see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2003.07.038

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mainly in terms of the photosensitive devices
employed. The RD17 (FAROS) Collaboration at
CERN has demonstrated particularly high perfor-
mances of SciFi detectors with position-sensitive
photomultipliers (PSPM) [8]. Its work has conse-
quently led to new applications of the SciFi
technique in tracking detectors and also in
topological trigger devices.
The idea to develop SciFi detectors with PSPMs
in order to satisfy stringent conditions in recent
high-rate experiments is essentially based on their
two prominent features: good time resolution and
high-rate capability due to the fast response of
SciFis and PSPMs. Thanks to rapid progress
achieved in recent years in the technology of both
SciFis and PSPMs the performances of SciFi
detectors have significantly improved, as demon-
strated in the DIRAC (PS212) experiment at
CERN [9].
In order to use this progress to benefit the
COMPASS (NA58) experiment at CERN [10],
research and development work has been done
[11,12]. The final detectors have been in operation
since 2000 and preliminary results are reported in
Ref. [13]. The purpose of the present paper is to
present some basic results which have demon-
strated the feasibility of constructing such detec-
tors to work under the severe conditions imposed
by COMPASS.
One of the major physics goals of COMPASS is
to determine the gluon polarization DG=G in the
nucleon by measuring the cross-section asymmetry
for open charm production in deep inelastic scatter-
ing of polarized muons on polarized nucleons. The
low cross-sections for the interactions to be studied
require high beam rates of the order of 2 ? 108
muons/2.4-second spill. Covering the beam regions,
SciFi trackers should be capable of measuring the
incident and scattered muon tracks with a precision
of a few hundred micrometres. In addition, accurate
timing determination is particularly important for
these trackers to select the triggered events amongst
108incoming muons/s by means of the timing
correlation. Time resolution better than 500 ps
(r.m.s.) is thus required in order to keep the
probability of accidental coincidences down to 10%.
In the present study, we investigate the overall
performance of SciFi detector prototypes by the
simultaneous
meters: time and spatial resolutions, detection
efficiency and hit multiplicity. One of the most
important subjects in the present paper is the test
of the prototype under high-rate conditions. The
prototypeusingHamamatsu
equipped with a booster was tested under high
rates up to 4 MHz=fibre-channel, showing stable
performances. In parallel, Bisplinghoff et al. [14]
performed high-rate tests of a SciFi detector
consisting of SciFis of 1 mm diameter. They
showed that the output pulse height is stable up
to rates of several 106Hz=fibre-channel when the
PSPM tube is equipped with a booster base. This is
in agreement with the stability in detector perfor-
mances we present here. Radiation hardness of the
fibres, which is the other essential issue for the
high-rate application of SciFi detectors, was also
studied in detail in Ref. [14].
In Section 2, we describe the detector design and
the readout electronics. In Section 3, we present
test results of basic performances obtained from
the detector prototypes with different configura-
tions. Section 4 is devoted to test results under
high-rate conditions with a beam flux of up to
1:4 ? 108muons/spill. Some discussion and con-
clusions are given in Section 5.
measurements of variouspara-
H6568PSPMs
2. Description of the detector
2.1. Structure of the fibre bundle
Two different fibre bundles, H7 and H13, were
prepared especially to study the dependence of the
time resolution on the number of photons
generated in the fibres. They differ in the number
of SciFis aligned along the direction of incident
particles, and in the length of clear fibres
connected as light guides between SciFis and a
PSPM.
The structure of a SciFi array is shown in Fig. 1.
Both H7 and H13 consist of two precisely over-
lapped planes, A and B, each containing 16 fibre
columns of Kuraray SCSF-78MJ multi-cladding
SciFis of 0:5 mm in diameter [15]. We previously
demonstrated that the use of multi-cladding fibres
ensures higher light yield and is a good solution to
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improve the time resolution [12]. The number of
layers per plane is 7 and 13 for H7 and H13,
respectively. Each pixel of the detectors is there-
fore defined by 7 or 13 fibres precisely aligned
along the incident particle direction. The fibre
columns are most closely packed in the direction
perpendicular to the beam, resulting in a pitch of
0:43 mm between adjacent fibre columns. The
scintillating cores (0:44 mm in diameter) of SciFis
in adjacent columns are thus slightly overlapped so
that the fibre array is continuously sensitive to
incident particles without dead space.
Fig. 2 shows the basic concept of a single-
detector plane. Each individual SciFi is connected
with optical epoxy to a clear fibre (Kuraray Clear-
PSM) of the same diameter. Connection of
scintillating and clear fibres is made with the aid
of a jig consisting of a brass plate with grooves of
pitch 0:86 mm precisely machined on its surface.
The two fibres, rectified at the ends, are introduced
into grooves from opposite sides, and glued with
optical epoxy. During hardening of the epoxy,
fibres are kept tightly inside the grooves by other
parts of jig so that the fibre ends are precisely
placed face-to-face over all the pairs of fibres to be
glued. Using this technique we were able to glue
precisely more than 50 pairs of fibres with a single
cycle of the procedure, resulting in a large
time saving compared to the welding of fibre pairs
one by one. The same technique was recently
employed with success in the construction of high-
resolution hodoscopes using fibres of 0:28 mm
diameter [16].
The far ends of clear fibres are grouped for each
fibre column and glued inside a hole perforated in
a black plastic diskette designed to keep the fibre
ends precisely positioned at the centre of each pixel
of the PSPM and in good contact with the
photocathode window. The light pulses yielded in
a fibre column are thus led to the corresponding
PSPM pixel, and then read out by an individual
ARTICLE IN PRESS
1
3
5
7
9
11
13
15
2
4
6
8
10
12
14
16
Fibre column
Layer
0.43 mm
∅ 0.5 mm
BEAM
ABAB
H7 (2 x 7 layers)H13 (2 x 13 layers)
Fibre column No.
Fig. 1. Structure of a SciFi array.
189 16
271015
361114
451213
Fibre position on the PSPM
photocathode (H6568)
2
4
6
8
10
12
14
1
3
5
7
9
11
13
1516
Cross section at A
Cross section at B
Diskette
Clear-fibre light-guideStacked section
5 cm
Juncture
(Glued using optical epoxy)
PSPM
Hamamatsu
H6568
SciFi array
BA
Al. Mylar
Fibre column No.
Fig. 2. Concept of a single detector plane.
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channel of associated electronics. A sheet of
aluminized Mylar is glued using optical epoxy on
the open side edge of the fibre array and serves as a
reflector.
Some of the SciFi trackers in the COMPASS
spectrometer are located close to the super-
conducting solenoid magnet of the polarized
target. Light guides with a length of a few metres
are therefore needed for these trackers to keep
the PSPM away from the magnets to prevent the
deterioration of the PSPM output signal due to the
strong fringe field. The feasibility of using long
light guides for this purpose was demonstrated in
Ref. [11]. For the present study, we prepared
additional light guides of 3 m length to be inserted
between the H7 fibre bundle and the PSPM so that
we can investigate directly the effect of the long
light guides. Such a light-guide extender is made of
16 Kuraray Clear-PSM fibres 2:0 mm in diameter
for each plane of H7. As illustrated in Fig. 3, seven
clear fibres, corresponding to each SciFi column,
are connected to an individual clear fibre of the
extender with the aid of precisely perforated holes
on the diskettes. For reasons of mechanical
stability, connection of the fibre components as
mentioned above is made without any optical
medium, such as optical grease, following experi-
mental confirmation that the loss of light across
the air gap at the joints is only of the order of 10%.
This is a far simpler solution than the connection
of 0:5 mm fibres as described in the preceding
paragraph when there is no strict constraint on the
lateral position of fibres to be connected.
Table 1 summarizes the different detector
prototypes investigated in the present study.
2.2. Position-sensitive photomultiplier
The choice of the photomultiplier tube was made
taking into account the excellent characteristics of
ARTICLE IN PRESS
BA
2.0 mm ∅ clear fibres
Fibre bundle (H7)
PSPM
Hamamatsu
H6568
3 m
Projection of 0.5 mm ∅ ClFi ends
Cross section at B
0.5 mm ∅
clear fibres
Diskettes
Cross section at A
2.0 mm ∅
clear fibre
0.5 mm ∅
clear fibres
2.0 mm ∅
clear fibres
Fig. 3. Concept of the light-guide extender.
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the H6568 from Hamamatsu Photonics [17]
already confirmed in the DIRAC experiment.
H6568 is a type number of a photomultiplier tube
assembly including the housing, integrated voltage
divider and signal cables provided by the manu-
facturer. The bare tube is called R5900-00-M16,
which is known for its high performances in single
photon detection with practically no noise and has
been used for various experiments in, for example,
RICH counters [18]. It also has good time
characteristics, such as a typical anode pulse rise
time of 0:83 ns with a transit time spread of 0:3 ns
(FWHM). The typical gain of the tube is about
3 ? 106at a high voltage of HV ¼ 800 V and
about 106at HV ¼ 700 V; which allow us to use
them without amplifiers. In addition, this tube
shows a high immunity to the external magnetic
field as described in Appendix A.
Our PSPMs differ from the tube commercially
available in this series by the following specifica-
tions. They are equipped with an output of the last
dynode signal, and four booster cables from the
last four dynodes. The last dynode output provides
the analog sum of all 16 anode signals, which can
be used as a monitor of signal amplitudes as well
as for trigger purposes. The four booster cables
allow us to provide independent booster voltages,
stage by stage, in order to minimize the gain drop
due to the increase of the dynode current under
very high-rate conditions.
2.3. Peak-sensing circuit discriminator
A peak-sensing circuit (PSC) is a type of
discriminator sensitive to the variation of pulse
amplitude across adjacent channels [19]. It is
designed for the readout of anode signals of
PSPMs, particularly in combination with SciFi
detectors. The initial motivation for PSC develop-
ment was to treat in real time a strong cross-talk
between adjacent channels of first-generation
PSPMs. However, recent progress in the PSPM
technology has reduced the internal cross-talk due
to the dispersion of secondary electrons to a few
per cent. The main source of cross-talk in the
present case is, therefore, the tracks traversing the
slightly overlapping adjacent fibre columns and/or
transmission of photons between them (see Section
3.2). With properly tuned thresholds, a PSC allows
the annoying effects of the cross-talk between
adjacent channels to be suppressed and the track
position to be defined in real time as demonstrated
in Ref. [19]. In comparison with a conventional
threshold discriminator, a PSC gives fewer multi-
hits while better maintaining the detection effi-
ciency. Such an electronics is particularly useful to
define a topological trigger in high-rate experi-
ments without the hindrance of a high multiplicity
of hits due to the cross-talk. In the present
application, a low multiplicity of hits per event
significantly helps to simplify the position infor-
mation of incident particles.
For the first part of the present study, we used a
PSC module designed for the DIRAC experiment.
For the tests under high-flux beam, described in
Section 4, PSC prototypes recently developed for
the COMPASS experiment were used. The new
module, realized in the 3U euro-mechanics stan-
dard, houses 16 inputs and 16 digital outputs. The
output pulse is in the LVDS level and the width is
B8 ns: Dead time is reduced to B7 ns for
application in high-rate experiments, resulting in
a double pulse resolution of the order of 15 ns: The
discriminator threshold in each channel can be
ARTICLE IN PRESS
Table 1
Detector configurations
DetectorFibre bundleLG extender
Description No. of layers
per plane
SciFi length
ð0:5 mm fÞ
ClFi length
ð0:5 mm fÞ
ClFi length
ð2:0 mm fÞ
H7
H7L
H13
H7
H7
H13
7
7
13
5 cm
5 cm
5 cm
50 cm
50 cm
25 cm
—
300 cm
—
Abbreviations: ClFi—Clear Fibre; and LG—Light Guide.
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