Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
A Multi-strip Multi-gap RPC Barrel for Time-of-Flight Measurements
M. Kiš a,d,, M. Ciobanu a,b, I. Deppner b, K.D. Hildenbrand a, N. Herrmann b, T.I. Kang a,e, Y.J. Kim a,
P. Koczon a, Y. Leifels a, M. Marquardt a, M. Petrovici c, K. Piasecki b,g, M.S. Ryu a,e, A. Schüttauf a,
V. Simion c, J. Weinert a, X. Zhang a,f
a
GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
Physikalisches Institut der Universität Heidelberg, Heidelberg, Germany
c
National Institute for Nuclear Physics and Engineering, Bucharest, Romania
d
RuXer Bošković Institute, Zagreb, Croatia
e
Korea University, Seoul, South Korea
f
Institute of Modern Physics (IMP), Lanzhou, China
g
Faculty of Physics, University of Warsaw, Poland
b
The FOPI-Collaboration
a r t i c l e i n f o
abstract
Article history:
Received 4 August 2010
Received in revised form
18 February 2011
Accepted 22 February 2011
Available online 3 March 2011
The FOPI detector [1] at the heavy-ion synchrotron SIS-18 at GSI in Darmstadt has upgraded part of its
time-of-flight (ToF) system by adding a new sub-detector shell (called the MMRPC Barrel) that is made
out of Multi-strip Multi-gap Resistive-Plate Counters (MMRPCs). The MMRPC Barrel has an active area
of 5 m2 covered by 2400 individual anode strips [2,3] which are read out at both ends by customdesigned electronics [4,5]. With these multi-strip anodes we have obtained an enhanced detector
granularity with 95% efficiency and a ToF resolution sToF better than 90 ps which increases the
identification limit for charged kaons up to laboratory momenta of at least 1 GeV/c. In this paper we
report on the design, construction and operational characteristics of the MMRPC Barrel and describe its
performance during the first experiments.
& 2011 Elsevier B.V. All rights reserved.
Keywords:
FOPI
MMRPC
Timing RPC
ToF
Multi-strip
Segmented anode
1. Introduction
Within the last two decades FOPI (an acronym for FOur-PI
detector) has collected a substantial amount of new data in the
field of heavy-ion (HI) collisions at relativistic energies up to 2 A GeV
(see e.g. Ref. [6]). In the past years the research program has focused
on strangeness production in hot and dense nuclear matter, with the
aim to verify and study the existence of in-medium effects. The
production of kaons and anti-kaons close to and below the production threshold and their phase-space distributions offer unique
possibilities to investigate these aspects [7,8]. In order to extend
the phase-space acceptance of the system the particle identification
(PID) capability for kaons was improved by adding a new detector
shell of Multi-strip Multi-gap Resistive-Plate Counters [2,3], whose
Corresponding author at: GSI Helmholtzzentrum für Schwerionenforschung
GmbH, Planckstraße 1, D-64291 Darmstadt, Germany.
E-mail address: M.Kis@GSI.de (M. Kiš).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2011.02.076
performance in terms of granularity and time resolution is superior
to the existing scintillator time-of-flight detector array [1].
The FOPI detector system is depicted in Fig. 1.
Its main components are two pictorial tracking chambers, the
Central Drift Chamber (CDC) and the forward drift chamber
named HELITRON, both placed inside a solenoid magnet with
an axial field of 0.6 T. Behind each chamber the particles hit a
scintillation ToF detector: Plastic Wall and Zero Degree Detector
(ZDD) are placed behind the HELITRON, while the Plastic Barrel
surrounds the CDC in a cylindrical geometry. PID uses the combined
dE/dx and Br (i.e. momentum) information from the drift chambers
and the time of flight from the scintillator devices behind. During
the upgrade of the ToF system the Plastic Barrel was modified and
shifted upstream, covering now polar angles beyond 701 relative to
the nominal target position. Polar angles between 381 and 681 are
covered by the new MMRPC Barrel, which surrounds the CDC in a
quite similar cylindrical geometry.
In this paper we will first present the physics goals and the
resulting performance requirements and design criteria for the
new Barrel, followed by a detailed description of the construction
28
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
Fig. 2. One end of an assembled anode PCB showing the capacitor block, the
multi-pin connector, and the layout of strips (see text for details).
Fig. 1. Setup of the FOPI detector. The main detector subsystems are labeled (see
text for details). The target (indicated by the three coordinate arrows) is placed
inside the Central Drift Chamber (CDC). The in-beam Start detector (not shown) is
placed some 2 m upstream of the target.
with emphasis on peculiarities of the multi-strip design. The
operational characteristics of the counters will be described
briefly together with the custom-designed electronics. The counters’ performance and their resolution in test experiments are
outlined next. Finally, we will show results of the Barrel in the
first experiment after its full implementation (Ni on Ni collisions
at a beam energy of 1.9 A GeV).
2. Upgrade requirements
The main goal set by physics was an enhanced kaon identification up to a momentum of PLab ¼ 1 GeV/c. This requires a fullsystem time resolution of sToF r100 ps to have kaons separated
from pions and protons above a 3s level. Assuming a time
resolution of the in-beam Start counter of stStart r 50 ps as
reference, the RPCs must provide a resolution of stRPC r 85 ps in
order to meet the full-system value. Due to the low production
probability ( 104 ) of anti-strange particles at the maximum SIS-18
energy of 2 A GeV, the geometrical acceptance of the system
should be maximal and the detector efficiency has to be high
( e Z 98%).
For the PID the particle tracks from the CDC have to match with
the hits in the MMRPC Barrel; therefore the azimuthal resolution of
the latter should be comparable to the CDC position resolution in
the bending x–y plane ðsx,y r 250 mmÞ. The z-coordinate parallel to
the beam axis is less critical; a resolution of a few cm is sufficient,
since this is the order of the CDC resolution.
The necessary Barrel granularity (number of individual cells)
was dictated by the expected multiplicity in central collisions of
Au on Au at 1.45 A GeV: with up to 60 charged particles emitted
into the acceptance of the CDC a granularity of about 700
individual cells is necessary to keep the double-hits on a fewpercent level.
All these requirements are met best by counters with long,
narrow anode strips which are read out at both ends. As in the
case of a scintillator strip the time of flight is given by the sum of
the times measured at both ends and the position along the strip
by their difference. The transverse position is given by the fired
strip, but with the signal expanding over a few neighboring strips
the position can be determined even better via the centerof-gravity of these strips. The small pitch of 2.54 mm should
guarantee an azimuthal resolution of the required accuracy.
With this geometry (and the chosen glass stack configuration)
the impedance is already close to 50 O; fine tuning can be
obtained by varying the strip to gap ratio. Hence from the very
beginning the R&D work has concentrated on multi-strip counters
with double-hit capability of the individual detectors i.e. two hits
on different strips should be detectable separately without
degradation of their time resolutions.
The angular range between 381 r y r 681 was dictated by space
requirements; it covers a fair amount of mid-rapidity kaons except
for a cone of low transverse momenta. Finally the whole new
MMRPC Barrel including the electronics had to fit into the radial
gap of 25 cm between the CDC and the Plastic Barrel light guides
(cf. Fig. 1).
3. Multi-strip MRPC construction
The counters are of Multi-gap Resistive Plate (MRPC) type [9–11];
which normally feature single pads or strips as anodes. In our case
signals are induced onto a multi-strip anode, hence the term
MMRPC [2,3]. The single detectors are 46 mm wide with anodes
segmented into sixteen 90 cm long strips of 1.64 mm width separated by 0.9 mm wide gaps (pitch of 2.54 mm), cf. Fig. 2.
The anode board is a standard 0.6 mm thick double-sided
printed-circuit board (PCB); the corresponding strips on the upper
and lower PCB sides are through-connected at both ends from
where 50 O signal lines lead to a multi-pin connector. From there
coaxial double-shielded cables lead to the readout electronics.
The connectors1 as well as the cables have 80 pins/lines of 0.8 mm
pitch from which only 16 are used, while unused lines are
grounded by 50 O resistors. In order to minimize cross-talk
within the connector, a special distribution scheme (16 signals
over 80 pins) is chosen. Both ends of the anode board are covered
by capacitor-blocks placed there for mechanical reasons and as
part of the signal transmission line between the active counter
and the connectors.
As resistive plates we use common float (window) glass of
7 1012 O cm bulk and 5 1014 O=& surface resistivity, both
values measured in dry N2-atmosphere at room temperature. This
material was chosen because of its availability and the low costs;
the high resistivity which might deteriorate the resolution and
degrade the efficiency at high rates does not pose a problem, since
the maximum rates in our experiments stay well below a full
1
SAMTEC, http://www.samtec.com.
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
SCREW
ANODE
STRIP
CATHODE
STACK
SUPPORT
SM
BOX
GAS
29
MMRPC
GLASS
HV
PCB
GAS
GAP
Fig. 3. Cross-section of the MMRPC. The whole stack is held together by nylon
screws placed on both sides along the counter; they are also used as supports to
span the fishing line spacers. The outermost two strips are not read out; they are
grounded and should improve the uniformity of the field across the stack. For
details see text.
READOUT
ELECTRONICS
LV
2
counter illumination of 100 Hz/cm . We apply the common
double-stack configuration (see Fig. 3) of 2 4 gaps with the
anode in the center, where the gaps are defined by spacers of
220 mm thick fishing line. The stacks contain 1.1 and 0.55 mm
thick glass plates in an alternating way.
The two outermost plates serve as cathodes; their outer
surface is covered by self-adhesive copper foil powered by high
voltage. This symmetric construction allows to operate the
counters under high electric fields of E Z 100 kV=cm at moderate
voltages of U r 10 kV only. Finally, the whole stack is mechanically stabilized by two 3 mm thick support plates.2
With the described layout (strip/gap width, total glass thickness and gas–gap width) the single anode strips are almost ideal
50 O transmission lines. The same is true for the signal connections leading to the multi-pin connector: here material and
geometry of the capacitor block (cf. Fig. 2) are chosen in a way
to guarantee an impedance of 50 O as well. As a consequence the
impedance stays within 50 72 O throughout the counter.
Five of these counters are placed together in a carbon-fiber box
of 0.6 mm wall thickness that we will refer to as a Super-Module
(SM), Fig. 4. The front faces of the box are closed by aluminum
flanges through which the capacitor-blocks of individual counters
protrude; O-rings around each capacitor block keep the SM gastight. On one side (shown in Fig. 4) each SM has a gas inlet and
outlet and a HV connector. All five counters within the SM are
powered by one single HV channel (CAEN mod. A1526N) via a
filter/divider circuit inside the box. Within the SM the five
counters are mounted in partially overlapping layers, three
counters facing the target (upper row in Fig. 4) with two behind
them. At each end the counters are connected to the readout
electronics placed on an aluminum plate below the box, which
also supports the carbon box through four legs in the corners. In
its position inside the magnet each SM is movable separately in
z-direction on stainless steel rails, rolling on wheels fixed to its
support plate.
An MMRPC Barrel with full azimuthal coverage would consist
of 32 SMs placed side by side on a cylindrical shell of 94 cm radius
around the beam axis. However, due to space constraints imposed
by the CDC mechanics only 30 can be mounted, of which 28 SMs
have been installed to date (the two missing ones require a
different placement of electronics). The full number of 30 SMs
accounts for 150 single counters with a total of 2400 individual
strips (or 4800 electronic channels). The photo in Fig. 5 shows the
assembled MMRPC Barrel.
2
Stesalit 4411.
Fig. 4. A single SM mounted with the readout electronics below. For details see
the description in the text.
Fig. 5. Photo of the MMRPC Barrel.
4. MMRPC operation
The detector construction was preceded by an extensive R&D
phase [2,12–14]. Although the operation principles of MRPCs [9,10]
were already established at the time, the peculiarities of the MMRPC
design required additional effort to realize the concept. In multi-strip
counters with narrow strips the induced signal is shared between
neighboring strips [15], and the resulting signal is smaller in
comparison to a standard pad design. In order to keep the efficiency
for double-hits high the avalanche of a traversing minimum-ionizing
particle should fire not more than three to four strips. Therefore the
induced signal was even lowered by constraining the avalanche in
space through a reduction of the gas-gap width to 220 mm and by
using an SF6-rich gas mixture (80% R134a, 15% SF6, and 5% Isobutane) as reported in Ref. [16]. The SF6-rich mixture allows to
operate the counters at rather high fields without any problems
related to streamers.
With such low signals the design of a sensitive fast front-end
electronics (FEE) was a major task [5]. The result of the collaboration’s extensive R&D effort to develop a high-bandwidth multichannel FEE-card able to cope with the fast MMRPC-signals is
summarized below. In order to transfer the signal to the electronics as efficiently as possible and to minimize reflections, the
30
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
1
110
0.5
100
105
110
160
proton 2 GeV
140
120
100
Efficency [%]
-1.5
60
40
20
-2
0
0
50
-2.5
100 150 200 250 300
Q [fC]
70
100
60
80
50
60
Efficiency
σt (RPC-RPC)
σt (RPC)
40
0
5
10
15
20
25
30
35
40
45
σt [ps]
80
80
-1
-3
100
90
-0.5
counts
V [mV]
0
95
50
t [ns]
Fig. 6. A typical cosmic-ray signal on a single MMRPC strip measured at an
electric field of 110 kV/cm directly with the DSO (Tektronix TDS 85104 with 1 GHz
analog bandwidth). The inset shows the charge distribution of recorded signals
with a mean of 95 fC.
anode strips and the whole signal paths were designed as 50 O
transmission lines and optimized by using time-domain reflection
techniques. The quality of this impedance matching is shown
in Fig. 6, there are practically no reflections visible after a typical
signal.
4.1. Readout electronics
The readout electronics consists of the FEE (version FEE5 [5])
followed by the Time-to-Amplitude Converter (TAC) based digitizer
(TACQUILA3) [4]. Both house 16 channels, hence 10 such systems
are needed to read out one SM (see Fig. 4) from both ends. FEE5 is an
amplifier/discriminator card with both timing and analog output for
each channel; it comprises high gain ( DG 160, optimized for our
typical signals) at high bandwidth ðdf 1:3 GHzÞ and an excellent
electronic time resolution ðseFEE r18 psÞ. The card is built with
discrete elements that causes a rather high power consumption of
0.5 W/ch.
The TACQUILA3 digitizer is operated in Common-Stop Mode,
with a distributed free-running 40 MHz clock as stop signal. The
intrinsic resolution of the TAC chips reaches seTAC r10 ps. An
additional piggy-back QDC card on the TACQUILA3 card digitizes the
analog signals from FEE5. The charge provides information on the
signal distribution over neighboring strips (‘‘cluster’’) and it is also
used to correct the time-walk. In order to reduce the number of
electronic channels the charge is measured at one strip end only. Up
to 30 TACQUILA3 cards can be daisy-chained and readout by the
FOPI data acquisition, which is based on the Multi-Branch-System
(MBS) [17] developed at GSI. Contributions to the overall electronic
time resolution come also from the clock jitter sjCLK r 8 ps (due to
the distribution of the clock) and from individual card-to-card
variations sjCRD r10 ps, yielding a total ToF-electronics resolution
of se r25 ps.
In such a free-running system both start and stop signals have
to be measured vs. the same clock signal. Therefore one additional
TACQUILA3 board is used for the in-beam Start detector that is
read out via a modified FEE5 card.
4.2. MMRPC performance and operating characteristics
Extensive tests were performed during the prototyping phase
in order to gain better understanding of the MMRPC concept. The
30
20
95
100
105
40
110
20
E [kV/cm]
Fig. 7. Time resolution and efficiency of an MMRPC measured in a direct proton
beam (spot illumination) at a rate of 100 Hz/cm2. Open symbols represent the
combined resolution of two overlapping MMRPCs, closed symbols show the
individual contribution of each detector (assuming equal contribution
pffiffiffi to the time
resolution for both counters, i.e. dividing the combined resolution by 2). The errors
bars are dominated by estimated systematic errors and hence equal for all points,
curves are eye-guides.
typical performance of the MMRPCs is shown in Fig. 7 in terms of
efficiency and resolution as a function of the field strength over
the gaps; the curves have been measured with two of the first
mass-produced counters in a direct 2 GeV proton beam.
For electric fields E Z108 kV=cm an efficiency of e Z99% is
reached; at the same time the single counter time resolution
(measured between two MMRPCs) levels off at stRPC r 60 ps.
Based on these results the operating HV of SMs and hence all
single MMRPCs has been fixed to 9.6 kV which corresponds to a
field of 109 kV/cm. The beam rate in these measurements was
100 Hz/cm2, which is about the maximum expected rate in our
experiments.
During the mass production only a few counters could be
tested in such an extensive way. All SMs, however, were inspected
with respect to gas-tightness of the box and stability of the HVcircuit. Operation and stability were checked by measuring the
dark-rate of each counter (with all anode strips fed into an OR) as
a function of the HV; at the chosen operating voltage all stayed
below 0.2 Hz/cm2, also demonstrating the quality of the counter
mounting in our clean room. In additional g-source tests signals
and electronics were checked under experimental conditions.
4.3. Data calibration
The initial step of the time calibration assumes equidistant
bins in a clock cycle. The most important correction of the
measured time concerns the time-walk (or slewing), the dependence on the signal height caused by the leading-edge discrimination. In our case we use an arbitrary charge calibration of the
signals for the correction; details can be inferred from Fig. 8. The
charge Q also serves for a proper weighting of neighboring strips
in clusters in a particle hit finder as explained further below.
The initial assumption of equal gains (in ps per channel) for all
channels of a single TAC spectrum is not justified: The TAC chips
exhibit intrinsic integral non-linearities which are reflected in a
feature that we will refer to as wiggles due to the shape of the
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
0.2
31
300
[counts]
0.1
dt [ns]
0
200
100
-0.1
-0.2
strip 1
0
strip 8
100
sumr [x103 counts]
-0.3
-0.4
0.2
0.1
75
50
25
0
1000
-0.1
-0.2
diff [counts]
dt [ns]
0
strip 1
strip 8
-0.3
0
-1000
-0.4
7.5
-0.5
3
3.5
4
4.5
5
5.5
6
10
12.5
15
Fig. 8. Leading-edge discrimination techniques make the time measurement
sensitive to the amplitude of the signal. This effect (known as time-walk or time
slewing) can be corrected by a measurement of the charge. The upper panel
(a) shows the typical Q-dependence (one outer and one inner strip) of the time
difference (channel time vs. reference). After correction the walk effect is
minimized (b).
spectra. These wiggles are corrected in one of the next calibration
steps; details are explained in Fig. 9. The correction has to be
performed in principle for each channel separately, and it has to be
repeated in certain time intervals. The reason is the temperature
sensitivity of the TAC chips: temperature monitors show typical
day-to-night temperature variations of the order of 1–2 1C, and
these changes in temperature already cause changes in the gains of
the time digitizers and consequently in their effective range.
A decent correction, however, requires adequate statistics.
Hence, since the characteristic behavior of the chips is similar
the correction is performed in common for all channels on one
side of a counter i.e. averaged over the 16 TAC chips of one
TACQUILA board. The result is shown in Fig. 10. In this way the
integral non-linearity is corrected; slight differential non-linearities remain.
After calibration the two times (from both ends of the strip)
and one charge (measured at one strip end only) are available. In
first order it is assumed that the signal propagation velocity
is equal on all strips, in our case bsig ¼ 0:5. The time of flight is
then given by the difference between the mean value tmean ¼
(tleft þtright)/2 and the reference time of the in-beam Start counter;
the difference tleft tright delivers the position along the anode
20
22.5
25
27.5
30
2
t [channels x 10 ]
6.5
ln(Q) [arb. units]
17.5
Fig. 9. Integral non-linearity of TAC chips (wiggles) in the time spectra: panel (a)
shows a single TAC chip spectrum (a time signal vs. random clock), indicating a
good uniformity except at the extremes. In panel (b) this upper spectrum has been
converted into a cumulative sum with the expected linear shape from which one
can calculate the time gain per channel. Panel (c) results from a subtraction of a
fitted straight line from histogram (b). It indicates a systematic deviation from a
constant; by assuming a fixed time gain for the single TAC channel we obviously
overestimate the lower part of the spectrum and underestimate the higher part
(the time bins are narrower and wider, respectively).
strip. A common offset for ToF is derived from the fastest ðb ¼ 1Þ
particles, in our case pions identified by the CDC.
Mean time and position are used by an algorithm which
identifies clusters of signals which form single hits. The transverse
position is then calculated from the center-of-gravity of the charge
recorded on neighboring strips. So finally each hit has its ToF and
position defined. The hits are matched to identified particles from
the CDC and, by taking into account the various distances between
target and hit position, the ToF can be converted into the velocity
of the reaction product.
5. Performance and results
The following results have been obtained by bombarding a
natural Ni-target with a 56Ni beam of 1.91 A GeV.
After the described corrections one obtains a ToF resolution
sToF r 90 ps, as shown in Fig. 11.
For this analysis a subset of minimum-ionizing particles was
used (fast pions). The ToF resolution is the full-system resolution;
it includes contributions from the Start detector and the MMRPC
Barrel and corrections for e.g. the different flight-path lengths
32
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
9000
0.4
σy = 0.169±0.001 cm
8000
0.3
7000
0.2
6000
counts
dt [ns]
0.1
0
5000
FWHM
4000
-0.1
3000
-0.2
2000
1000
-0.3
0
-0.4
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Δy [cm]
0.4
1200
0.3
0.2
1000
800
0
counts
dt [ns]
0.1
-0.1
600
400
-0.2
-0.3
σz = 1.53±0.01 cm
200
-0.4
7.5
11.25
15
22.5
18.75
t [channels
26.25
30
x102]
40
35
counts x 103
30
25
20
15
10
5
-0.8
-0.6
-0.4
-0.2
dt [ns]
-40
-20
0
20
40
z [cm]
Fig. 10. The wiggles correction is performed for 16 channels in common (see text
for details). On the upper panel (a) one can observe the actual effect on the timing,
caused by the non-constant time bin size (cf. Fig. 9). After correction (b) this effect
is minimized.
0
0
0
0.2
0.4
Fig. 11. A Gaussian fit of the full-system time-of-flight resolution shows a sToF
below 90 ps for fast pions. The non-Gaussian tails (hatched areas in the inset)
outside the shown 7 3s interval contain less than 0.6% of all particles.
Fig. 12. Upper plot: Dy is the measured azimuthal difference between the CDC
and the MMRPC Barrel at the RPC radial position. The transversal position
resolution sy is obtained by a Gaussian fit shown. Due to the tails in the
spectrum, only the region above the FWHM was fitted while the region below
(indicated by dashed line) was ignored. Lower plot: z-position distribution; the
longitudinal resolution sz is obtained by fitting a modified box shaped function to
the distribution. The edges are described by the error function from which sz is
derived.
between target and impact point in the detectors. By selecting
hits in the overlap regions of neighboring RPCs one can rule out
Start detector and flight-path contributions, which results in
an MMRPC Barrel resolution of stðBarrelÞ r 70 ps. Hereupon we
obtain an independent estimate of a Start counter resolution of
stðStartÞ r 55 ps. In the present experiment the Start detector was a
2 cm 2 cm poly-crystalline diamond with an active area of
1 cm2 and thickness of 150 mm.
The position resolution in azimuthal direction is measured
relative to the CDC, i.e. to the extrapolated hit position in the
MMRPC Barrel. Due to the extrapolation over the distance of more
than 25 cm between both detectors the resulting distribution
(Fig. 12) shows tails. Nevertheless, the estimated transversal
position resolution is sy r1:7 mm, which agrees with a limit
given by the pitch of 2.54 mm.
Due to the poor longitudinal resolution of the CDC the longitudinal position resolution was estimated by fitting the distribution edges with an error function, cf. Fig. 12. While the resulting
longitudinal resolution of sz r 1:55 cm is about 50% larger than
one that could be inferred from the MMRPC Barrel time resolution
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
1.5
5.5
105
p
K
104
π+
0.5
p [GeV/c]
+
103
0
-0.5
K
-
π-
-1
0
5
10
15
20
25
30
35
strips in cluster
d
1
4.5
3.5
2.5
102
1.5
10
100
1
σss [ps]
t
-1.5
33
50
v [cm/ns]
Fig. 13. Particle identification in a momentum vs. velocity plot. It combines the
tracking information from the central drift chamber (CDC) and the ToF measurement in the MMRPC Barrel. The particle species are labeled.
5.1. PID
The measured ToF allows us to calculate particle velocities
with a correspondingly high resolution. These are plotted
in Fig. 13 against the momenta p from the Br analysis of the CDC
tracks.
In this plot K þ can be differentiated from p þ and protons up
to momenta of 1.2 GeV/c, which is in full accordance with the goal
of our ToF upgrade. Negative kaons (K ) are produced about
1/100 times less abundantly, so their statistics is rather poor.
Hence one cannot really demonstrate an equivalent PID capability
for K , nevertheless, the data indicate that we can differentiate
K and p up to at least 0.8 GeV/c.
5.2. Multi-strip features
Because of the multi-strip design our RPCs have particular
features that are related to the charge production and signal
generation in such a structure. In Fig. 14 we show three different
observables as a function of the position across the counter (strip
number 1–16). In panel (a) the number of strips fired in a hit
(cluster size) is shown, indicating a central plateau of 4.6 and
4.2 for all and minimum-ionizing particles, respectively. The drop
at the edges is caused mainly due to the geometry, since there
fewer strips generate the signal. Panel (b) shows the electronics
resolution of a single strip containing contributions from signal
propagation through the counter strip structure, the response of
the preamplifier/discriminator and the digitizer. It is measured by
the RMS of all the timing signals contributing to a single hit
originating from the same avalanche. Here the kind and velocity
of the particle obviously does not influence the result. The central
strips exhibit a plateau at 25 ps; the increase toward the edges is
mainly due to the smaller clusters and therefore a worse time
reference; the outermost strips have higher noise and may
deteriorate the resolution further.
0
200
Q [arb. units]
and the signal propagation velocity, it is more than sufficient to
allow for a proper matching between CDC and MMRPC Barrel. For
the determination of these position resolutions all charged
particles measured in the CDC and the MMRPC Barrel were used.
all particles
fast pions
100
0
1
2
3
4
5
6
7
8 9 10 11 12 13 14 15 16
strip no.
Fig. 14. Multi-strip features: three observables as a function of the position across
the counter (strip number). Panel (a): cluster size (number of adjacent strips
which fire in a hit), panel (b): electronics resolution of single-strip sss , panel (c):
measured charge Q. Open symbols refer to all detected particles, closed ones to
Minimum-ionizing pions only.
The average charge Q measured on a single strip (panel (c))
shows a uniform difference between minimum-ionizing pions
and all particles. From a central plateau the charge drops a bit
toward the edges followed by a slight rise at the extremes. The
overall drop toward the sides is caused by the decrease of the
effective field strength in the counter; the rise in the most outer
strips is attributed to the particles that are traversing the counter
through an edge of the active area; for those the charge sharing
between the strips is weighted toward the edge strips.
6. Conclusion
In an upgrade program our FOPI detector system has been
equipped with 150 Multi-gap Resistive-Plate-Counters (eight gaps
in this case), arranged on a cylindrical shell (or Barrel) of 5 m2
area. Each MRPC has an active surface of 900 46 mm2 covered
by 16 individual 900 mm long strip anodes; they are read out at
both ends, a novel feature in RPC techniques, hence we have
adopted the name ‘‘Multi-Strip-MRPC’’ or MMRPC. About 4800
electronic channels are read out and digitized by a fast electronics, also designed and adopted to the counters in our institute.
34
M. Kiš et al. / Nuclear Instruments and Methods in Physics Research A 646 (2011) 27–34
The full-system resolution (all counters overlaid in a production run) for the MMRPC Barrel alone is stðBarrelÞ r70 ps; including
the time reference from our in-beam Start detector we obtain a
total resolution in the experiment of sToF r90 ps, in full agreement with the design goals of the project. Due to the special
anode structure we also find an excellent resolution for the
particle’s hit position: The time difference of the two ends yields
about 1.5 cm resolution along the counter axis (defining the polar
angle in our experiment). The perpendicular position (azimuth) is
given by the mean of the deposited charge on neighboring strips
with a precision of better than 1.7 mm.
Meanwhile, we have run the MMRPC Barrel in a couple of
experiments (few months of running time) under very stable
conditions and uniform performance. In our opinion this timing
RPC technology is mature enough to be used in a wide range of
applications. It offers a good choice for any experiment where
similar features for particle detection are required, especially if
the experiment is operated in a strong magnetic field. As far as the
investment costs are concerned, RPCs may be the only affordable
solution in very large detector systems. Furthermore, the development of an own (detector-customized) read out electronics can
significantly reduce the costs and even improve the performance
of the detector.
Acknowledgments
This work was supported by the German BMBF under Contract
No. 06HD190I, by the Korea Science and Engineering Foundation
(KOSEF) under Grant No. F01-2008-000-10007-0, by the Polish
Ministry of Science and Higher Education under the Grant No. DFG/
34/2007 by the mutual agreement between GSI and IN2P3/CEA, by
the Hungarian OTKA under Grant No. 47168, within the framework
of the WTZ program (Project RUS02/021), by DAAD (PPPD/03/
44611), and by DFG (Project 446-KOR-113/76/04). We have also
received support by the European Commission under the 6th
Framework Program under the Integrated Infrastructure on:
Strongly Interacting Matter (Hadron Physics), Contract No. RII3CT-2004-506078.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
A. Gobbi, and the FOPI Collaboration, Nucl. Instr. and Meth. A 324 (1993) 156.
A. Schüttauf, et al., Nucl. Phys. B (Proc. Suppl.) 158 (2006) 52.
A. Schüttauf, et al., Nucl. Instr. and Meth. A 602 (2009) 679.
K. Koch, et al., IEEE Trans. Nucl. Sci. NS-52 (3) (2005) 745.
M. Ciobanu, et al., IEEE Trans. Nucl. Sci. NS-54 (4) (2007) 1201.
W. Reisdorf, et al., Nucl. Phys. A 781 (2007) 459.
E. Bratkovskaya, et al., Nucl. Phys. A 622 (4) (1997) 593.
C. Hartnack, et al., J. Phys. G 35 (4) (2008) 044021.
E.C. Zeballos, et al., Nucl. Instr. and Meth. A 374 (1996) 1322.
P. Fonte, et al., Nucl. Instr. and Meth. A 443 (2000) 201.
W. Riegler, C. Lippmann, Nucl. Instr. and Meth. A 508 (2003) 14.
M. Petrovici, et al., Nucl. Instr. and Meth. A 487 (2002) 337.
M. Petrovici, et al., Nucl. Instr. and Meth. A 508 (2003) 75.
A. Schüttauf, et al., Nucl. Instr. and Meth. A 533 (2004) 65.
W. Riegler, Nucl. Instr. and Meth. A 491 (2002) 258.
I.M. Deppner, Diploma Thesis, Rupprechts-Karls University, Heidelberg, 2008
/https://www.gsi.de/documents/DOC-2008-Apr-11.html S.
[17] H.G. Essel, N. Kurz, IEEE Trans. Nucl. Sci. NS-52 (3) (2005) 745.