Detector (draft in 2014.8-2014.10)
1. Overview
1.1
Introduction
The High Intensity Electron-Positron Accelerator Facility (HIEPAF) will work at
the c.m.s. energy of 2-7GeV and reach the luminosity of L=1035cm-2s-1. The
requirements of the detector system on it must match the event characteristics, with
emphasis on those demands crucial to achieve the most important physics objectives.
There have been several workshops on the super tau-charm (-c) factory (STCF)
physics and detector [1,2…]. The physics requirements and the general considerations
on the detector components have been spelled out by previous works, such as the Elba
/c workshop in 2013 that put forward an STCF detector conceptual design, which is
a valuable reference for the HIEPAF detector system. Two earlier B Factory detectors,
the BaBar [3] at SLAC and the Belle [4] at KEK, and the upcoming super B factory
detector, the Belle-II, which have detailed the technical components, have
accumulated invaluable experience of designing the detector system. Their choices of
the major detector components are quite similar in principle to those needed in STCF.
Following their technical achievements and developments will help us figure out the
schematic detector, and find out the key technical difficulties in each sub-detector and
the way to overcome them. In another important case the experience learned from
building the BESIII [5] at BEPC-II will be helpful for the HIEPAF detector design
and construction.
However, comparing to the B factories, the HIEPAF is different both in the
physics requirements and the practical application. The HIEPAF detector should be
designed based on a fully understanding of its requirements from the key physics and
technical approaches. The choices for detector subsystems should also consider the
possible upgrade in the future.
1.2 Physics Requirements (assigned to Haiping)
1.3 General Considerations
The HIEPA detector system will face critical challenges from the Particle
Identification (PID) in a large kinematic range with the high luminosity background.
Data will be taken at a rate 2-3 orders of magnitude higher than the current /c factory,
and rare processes will become accessible.
For many physics studies at HIEPA, the systematic error will be the dominant
factor that limits the measurement precision, which may come from: a) uncertainty in
detector acceptance, either from the uncertainty in geometrical acceptance or from
uncertainty in detector response, such as the detector efficiencies and the nonlinear
energy deposit; b) detector mis-measurements, such as mis-tracked particles, fake
photons, particle mis-identification and detect electronics noise; c) the luminosity
measurement error which affect the overall normalization.
For efficient exclusive event reconstruction, background discrimination, and
reduction of the detector related systematic error, general detector requirements
include:
a) (nearly) 4π detector solid angle coverage for both charged and neutral particles,
and uniform response for all particles;
b) high resolution of momentum and angular for charged particles; and high
resolution of energy and position for photons ;
c) superior PID ability (e///K/p/g) and high detection efficiency for low
momenta particles;
d) precision luminosity measurement;
e) work under high luminosity environment.
Particularly, there are practical considerations in the detector design. Because
most of the particles to be measured are below 1GeV/c, the multiple scattering will
dominate the momentum resolution, which requires a light material budget in tracking
system. Also for measuring the low energy gammas, as little material as possible
before the Electro-magnetic Calorimeter (EMC) should be pursued for. The detector
may encounter significant radiation dose, which can be quite high in the forward
regions. So the chosen detector components should withstand the expected dose.
Furthermore the detectors and the electronics should be reliable and can be produced
with reasonable cost.
The conceptual layout of the HIEPAF detector system is shown in Fig.1. Along
the radial direction starting outwards from the interaction region, the major detector
components are: an Inner Silicon Detector (ISD) composing of several layers of Pixel
Detector (PXD) and Silicon Strip Detector (SSD) closest to the beam pipe, which is
made of beryllium (<0.5mm think) with a radius of <2cm in order to minimize the
multiple scattering; a Main Drift Chamber (MDC) in a 0.5-1.0Tesla solenoidal
magnetic field provides precise trajectory measurement for charged particles; a
Cherenkov system with the RICH technology is the main component for PID; a
homogeneous EMC composed of trapezoid-shaped crystal (BSO/PWO/LYSO)
scintillators determines precisely the photon energy; outside the EMC a
superconducting solenoid producing the magnetic field; a multi-layer flux return york
instrumented with large area Resistive Plate Chambers (RPC), taking the role as a
Muon Counter (MUC) to provide sufficient / suppression power.
Fig.1: Schematic layout of the detector system at HIEPAF.
1.4 Expected Performances
The expected key features of the HIEPAF detector system are listed below.
1) Vertexing performance and low-momenta tracking eff.
2) Tracking system: pT resolution 0.5~0.7% @1GeV/c and dE/dx resolution <7%,
low material budget ().
3) PID π/K (and K/p) 3-4 separation up to 2GeV/c with modest material budget
(<0.5X0)
4) EMC stochastic term <2%/√E and constant term <0.75%, angular resolution?
5) MUC / suppression power >10, down to p=0.5GeV/c.
2. The Inner Silicon Detector (ISD)
2.1 Introduction
The Inner Silicon Detector (ISD) is a silicon detector utilizing advanced active
pixel sensors and/or silicon strip technology. Its inner layers closest to the beam pipe
provide additional precise hits that connect the particle track reconstructed in the
Main Drift Chamber (MDC) and the collision point. The conceptual design of ISD
aims to improve the track momentum resolution and to improve the tracking
efficiency, in particular for low transverse momentum (pT < 500 MeV) particles.
Therefore, minimizing the multiple scatterings requires very low material budget.
Although there is no requirement to reconstruct secondary vertices for the
heavy-flavor particles, the sufficient spatial resolution is still needed to achieve high
momentum resolution. In addition, the ISD should response fast to survive in high
luminosity environment (~1035 cm-2s-1). The following sections will introduce options
of the existing vertex detectors in high energy experiments, the conceptual design of
the ISD and its performance estimated via simulation and theoretical calculations.
2.2 Options for vertex detector
Introduced in this section are two successful designed and tested vertex detectors
recently used in high energy experiments, with low material and high resolutions, the
Heavy Flavor Tracker (HFT) in STAR experiment and the Vertexing Detector (VXD)
in Belle-II. They both show expected performance during the beam-on operations.
2.2.1 STAR Heavy Flavor Tracker
The STAR HFT consists of 4 layers of silicon detectors grouped into three
sub-systems with different technologies, guaranteeing increasing (better) resolution
when tracking from the Time Projection Chamber (TPC) towards the vertex of the
collision. The Silicon Strip Detector (SSD) uses an existing detector in double-sided
strip technology. It forms the outermost layer of the HFT. The Intermediate Silicon
Tracker (IST), consisting of a layer of single-sided strip-pixel detectors, is located
inside the SSD. Two layers of silicon pixel detector (PXL) are inside the IST. The
pixel detectors have the resolution necessary for a precision measurement of the
displaced vertices of open heavy-flavor hadron decays. The table below lists the key
parameters of the HFT design
Detector
Radius
(cm)
Hit Resolution
R/ - Z (m - m)
Radiation length
SSD
22
30 / 860
1.5 %X0
IST
14
170 / 1800
1.32 %X0
8
12 / 12
~0.37 %X0
2.6
12 / 12
~0.37 %X0
PIXEL
The PXL detector is designed with 10 sectors with 4 ladders (1 inner ladder and
3 outer ladders) in each sector, supported by carbon fiber frame. For each ladder 10
pixel pitches with 20.720.7 mm2 size each are attached on top of the cupper or
aluminum cable, resulting in 0.52%X0 or 0.37%X0, respectively. Signals are readout
directly by the cable and collected at the end of the ladder with RDO (?) buffers and
drivers.
Figure 1: PXL ladder and sensor pitches.
The PXL detector used an advanced commercial CMOS technology, Monolithic
Active Pixel Sensors (MAPS). The sensor and signal processing are integrated in the
same silicon wafer with digital output. The sensor is thinned to 50 m. Signal is
created in the low-doped epitaxial layer (typically ~10-15 m). MIP signal is limited
to <1000 electrons. Charge collection is mainly through thermal diffusion (~100 ns),
reflective boundaries at p-well and substrata. The power dissipation is ~170mW/cm2,
which is similar to sun light.
Figure 2: MAPS operation algorithm.
The table below lists the performance parameters of PXL detector.
Pointing resolution
(13  22GeV/pc) m
Layers
Layer 1 at 2.6 cm radius
Layer 2 at 8 cm radius
Pixel size
18.4 m × 18.4 m
Hit resolution
10 m rms
Position stability
6 m (20 m
Radiation thickness per layer
X/X0 = 0.37%
Number of pixels
~436 M
Integration time (affects pileup)
200 s
Radiation tolerance
300 krad
envelope)
Rapid installation and replacement to cover Installation and reproducible
radiation damage and other detector failure
positioning in 8 hours
The read out time per sensor is limited by the frame integration time of 100-200 s.
The operation mode is column parallel readout with integrated serial data
sparsification. Figure 3 and 4 shows some performance examples of the HFT detector.
Figure 3: Left: Number of hits per sensor during the data taking in 2014 run. Right:
TPC tracks and PXL hits association.
Figure 4: Impact parameter (DCA) resolution as a function of momentum. Data
show good agreement with MC simulation.
In summary, the PXL detector based on MAPS technology delivers ultimate
resolution (DCA ~ 40 m for 1 GeV/c tracks) and low material (0.37%X0 per layer),
which is ideal for secondary vertex reconstruction and minimizing the momentum
resolution. However, its read out time is limited to 100-200 s, but still has room for
improvement.
2.2.2 Belle-II Vertex Detectors
Compared with STAR HFT, the vertexing detectors in the Belle-II experiment,
consisting of 4 layers of Silicon Vertex Detector (SVD) and 2 layers of Pixel Detector
(PXD), is with a bit worse resolution but faster readout.
Belle-II operates in the similar low energy environment of KEKB (10.58 GeV) as
HIEPAF (2-7 GeV) and with higher luminosity (~81035 cm-2s-1). This requires very
low mass and fast detector. The two layers of PXD are 1.4 cm and 2.2 cm in radius,
consisting of 8 and 12 modules for the innermost layer and the second, respectively.
In the active pixel matrix region, the thickness is ~75 m. Note the innermost layer is
very close to beam pipe. This provides precise constrains on the track projecting to
the collision point but is also technically challenging due to high radiation
environment.
Table below lists the key parameters of PXD design.
Number of pixel per module
250 × 1536
Total number of pixels
8M
Layers
Layer 1 at 1.4 cm radius
Layer 2 at 2.2 cm radius
Pixel size (r-phi, z)
50 m × (60 - 75) m
Resolution (r-phi, z)
~12 m, < 20 m
Radiation thickness per layer
X/X0 = 0.2%
Occupancy for innermost layer
0.2 hits m-2s-1
Integration time (affects pileup)
20 s
Radiation tolerance
~10 Mrad
The PXD consists of a fully depleted silicon substrate and is equipped with a
p-channel MOSFET structure with an internal gate where the electrons liberated by
traversing charged particles are collected. The internal gate modulates the current
through the MOSFET at readout time. This kind of DEPFET (DEPleted Field Effect
Transistor) pixel sensor is a monolithic structure, with current-digitizing electronics at
the ends of the sensor, outside of the acceptance region.
The inner layer consists of 8 planar sensors (“ladder”), each with a width of 15
mm, and a sensitive length of 90 mm. The outer layer consists of 12 modules with a
width of 15 mm and a sensitive length of 123 mm. The sensitive lengths in each of the
layers are determined by the required angular acceptance of the tracker.
Figure 5: PXD layout and its prototype.
The DEPFET sensors are monolithic all-silicon sensors without need of
additional support and cooling material in the active region of the detector. The n-bulk
is fully depleted with a potential minimum below the strips and the structure of a field
effect transistor. The electrons created by a charged particle accumulate in the
potential minimum. The field configuration is such that the electrons drift underneath
the gate of the transistor modifying the source drain current. An active clear is
necessary to remove the electrons, as shown in the left panel of Fig. 6. (J.Kemmer and
G.Lutz, NIMA 253, p.365, 1987).
The DEPFET pixel modules are read out in a rolling shutter mode: a matrix
segment consisting of four multiplexed rows is selected by pulling the gate line to a
negative potential using the SWITCHER chip. The selected DEPFET pixels send
currents to the vertically connected drain lines. These currents are processed at the
bottom of the matrix by the Drain Current Digitizer (DCD) chips. The DCD performs
an immediate digitization of the current with 8 bit resolution and sends the data
serially through many low-swing single ended lines to a third chip, the Data Handling
Processor (DHP), which buffers and analyzes the digital data stream and performs
zero suppression. The remaining data are then sent to the off-module data handling
hybrid (DHH), see the right panel of Fig.6.
Figure 6: Left: Operating principle of DEPFET.
Right: DEPFET readout module.
In summary Belle-II PXD is a high resolution and fast detector with low mass.
Table below shows the comparison of Belle-II PXD and STAR PXL. (values are
different from previous tables)
Pixel detector
Resolution (m)
Total material (%X0)
Si thickness (m)
Occupancy (cm-2s-1)
Radius (cm)
Readout time (s)
STAR PXL
30
0.37-0.52/layer
50
>106
2.6/8
200
Belle-II PXD
50
0.15-0.2/layer
75
max. 5107
1.4/2.2
20
2.3 Conceptual Design for ISD
The scheme -pixels followed by strips - has been successfully applied for the
detectors at RHIC and LHC. With similar luminosity of KEKB and collision energy,
the beam background at small radius of HIEPAF is also similar. This requires ISD to
be very light to minimize the multiple scattering. Both STAR PXL and Belle-II PXD
are based on thin pixel sensor technology with readout electronics and active cooling
outside the acceptance. Both would be ideal options for the ISD consideration. The
advantage of DEPFET sensor is the ~10 times faster readout than MAPS.
The ISD will consist of 1 layer of SSD and 2 layers of PXD. The SSD located inside
the MDC at a radius of ~10 cm and 2 layers of PXD that are 3 and 6 cm in radius, as
shown in the conceptual layout in Fig. 7.
Figure 7: Conceptual layout of the inner tracking system.
In terms of comparison, two options are selected for performance study as below.
Option 1: MDC + HFT-like parameters.
Detector
Radius (cm)
Material (%X0)
Resolution (m)
MDC outer 9-48
23.5-82
0.0045/layer
130
MDC inner 1-8
15-22
0.0051/layer
130
SSD
10
1.5
250
PXL 2 layers
3/6
0.37/layer
30
Beam pipe
2
0.15
Option 2: MDC + PXD-like parameters.
Detector
Radius (cm)
MDC outer 9-48
23.5-82
MDC inner 1-8
15-22
rd
PXD 3 layer
10
PXD 2 layers
3/6
Beam pipe
2
Material (%X0)
0.0045/layer
0.0051/layer
0.15
0.15/layer
0.15
Resolution (m)
130
130
50
50
-
2.3.1 Performance of ISD
The analytic fast calculation is used to study the performance in tracking with the
designed MDC + ISD parameters. The probability to pick up the correct hit can then
be described by the following equation:
a
r
òs
2
0
2 æ
1 ö
e-r ç pr +
÷ dr .
è
2s 2 ø
(1)
The probability to pick up the wrong hit is described by the equation:
a
r
òs
0
2
e
-
r2
2s 2
(1- e ) dr ,
-p r2r
(2)
where σ is the uncertainty in the track extrapolation from the outer detectors to the
target layer,  is the background hits density, and ‘a’ (the upper limit of the integral) is
the search cone radius. The analytic calculation of the quantity σ includes
contributions from the multiple Coulomb scattering (MCS), the detector resolution,
and the track position uncertainty due to the quality of track fitting. The quantity 
includes contributions from the pile-up events and other tracks in the current event.
We have discovered that track fitting errors are the dominant term in the calculation of
σ while the detector resolution drops out due to the extremely high resolution of the
detectors. So in the simulation study, we have set σ equal to the search radius used by
the tracking software at each detector layer.
The tracking software is based on the Kalman Filter techniques from [Billoir
NIM 225 (1984) 352]. The detector geometry and hit resolutions as listed in Option 1
and 2 are used. The impact parameter (or DCA) resolution (accuracy of pointing to
the vertex) can be expressed in the following form:
s =
2
s 12 r22 + s 22 r12
(r2 - r1 )2
2
q mcs
r12
+ 2
sin (q )
(3)
where σ1 and σ2 are the position resolutions on each detector layer, r1 is the inner layer
radius and r2 is the outer layer radius and θmcs is the MCS angle in the first layer of the
detector. θ is the angle of entrance into the detector relative to the beam line. The
second term, the projection error due to MCS, is the parameter of interest and it is this
term that dominates the detector performance at low particle momentum.
The MCS effects are estimated by randomly smearing the scattering angle of the track
when projecting to the next layer. The scattering angle is sampled with Gaussian
distribution with the rms as:
q rms =
13.6
bp
L
X0
(4)
where L is the track path length along the incident direction. L/X0 is the fraction
radiation length. In the Kalman filter tracking process, a covariance matrix method is
used in the iteration:
*
I conv
= (DT )-1 (I *-1 + A)-1 D-1 + M
(5)
A is the angle matrix for MCS at each step. D is the distance matrix at each step,
which propagates the particle backwards along the track. M matrix stores the detector
resolution at each step. Note the following performance study are with 100% hit
finding efficiency, further study by taken into account hit finding probability from Eq.
(1) and ghosting rate from Eq. (2) is ongoing.
Figure 8 and 9 show the momentum resolution and position resolution performance of
the inner tracking system with Option 1 and 2, respectively. With MDC + PXD (or
PXL) the momentum resolution is improved by about a factor of 2 at 3-4 GeV/c
compared with MDC tracking only. The position (DCA) resolution, also improved by
more than a factor of 2, is ~60 m at 1 GeV/c. The two options give similar results.
This is likely because the worse resolution in Option 2 is compensated by the
reduction of the material compared with Option 1.
Figure 8: Momentum resolution (left) and position resolution (right) from the analytic
calculation with Option 1. (change PVD to PXL)
Figure 9: Momentum resolution (left) and position resolution (right) from the analytic
calculation with Option 2.
In Fig. 8, the third layer, SSD, helps little in terms of momentum resolution and
position resolution, but it helps to connect PXL hits and MDC tracks thus will
improve the tracking efficiency. Since efficiency study is more complicated and need
further full Geant simulation, here we only use the existing experiment for reference.
For example, Figure 10 shows how the STAR SSD helps in the tracking efficiency
with full Hijing + Geant simulation.
Figure 10: STAR HFT tracking efficiency.
2.4 Summary
Silicon pixel + strip detectors are now commonly used as inner tracking system
in high energy experiment. The concept of these detectors and related technology are
mature and under continuous developing. The expected performance of these
detectors with low material budget and high resolution is appropriate for the HIEPAF
ISD consideration. The inner tracking helps to improve the momentum resolution,
position resolution and tracking efficiency, especially for low momentum tracks.
However, the man power and knowledge for the silicon detector technology and
related electronics in China are far from ready yet.
3. Main Tracing Detectors (MTD) (assigned to Jianbei)
4. Particle Identification (PID)
4. 1 Introduction
To fully exploit the physics capability at HIEPAF, it’s essential to have excellent
PID performance. From the simulation study in the previous (physics and accelerator)
sections, a series of stringent requirements are imposed to the PID detectors. Charged
hadrons (, K, p) are to be distinguished up to p=2GeV/c to cover all available
kinematics, and need good separation from electrons and muons down to low
momentum range. e/h and / rejection power better than 100 and 10 are pursued.
Identification of neutral particle (0, neutron) and photon is another important aspect
of the experiment, as well as the precision measurement of their energies or masses.
In this section, we mainly focus on the PID of charged hadrons. The identification of
electrons and photons, as well as 0 and neutron, are primarily done by the EM
calorimeter, which will be discussed in detail in the EMC section. Distinguishing
muon from hadrons (especially pion) is briefly introduced in this section, but the
majority of the discussion will be left in the MUC section.
As depicted in Fig. 1.1, the charged hadrons identification is achieved mainly
through the PID detector, which is located between the tracker (MDC) and the EM
calorimeter. Since the luminosity at HIEPAF is expected to be high (1035cm-2s-1), fast
and radiation hard detectors are preferred, especially in the endcap region. The
material budget must be minimized (<0.5X0) in order to ensure good EMC energy and
spatial resolution. To reduce the total cost of the detector system, the inner radius of
the EMC can’t be too large thus the PID detector is required to be compact to fit the
~20cm space between the MDC and the EMC.
4.2 Detector Options
According to the basic detector concept, charged particle ID can be obtained by
the specific energy loss (dE/dx) measurement at low momentum. With a dE/dx
resolution of 6-7%, clean /K/p ID for p<0.8/1.1 GeV/c is achievable. Further
charged particle ID up to p=2GeV/c requires a dedicated PID detector system. At
HIEPAF energy, two types of detector techniques can be chosen to improve the PID at
intermediate momentum range, namely time-of-flight (TOF) and Cherenkov detector.
4.2.1 TOF
A TOF utilizes the difference mass, thus difference velocity at a given
momentum, to distinguish the particle species. Excellent time resolution is crucial to
extend PID by TOF to high momentum. The basic formulae for TOF PID are listed
below,
,
where T is the time of light,  is the velocity, m is the particle mass and p is the
momentum. T,  and m2 are the differences of T,  and m2 between different
particle species. Thus
,
where L is the path length of the particle trajectory and c is the speed of light in
vacuum. As shown in Fig. 1, L may be down to 1m.
For /K and K/p at p=2GeV/c, the time difference T are around 0.1 and 0.27ns. So
to achieve 3 /K/p separation an overall TOF time resolution better than 30/90ps is
needed. Considering the available choice on detector technology, a TOF alone cannot
identify /K to p=2GeV/c.
4.2.2 Cherenkov detector
The Cherenkov radiation is commonly used in high-energy experiments to
identify particles at high momentum. This specific radiation happens if a charged
particle travels faster than the speed of light in the medium, as illustrated in Fig. 1. By
choosing different types of medium, often called Cherenkov radiator, particle species
can be distinguished in a range from several to hundreds GeV, depending on the
refractive index of the radiator. As shown in Fig. 1 is the principle of Cherenkov
radiation. With a refractive index n of the radiator and particle speed v=c, the
Cherenkov radiation emits at an angle c relative to the particle moving direction.
Some relevant formulae are listed below.
Fig. 1: The principle of Cherenkov radiation and main parameters.
,
where and is the Cherenkov emission angle;
and ,
which denote the threshold velocity and Lorentz factor for Cherenkov radiation
emission; and the differential Cherenkov photon yield is expressed as:
,
,
in which N, E and  are the photon yield, energy and wavelength,  and h are the fine
structure constant and Planck constant respectively.
To identify different particle species by using Cherenkov radiation, the usual
ways are:
a) to check if there exists Cherenkov radiation at given momenta, i.e. if the particle
has reached its threshold velocity. In this way the detector is called threshold
Cherenkov detector, which is usually technically simple to build but often applicable
limited;
b) to determine the Cherenkov emission angle precisely, and compare to expected
values of different particle species to find the most possible candidate. In this case the
momentum vector when the particle hits the Cherenkov detector must be known with
good precision, and the spatial resolution of the Cherenkov light detector should be
good enough. The detector design is usually more complex than the threshold
Cherenkov detector, but with much wider kinematic coverage and more diverse PID
capability. There exist various methods to experimental realize such kinds of
Cherenkov detector, such as the ring imaging Cherenkov detector (RICH) and the
detection of internally reflected Cherenkov light (DIRC).
Fig. 2: The separation of different particle species by Cherenkov angle and
momentum measurements.
Shown in Fig. 2 is the idea to separate different type of particles through
Cherenkov emission angle and momentum measurements. The blue lines in the plot
denote the expectations of charged particles e, , K and p due to the equation below,
,
where is the Cherenkov emission angle at high velocity limit (v=c) and . The
separation of between two particles with mass difference of is written as
.
the grey bands in Fig. 2 simulate the experimental results, with an uncertainty
estimation of
.
with the two parameters and , the PID capability can be evaluated. The right vertical
coordinate shows the number of Cherenkov photon , where is XXX and T is the
thickness of the radiator.
Options for the PID detector
4.3.1 Barrel
RICH is a suitable Cherenkov detector for PID at barrel. To avoid space
consumption and relatively complex optical design, the proximity-focusing RICH is
found to be an appropriate candidate. Due to the limited space, the proximity gap
should not be very large. A 10cm gap is considered here. Without TOF, the dE/dx
measurement only (7% resolution) can separate /K to p=0.8GeV/c. To ensure full
PID coverage, the Cherenkov radiation measurement should be applicable at this
momentum, indicating that the refractive index of the Cherenkov radiator must be
larger than 1.18. This requirement practically removes all gas mixture and aerogel as
Cherenkov radiator, leaving only liquid and solid radiator material.
Liquid C6F14, a Cherenkov radiator with n=1.3 at 175nm wavelength, is a proper
candidate already proven in previous high energy experiments. A possible design is
that similar to the high momentum particle identification detector (HMPID) currently
operating at the ALICE experiment, as shown in Fig. 3. The liquid radiator is
encapsulated in an UV-transparent container (e.g. quartz). The generated Cherenkov
photons are detected by the photon sensor array after an 8cm proximity gap. The
photon detection is achieved by multi-wire proportional chamber (MWPC) with its
cathode covered by CsI film. The readout pad size of around 8*8.4mm2 is tuned to
ensure 3 separation for /K/p up to p=3/5GeV/c.
LiF is another commonly used Cherenkov radiator in high energy experiment,
with n=1.46 at 7eV. Compared to liquid radiator, it is relatively easier to maintain and
operate. LiF is more UV transparent than quartz (or fused silica). However for
charged particles at high momentum, the generated Cherenkov light will suffer total
reflection inside the LiF geometry thus the detection efficiency is low. CLEOIII tried
a novel surface processing method to improve this situation – the rear surface of the
LiF radiator is machined to be sawtooth like so that the outgoing Cherenkov photons
suffers less total reflection. With this method an 4 separation for /K within
0.47-2.65GeV/c momentum range is achieved at CLEOIII.
Fig. 3: The structure of ALICE HMPID and its PID capability for charged hadrons.
4.3.1.2 TOF + Threshold Cherenkov Detector
An alternative PID detector technique involves TOF. As discussed earlier, a TOF
system with an overall time resolution of ~90ps provides required performance for
K/p separation. However for /K separation the needed ~30ps time resolution is very
challenging to achieve. One of the reasons is the precise determination of the collision
time (start time), which is roughly 55ps at BESIII. So with TOF alone it’s not
sufficient to identify all charged hadrons in the kinematic range at HIEPAF.
In order to further distinguish /K, additional detectors may be required. A possible
candidate is the threshold Cherenkov detector, which is relatively simple to build. In
Fig. 4 the threshold momenta of , , K and p are plotted as a function of the
refractive index n. It’s clear with n~1.03  and K can be identified in the momentum
range of 0.6-2GeV/c by checking whether the Cherenkov radiation happens.
A practical example of such approach is the TOF+ACC (aerogel Cherenkov counter)
PID system in the BELLE experiment. Sufficient /K separation in 0.8-2.5GeV/c with
n=1.03 and separation in 1.0-3.5GeV/c with n=1.01 have been experimentally verified.
Both TOF and ACC are technically easy to realize, but need adequate space to
accommodate all the detector components. The gap between MDC and EMC need be
increased to ~30cm to fit this PID option. The influence on outer detectors,
performance and cost, should be taken into account.
Fig. 4: The dependence of Cherenkov threshold momentum for different particles on
the refractive index n of the radiator.
4.3.1.3 TOP
Time of propagation (TOP) technique is a recent progress in the field of
Cherenkov detector. The principle of this technique is illustrated in Fig. 5. It makes
use of the total reflection inside the fused silica (served as radiator as well as
Cherenkov light guide) and precision measurement of both the 2-D hit position and
the timing information at the one or both ends of the radiator. In this way the final 2-D
spatial and 1-D time structure is combined to identify the particle species. This
technique is to some extent similar to the DIRC approach, but with 1-D spatial
measurement replaced by 1-D timing measurement. The modification of readout
method makes TOP an impact Cherenkov detector. The challenge in measuring
simultaneously the photon hit position and time with good precision is well within the
capacity of modern photon sensors.
The TOP technique has been extensively studied as a candidate detector for the
BELLE-II experiment. The technical principle has been proven in the beam tests. The
Cherenkov photons are detected by MCP-PMT. As shown in Fig. 5, the horizontal
coordinate represents 2-D position measurement (converted into 1-D position) while
the vertical coordinate shows the timing information. With a photon sensor size of
~5.3*5.3mm2 and ~50ps timing resolution, the Cherenkov radiation structure is clear
seen in this time-space plot. The agreement with Monte Carlo (MC) simulation is also
excellent, which is essential for likelihood PID analysis. Nevertheless adapting this
technique to the PID detector at HIEPAF requires further experimental verification.
Fig. 5: The principle of TOP and its experimental measurement.
4.4 Endcap
4.4.1 RICH
Similar to the RICH used in the barrel, this technique is applicable at the endcap
region. Due to longer track length in the MDC, the energy loss resolution may be
improved, thus allowing wider identification range by dE/dx only. If the dE/dx
resolution is good enough so that /K separation can go up to ~1GeV/c, there will be
more choices to realize the RICH. According to Fig. 4, kaon starts to generate
Cherenkov radiation at p~1GeV/c with n~1.13. Such a refractive index may be
obtained by aerogel technique, adding more radiator candidates to those introduced
previously for the barrel. Furthermore, this RICH serves as a threshold Cherenkov
detector for proton identification since the threshold momentum for proton at n=1.13
is close to 2GeV/c.
BELLE-II experiment uses this approach as the baseline design in the endcap region,
as shown in Fig. 6. Double aerogel layers are used with the refractive index of the
second layer slight higher than the first one n2>n1. This helps improve the spatial
resolution of the Cherenkov light ring thus better determine the Cherenkov angle.
n~1.06 and 20cm proximity gap are chosen so that /K separation can be obtained up
to p~4GeV/c (but with a kaon threshold momentum of 1.4GeV/c). The beam test
results confirm this expectation with >5.5 significance. Different kinds of photon
sensors are studied, including HAPD, MCP-PMT and SiPM. Each of them provides
satisfying photon detection, while the later two need further study especially on the
radiation tolerance and aging effect.
Fig. 6: Schematic view and experimental test setup of the BELLE-II ARICH in the
endcap region.
4.4.2 TOF+ACC
The PID method with threshold Cherenkov detector and TOF is relevant in the
endcap region, just similar to that in the barrel. However, to ensure good time
resolution the endcap TOF (ETOF) should be carefully designed. Plastic scintillator
coupled with fast PMT is used in the endcap at the BESIII experiment. The data taken
in e+e- collisions reveals that the TOF resolution is around 110-130ps for charged
particles other than electron. To get better time resolution the new type gaseous
multi-gap resistive plate chamber (MRPC) is proposed to replace the current ETOF.
Beam test has proven the choice and an overall TOF resolution <80ps is foreseen. If
successfully operated in BESIII, this technique can be adapted to the experiment at
HIEPAF.
4.5 Conceptual Design and Expected Performance
Considering all the detector options and technical advancement, the baseline
design of the PID detector at HIEPAF is illustrated in Fig. 7. The basic structure is
similar to that of the ALICE HMPID, but with MWPC in the photon detection part
replaced by triple gaseous electron multiplier (GEM) layers. The GEM readout is
mechanically easier to maintain, meanwhile the position resolution and rate capacity
are much better than the MWPC. The surface of the first GEM layer is doped with a
CsI layer of several hundred nanometers thick to convert the Cherenkov photon to
electron (called photo-electron, or p.e.). The electric field in the ionization gap is
reversely biased so that most of the electrons from charged particle ionization are
removed. In this way the background noise is significantly suppressed. This readout
method has been used in the PHENIX hadron blind detector (HBD), also proposed for
the PANDA particle ID.
According to studies and operation experience at ALICE HMPID, the liquid
radiator C6F14 must be kept at high purity. The impurity, especially Oxygen, must be
less than 10ppm (?). This imposes the major technical challenge of the baseline design.
High qualities liquid recycle system with excellent purification and monitor function
is needed. Another important issue is to develop a dedicated CsI doping and testing
technique for the GEM foil. Long term stability and aging effect should be thoroughly
investigated.
The baseline design uses the same technique in both the barrel and the endcap
region. This reduces the complexity of combining different detection methods and the
risk to maintain the PID system.
Fig. 7: The baseline design of the PID detector.
The radiator thickness and the proximity gap should be tuned to optimize the
performance and the cost of the PID detector. At n=1.3 and p=2GeV/c, the /K
separation requires ~120mrad Cherenkov angle resolution . Take a photon yield of 10
and a proximity gap of 10cm, the estimated photon sensor size is around 1-1.5cm,
which should be straightforward to achieve.
The intrinsic spatial uncertainty from the proximity focusing technique should
also be taken in account. For n=1.3 the Cherenkov angle is at most . In this case a
radiator of T thick the smearing of the Cherenkov light cone will be around . If
T=1cm the spread is around 0.55cm. Thus it’s insignificant to have a photon sensor
smaller than 0.55cm.
4.6 Summary
According to the requirements on PID imposed by the physics program at HIEPAF,
various detector techniques applicable at this experiment are discussed. The baseline
design is presented and the estimated performance fulfills the PID capability target.
5. Electromagnetic Calorimeter (EMC)
5. 1 General Consideration
The electromagnetic calorimeter (EMC) is an array of scintillating crystals
readout by photo-sensors. It has a barrel EMC and two endcap EMCs to maximize the
coverage towards 4It measures the energy and direction of photons, electrons and
discriminates between electrons and charged hadrons. It is also helpful for the
identification of hadrons including anti-neutrons. To accomplish these jobs, the EMC
is required to have good energy and position/angular resolution. It is also helpful if it
has good timing resolution.
In the high luminosity era, the background in the EMC region is significantly
higher than the EMC currently operating in electron-positron colliders, such as the
EMC of BaBar, Belle and BESIII. The high background, on one hand, results in
radiation damage of the crystals. This radiation damage degrades the light yield of the
crystal thus worsen the EMC energy resolution. Furthermore, the degradation of the
crystal light yield is a function of radiation dose, or the running time. This will
requires much more efforts on the timely calibration and potentially introduce larger
systematic uncertainty. On the other hand, the high rate of photon background
produces lots of pile-up in EMC. They energy deposited in EMC by the high rate of
photon background fluctuates frequently with time. These noises will definitely affect
the energy and position resolution of the EMC.
With the new challenge in the high luminosity era, the EMC is required to have a
crystal with short decay time thus a short signal shaping time and charge integration
time is adequate to get high statistics of fluorescence photons. The crystal is also
required to be radiation hard. In the reality that the fast crystal in the market usually
has much less light yield than that of CsI(Tl) used by BaBar, Belle and BESIII EMC,
to construct a EMC with fast crystal and good energy resolution, a photo sensor with
high photon detection efficiency, high gain and small excess noise factor is needed.
5.2 Crystal Options
The properties of crystal significantly impact the performance of EMCs. Tabel 1
lists the properties of various crystals including doped CsI and pure CsI, BSO,
PbWO4 and LYSO crystal. The doped CsI crystal CsI(Tl) has very high light output. It
is widely used in electron-positron collision experiments such as CLEO, BaBar, Belle
and BESIII. Very good energy resolution was achieved. The major difficulty to use it
in the high luminosity era is to deal with its long decay time, as large as microsecond.
The small radiation resistance also prevents it to be used at a high background region.
The superB experiment think the radiation damage of CsI(Tl) is affordable at barrel
and proposed to reuse Babar barrel EMC as its barrel EMC, but pay the price of
higher noise equivalent energy due to shorter shaping and integration time. The Belle2
experiment also decides to reuse Belle barrel EMC and upgrade the electronics to
Flash ADC to suppress the pile-up effects.
5.2.1 CsI Crystal
Pure CsI is much faster than doped CsI, which make it a candidate of crystal at
high luminosity era although its light output is significantly lower than doped CsI.
The major difficulty is to find an appropriate photosensor with high quantum
efficiency at the wavelength of ~300 nm.
5.2.2 LYSO Crystal
The LYSO crystal has relatively high light output (about half of doped CsI), short
decay time (about 40 ns) and high radiation hardness (upto 108 rad). These properties
make it an ideal candidate of the EMC crystal at high luminosity era. However, the
high melting point makes it too expensive to be affordable for a big detector array.
5.2.3 PbWO4 Crystal
PbWO4 crystal is also a fast crystal. It has a major component with decay time of
30 ns and a smaller component with decay time of 10 ns. It has been used in CMS
experiment and provides good energy resolution at high energy. However, the low
light output limits it to be used at an electron-positron collision experiment where the
energy resolution at low and intermediate energy is important. The Panda experiment
at GSI is working with crystal vendors to improve the light output of the PbWO4
crystals and a 2 to 3 times of light output improved is achieved. They also proposed to
work at a temperature as low as -25 oC to further increase the light output.
5.2.4 BSO Crystal
Recently, a new type of crystal, BSO, is being developed. Its performance is
between doped CsI and PbWO4. The density is higher than CsI(Tl) and close to that of
PbWO4. Its radiation length and Moliere radius are about 40% lower than doped
CsI(Tl) and close to PbWO4. This allows a more compact EMC with higher
granularity, which is import to improve the position/augluar resolution and suppress
the pile-up effect as the area of surface of single crystal is significantly reduced. Its
decay time is about 100 ns, an order or magnitude smaller than that of CsI(Tl) and a
few times larger than that of pure CsI, PbWO4 and LYSO crystals. Its light output is
about a factor of 50 lower than CsI(Tl), but about 10 times higher than PbWO4. The
light output of the crystals within a window of 100 ns is also listed in table 1 for
comparison. Note that although the light output of CsI(Tl) within 100 ns is still high
to be able to give good statistics, the long decay time will produces significant pile-up
to the upcoming events and significantly reduce the EMC performance. The radiation
hardness is also reported to be as high as of 105-7 rad. It turns out that the BSO crystal
is a very good candidate for the high intensity electron-positron collision experiment.
One concern is that this crystal has not been widely used in high-energy experiments
and the mass production capability is not proven yet. However, the Shanghai Institute
of Ceramics, Chinese Academy of Sciences (SICCAS) has successfully produced 9
crystals for us for properties test. Figure 1 shows the 9 crystals with size of 2x2x20
cm3. Figure 2 shows the light output measured with a PMT XP2262 at room
temperature. A good trend is observed that the light output increases with production
time. The last 3 crystals have light output of around 90 photoelectrons per MeV
deposited by a -ray. This light output is comparable to the improved PbWO4 at
operation temperature of -25 oC. The right panel of Fig. 2 shows that the decay time
of BSO is about 100 ns. Figure 3 shows the normalized light output as a function of
time in the presence of  irradiation source at different dose rate. There is still room to
improve the radiation hardness.
Table 1 Properties of different crystals
Figure 1 BSO crystals produced by SICCAS.
Figure 2: The light output of the 9 BSO crystals and as a function of charge
integration time.
Figure 3: The radiation
damage test result obtained
by CalTech group. The
upper panel shows the
normalized
emission-spectrum-weighted
longitudinal
transmission
efficiency. The lower panel
shows the normalized light
output.
5.3 Readout Methods
At low and intermediate energy, the energy resolution is not dominantly
determined by the statistics of photon produced in the crystal. For example, A BSO
crystal with light output of 100 p.e./MeV produces about 10,000 photoelectrons when
hit by a 100 MeV photon and gives energy resolution of 1%. The noise from the
electronics plays an important role in the energy resolution and high
signal-to-background ratio is essential to achieve good energy resolution at the energy
range of tens MeV to a few GeV. The signal-to-background ratio depends on the light
output of the crystal and the gain of the readout photosensor. A crystal with high light
output readout by a photosensor with high gain gives high signal-to-background ratio
and, as a consequence, gives small equivalent noise energy. To achieve good energy
resolution, the noise from the photosensor itself is also required to be small.
5.3.1 Silicon Photodiodes
Silicon photodiodes (PD) are widely used in high-energy physics as light detectors,
such as BaBar, Belle, BESIII, CLEO, L3. Its quantum efficiency can exceed 90%.
However, its gain is only 1. Low-noise amplifiers are necessary and low
signal-to-background ratio is expected anyhow, unless the crystal has light output as
high as CsI(Tl).
5.3.2 Avalanche Photodiodes
In comparison with PD, the avalanche photodiodes (APD) have high gain
(~10-1000) .This can suppress the equivalent noise energy caused by the electronics.
However, the APD itself have large noise thus large excess noise factor (ENF) and
substantial cooling is often necessary.
One of the most promising recent developments in the photosensors is that of devices
consisting of large arrays of tiny APDs packed over a small area and operated in a
limited Geiger mode. It is usually called Silicon Photomultiplier (SiPM). It offers gain
of 105-6 at moderate bias voltage (~30-100 V) which helps to suppress the equivalent
noise energy and simply the electronics. It also has very good timing resolution (~0.2
ns for single photoelectron). It can be made in a size of as large as 3mm x 3mm per
channel with a price of O(10$), and can be easily butted to provide a larger
photosensitive area. The number of pixels can reach as high as 10,000 per mm2,
which provides a dynamic range of a few photoelectrons to 1,000,000 photoelectrons
for a 1 cm2 array. One major concern is its tolerance to neutron radiation and many
groups are working with the producers to verifying and improving it. Results show no
sign of preventing it to be used as crystal calorimeter readout in high-energy physics.
This device can provide compact, economical readout of EMC with high
signal-to-background ratio and low excess noise factor.
Figure 4 shows the charge spectrum measured by a 3x3 mm2 SiPM (Hamamatsu
S10362-33-050C) coupled to a 3x3x15 mm3 BGO crystal irradiated by a 137Cs and
22
Na  radioactive source, respectively. The SiPM has 3600 pixels in total. The
number of fired pixels as a function of the -ray energy is shown in the left panel of
Fig. 5. The right panel shows the corrected number of detected photons according the
formula:
.
The numbers shown in the figures indicate the deviations of the last data point
from the expectation based on the data points below 700 MeV. The uncertainty of the
expectation is estimated to be 2.5% and 3.8% for BGO and CsI(Tl) crystals,
respectively. The energy resolution as a function of energy and number of fired
pixels are shown in Fig. 6. A few percent energy resolution is achieved at energy
about 1 MeV. Our LYSO+SiPM results are similar as LYSO+PMT results shown in
SuperB Technical Design Report. A BSO+SiPM is to be tested in the near future. The
new generation of SiPM produced by Hamamatsu is to be pursued and tested to verify
the improvements on the suppression of dark noise, after pulse and radiation hardness.
Figure 4 Charge spectra measured by BGO+SiPM with different  sources.
Figure 5 The linearity of the SiPM.
Figure 6 The energy resolution of crystals readout by SiPM.
5.4 Barrel and Forward Calorimeters (Design)
The EMC for the super tau-charm factory is required to be fast, radiation hard,
have good energy and positioning resolution and be cost effective. For the barrel
portion of the EMC, where the radiation and background is relative low, we proposed
to use BSO crystal readout by SiPM. Although the light yield of BSO crystal is lower
than CsI(Tl) crystal used by the BaBar, Belle and BESIII EMCs, a higher
signal-to-background ratio can be achieved by using the SiPM photosensor with gain
of 105-6. Since the noise term contribute significantly to the EMC energy resolution at
low and intermediate energy where we are interested in, the signal-to-background
ratio is more important than the light output itself. We expect the BSO+SiPM EMC
has energy resolution no worse than CsI(Tl)+PD EMC.
For the forward portion of the EMC, we proposed the same option as the barrel
EMC. A careful calculation of the radiation flux at the forward region and detail
investigation of the radiation hardness of BSO crystal is to be performed to verify that
the BSO can survive in this region. In the case the radiation background is too high
for BSO, though the possibility is low, we proposed to use the improved PbWO4
developed for Panda experiment, and still readout by SiPM to avoid operating at
temperature below 0 oC.
5.5 Summary
The EMC at high luminosity requires the crystal being fast and radiation hard. The
BSO recently being developed is relatively fast (decay time ~100ns), dense (radiation
length and Moliere radius are about 40% larger than CsI(Tl)), radiation hard (reported
to be 105-6 rad) and cost effective (cost of raw material ~ PbWO4). The lower light
output compared to CsI(Tl) can be canceled out by the high gain of the photosensor
SiPM in terms of signal-to-background ratio, which contributes significantly to the
energy resolution at low and intermediate energy region. The Chinese vendor
produced up to 10 large size BSO crystals and showed good performance. The
performances of SiPM are also promising. Based on these, we propose the
configuration of BSO+SiPM as the option of EMC at the super tau-charm factory.
An alternative option is improved PbWO4+SiPM.
6. Muon Counter
6.1 General Considerations
The Muon Counter (MUC) locates at the outmost part of the detector system.
Using the magnetic flux return York as the hadron absorber, the active detector
elements are inserted into the gaps between the iron plates. The MUC detects the
muons and charged hadrons that escape from the EMC and identifies them mainly by
comparing the measured and predicted tracks. The basic requirements for the MUC at
HIEPAF includes high detection efficiency and good muon/hadron suppression power
(>10).
The Resistive Plate Chambers (RPC) is widely used as the active detectors for
muon detection at many experiments, such as BESIII and Belle [M. Ablikim et al.
(BESIII Collaboration), Nucl. Instru. and Meth. A 614 (2010) 345–399; A. Abashian
et al. (Belle collaboration), Nucl. Instru. and Meth. A 449 (2000) 112-124]. RPC can
be operated in either avalanche mode or streamer mode, with selected gas components
and working HV. With the streamer mode, RPCs output large enough signals which
can be discriminated directly without amplification. This is certainly a notable
advantage of simplifying the electronics system and saving money. The avalanche
mode has a weaker electron multiplication in the electric field and does not trigger the
streamer. Such a mode reduces the momentarily dead area around the avalanche and
potentially increases the rate capability. The typical resistive plates used in RPC
detectors are floating glass or bakelite plates. The glass electrodes have stable quality
uniformity and bulk resistivity around 5×1012 Ω·cm. The resistivity of bakelite plates
is typically lower than the glass plates by a magnitude of 1-2 and can be controlled in
some degree by the manufacture technique. In order to achieve good enough surface
quality the bakelite plates normally are treated with either linseed oil coating or
phenolic paper laminating.
For the muon identification in the τ-charm region like HIEPAF, the momentum
cut-off is an important consideration. In order to extend the muon identification range
to lower momentum, it is necessary to put the first layer of muon detector inside the
magnetic flux return plate. The thickness of the inner layers of the iron absorbers
should be made relatively thinner than the outer ones, namely, more detector layers
used. Besides, the first one or more detector layers could be made by TOF-like timing
RPCs, which provide extra precise time measurement and enhance the muon
identification capability.
Another consideration is the luminosity related background events at HIEPAF. As
reported by the Belle-II group, the observed efficiency drop on the outer layers of the
endcap muon detector at BELLE is associated with the accelerator beam background
and dominated by neutrons with an energy of about 10-100keV [Belle II Technical
Design Report, KEK Report 2010-1, October 2010]. These neurons generate ambient
flux illuminating the RPC detectors and the hit rate increases with the beam current.
Extrapolating the results from Belle to Belle-II luminosity, the conclusion is quite
negative: all layers of the endcap RPCs will be completely inefficiency. The predicted
ambient rate at the endcap region of Belle-II goes as high as 3 Hz/cm2. They are
considering replacing all the endcap RPCs and the inner layers of the barrel RPCs
with scintillator strip detector. The dependence of efficiency on the ambient hit rate is
more evident for the RPCs working in streamer mode, due to the intrinsic dead time
associated with the recovery of electric field near a discharge. RPCs working in
avalanche mode should be less affected by such background rate.
6.2 Possible Technical Choices
The basic detector element chosen for HIEPAF MUC is RPC made by glass
electrodes working in streamer mode, which has the advantages of simple
construction structure, mature technology and low cost. A cross section plot shows the
Belle glass RPC superlayer module with 2 RPC layers, as shown in Fig. 1. The
double-gap design provides higher efficiency (>98%) compared to single layer
(90-95%). The support spacers are offset in the two layers to minimize the associated
dead area. The signal pick up strips on the two readout planes are orthogonal to each
other, so that the two-dimensional position can be achieved in one superlayer. The
required spatial resolution for MUC is around several centimeters, considering the
multiple scattering of particles in the iron plates and other materials. The width of the
pickup strips will be in this order to save electronics channels. One of the most
important aspects of the readout design is the impendence matching with the signal
transmission cables and the front electronics. The double-gap design provides also the
operational redundancy with independently gas and HV supply to each RPC layer.
For a RPC working in streamer mode, the gas mixture contains 62% HFC-134a,
30% argon, and 8% butane-silver, for example. The working electrical field is around
4.3kV/mm. A typical efficiency plateau is show in Fig. 2, reported by the Belle KLM
group.
(plot not very clear)
Fig. 1: the cross section of a superlayer RPC module for Belle.
Fig. 2: The efficiency plateau of a single layer RPC and a superlayer with 2-gap.
Considering the low rate capability of streamer mode (~0.2Hz/cm2), the avalanche
mode operation is preferred for the endcap region. The detector structure could be the
same. A Freon based gas mixture will be used and higher electrical field is needed.
Additionally, amplifier will be used in the front-end electronics, which will increase
the cost.
6.3 R&D of Timing RPC
Timing RPC, namely Multi-gap Resistive Plate Chamber (MRPC), has already
been widely used as Time-of-Flight (TOF) system in many high energy experiments.
Recently, a large area Muon Telescope Detector (MTD) has been built with long-strip
MRPC detectors for the STAR experiment at RHIC. This novel muon system
identifies muons from hadrons using the good timing performance of MRPC, together
with its moderate spatial resolution and high efficiency. The STAR MTD locates
outside the magnetic return iron bars and contains only one layer of MRPC [L. Ruan
et al., J. of Phys. G 36 (2009) 095001].
The MRPC with long readout strips takes the advantages of good time resolution
and high efficiency, while having less readout channels to save the cost on electronics,
especially for large area coverage. The signals are read out from two ends of the strips
that make it possible to calculate the incident position with the measured time
difference. The MRPC module used for STAR MTD has 5 gas gaps of 250 μm. The
effective area is 8752cm2, which is read out by 12 strips as long as ~90cm. The
cosmic ray test and the in-beam test show the time resolution is about 60-70ps and
spatial resolution is better than 1cm, as shown in Fig. 3.
The MTD modules already installed on STAR have been calibrated with cosmic
ray data. The results show an overall time resolution of 108 ps and spatial resolution
of 2.6 cm (Z direction) and 1.9 cm (φ direction), as shown in Fig. 4 [C. Yang et al.,
Nucl. Instru. and Meth. A 762 (2014)16 –6].
Fig. 3: The efficiency, time resolution and spatial resolution of LMRPC prototype.
Fig. 4: The calibrated performance of the MRPC installed in STAR with cosmic ray.
Fast simulation shows that the time of light of punch-through pions and muons
coming out from the EMC show some difference at low momentum, as shown in Fig.
5. If MRPC was installed at the innermost layer of the muon counter at HIEPAF, it
will be very helpful for the identification of low momentum muons from pions. As
shown in Fig. 6. the muons can be clearly separated below 400 MeV.
Fig. 5: The time of flight differece between pion and muon (R=180cm).
Fig. 6: The separation of pions and muons with a “TOF” measurement of 50 ps
resolution.
6.4 Conceptual Design
The MUC at HIEPAF composes of one barrel and two endcap parts. The barrel
part is made up of 11 layers of active detectors and 10 layers of iron plates, which act
as both the hadron absorber and magnetic flux return. The first three layers of the iron
plates are 3 cm thick while the rest seven layers are 5 cm. The spaces between the iron
plates are 4 cm for the installation of detectors. Thus, the total thickness of barrel
muon counter in R direction is around 88 cm. The detectors used in barrel region are
mostly glass RPC in streamer mode except for the innermost layer which will be
made of long strip MRPC. For each layer of the detector, perpendicular readout strips
locate on the two surfaces with a pitch of 4 cm.
The two endcap parts also have sandwich structure with 10 layers of iron plates
and 11 layers of detectors. The thickness of iron plates are 4 cm. The ring-shaped
detector layer will be divided into 8 pie-shaped pieces. All the detectors in the endcap
region will be glass RPCs, but working in avalanche mode. The readout strips are in
the R and φ direction on the two surfaces with a pitch of around 4 cm.
6.5 Summary
The MUC for HIEPAF will identify muons from hadrons with a momentum
margin as low as 400 MeV and a / suppression power better than 10. RPC made of
floating glass will be used as the main detector element. In the barrel region, the RPC
detectors will be operated in streamer mode which has fewer requirements for the
electronics. For the two endcap parts, avalanche mode operation will be chosen to
deal with the high background related to the high beam luminosity. In order to further
extend the momentum limitation, timing RPC will be used in the innermost layer of
the barrel region, which provides extra time matching for muon identification.
In the next stage of investigation, some issues should be focused by simulation
and detector R&D. 1) Fast simulation based on the conceptual detectors design; 2)
rate capability of the RPC with avalanche mode; 3) the RPC prototype design and
test.