Detector (draft in 2014. 8-2014. 10)

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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. Physics Requirements (assigned to Haiping)

    1. 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:

  1. (nearly) 4π detector solid angle coverage for both charged and neutral particles, and uniform response for all particles;

  2. high resolution of momentum and angular for charged particles; and high resolution of energy and position for photons ;

  3. superior PID ability (e///K/p/g) and high detection efficiency for low momenta particles;

  4. precision luminosity measurement;

  5. 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




Hit Resolution

R/j - Z (mm - mm)

Radiation length



30 / 860

1.5 %X0



170 / 1800

1.32 %X0



12 / 12

~0.37 %X0


12 / 12

~0.37 %X0

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 Å 22GeV/p×c) mm


Layer 1 at 2.6 cm radius

Layer 2 at 8 cm radius

Pixel size

18.4 mm × 18.4 mm

Hit resolution

10 mm rms

Position stability

6 mm (20 mm envelope)

Radiation thickness per layer

X/X0 = 0.37%

Number of pixels

~436 M

Integration time (affects pileup)

200 ms

Radiation tolerance

300 krad

Rapid installation and replacement to cover radiation damage and other detector failure

Installation and reproducible positioning in 8 hours

The read out time per sensor is limited by the frame integration time of 100-200 ms. 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 ms, but still has room for improvement.
Detectors'>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

8 M


Layer 1 at 1.4 cm radius

Layer 2 at 2.2 cm radius

Pixel size (r-phi, z)

50 mm × (60 - 75) mm

Resolution (r-phi, z)

~12 mm, < 20 mm

Radiation thickness per layer

X/X0 = 0.2%

Occupancy for innermost layer

0.2 hits mm-2s-1

Integration time (affects pileup)

20 ms

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


Belle-II PXD

Resolution (m)



Total material (%X0)



Si thickness (m)



Occupancy (cm-2s-1)


max. 5107

Radius (cm)



Readout time (s)



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.


Radius (cm)

Material (%X0)

Resolution (m)

MDC outer 9-48




MDC inner 1-8








PXL 2 layers




Beam pipe




Option 2: MDC + PXD-like parameters.


Radius (cm)

Material (%X0)

Resolution (m)

MDC outer 9-48




MDC inner 1-8




PXD 3rd layer




PXD 2 layers




Beam pipe




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:

. (1)

The probability to pick up the wrong hit is described by the equation:

, (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:


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:


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:


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 p/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, b is the velocity, m is the particle mass and p is the momentum. DT, Db and Dm2 are the differences of T, b 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 p/K and K/p at p=2GeV/c, the time difference DT are around 0.1 and 0.27ns. So to achieve 3s p/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 p/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=bc, 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. 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 p/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. 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 22Na  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.

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