Barrel and Forward Calorimeters (Design)

Barrel and Forward Calorimeters (Design)

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. Summary

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