Basic Design Parameters

GEANT4 based simulation studies [4] of muon reconstruction, background rejection and hadronic energy leakage were used to support the geometry and segmentation chosen for the TCMT.

  1. 16 layers, each of active area 1m x 1m,
  2. Extruded scintillator strips 5cm wide and 5mm thick,
  3. Steel absorber with thickness 2cm (8 layers) and 10cm (8layers),
  4. X or Y orientation of strips in alternate layers,
  5. Silicon Photomultiplier (SiPM) photodetection.


The extruded scintillator strips will be produced at the Scintillator Detector Development Lab (SDDL) extruder facility operated jointly by Fermilab and NICADD [5]. The extruder uses polystyrene pellets and PPO and POPOP dopants to produce scintillator with good mechanical tolerances and an average light yield that is 70% that of cast scintillator. The strips produced will be 1m long, 10cm wide, 5mm thick and will have two co-extruded holes running along the full length of the strip. A 1.2mm outer diameter Kuraray wavelength shifting fiber will be inserted in each of the holes. Detailed studies of the strip-fiber system were carried out to converge on this solution [6]. Not only was the performance of this novel fiber-coextruded-hole configuration better than anything that could be obtained for a fiber-machined-groove geometry it is also significantly less labor intensive since no machining, polishing or gluing is involved. Due to the size of the die currently available the strips rolling off the extruder will be ten centimeters wide. To have the required five centimeter wide readout segmentation each of the strips will have a 0.9mm wide epoxy filled separation groove in the middle (see Fig. 1). Further R&D on the strip-fiber system optimization will continue in co-ordination with groups pursuing conventional photomultiplier readout [7].

\begin{figure}  \epsfxsize =4.5truein  \centerline{\epsffile{strp.eps}}\end{figure}

Figure: Strip processing stages.


We are using novel solid-state devices like SiPMs [8] or MRS (metal resistive semi-conductor) for photodetection. For the purposes of this discussion we will refer to these devices collectively as SiPMs. SiPMs are room temperature photo-diodes operating in the limited Geiger-mode with performances very similar to conventional photo-multiplier tubes i.e. they have high gain ($\approx 10^6$) but relatively modest detection efficiency (quantum x geometric efficiency $\approx$ 15%). Not only is the signal obtained for minimum ionizing particles with these devices large ($>$ 10 photo-electrons for our 5mm thick extruded scintillator strips), their small size (1mm x 1mm) and low bias voltage ($\approx$ 50 V) implies that they can be mounted in or very close to the scintillator strips. Consequently little light is lost since it does not travel large distances in the fiber to the photodetector, the need for interfacing to a clear fiber (connectors, splicing etc.) is obliterated and the quantity of fiber required is significantly reduced. Even more importantly, the generation of electrical signals, inside the detector, at or close to the scintillator surface eliminates the problems associated with handling and routing of a large number of fragile fibers. Our detailed investigations [9][10] into the characteristics of these photodetectors confirms their suitability for a dual purpose muon detector. While SiPMs are our preferred solution for the TCMT prototype we will remain active in evaluating the potential of new photodetector developments (for example [11]) as and when they become available.


The scintillator strips and their associated photodetectors in each layer will be enclosed in a light tight sheath which we refer to here as a cassette (see Fig. 2). The top and bottom skins of the cassette are formed by 1mm thick steel with aluminum bars providing the skeletal rigidity. The aluminum bars also divide the cassette into distinct regions for scintillator, connectors, cable routing and LED drivers such that they can be independently accessed for installation, maintenance or repairs.

\begin{figure}  \epsfxsize =4.5truein  \centerline{\epsffile{cass2.eps}}\end{figure}
Figure: Mechanical prototype of cassette.


One of the practical advantages of using the SiPMs is that we can use some of the electronics being developed for the scintillator-based hadron calorimeter, another project with which we are actively involved. Thus we will be using the preamplifier and DAQ boards already developed for the HCal. However the different structure and channel count of the device will necessarily lead to a different architecture of the electronics. This will necessitate the custom development of TCMT baseboards which will carry the preamplifier boards and communicate with the DAQ ones (see Fig. 3). We will carry out the design and fabrication of these boards in collaboration with DESY and Fermilab electrical engineering departments. The photodetectors inside the cassette will be connected to this baseboard with 50 ohm multi-coax cables with connectors at both the detector and board ends.

Figure: Electronics architecture for the TCMT

Figure: Electronics architecture for the TCMT.


The design of the absorber stack and table is being developed in collaboration with Fermilab mechanical engineering (see Fig. 4). The design foresees the welding of the steel absorber plates to a frame which also doubles as a lifting fixture. This structure will be then placed on top of a table capable of forward-backward and left-right motion with the help of Hillman rollers. The stack will have the capability of being rotated by $90^{o}$ for taking normally incident cosmics during beam downtime. The electronics crates will be attached to the stack to keep the cable lengths to a minimum. The drawings for the absorber stack and table are available and construction can commence soon. We have already located (Fermilab scrapyard) and reserved most of the absorber plates required for the TCMT. Some processing in the shape of flame cutting, welding etc. will however be required. Only a couple of plates will have to be bought outright.

Figure: TCMT absorber stack structure

Figure: TCMT absorber stack structure.