Inner Detector

The inner detector consists of the semiconductor pixel detector, the semiconductor strip tracker, and the transition radiation tracker. The inner detector system has a length of 5.3 m and a diameter of 2.5 m. Figure 3.5 shows a schematic view of the inner detector. These detectors measure tracks of charged particles. We calculate the momentum of a charged particle from a curvature of the track under the strong magnetic field and estimate a position of a primary vertex. We also employ the secondary vertex and the impact parameter information for the flavour tagging. In case of the electron identification, detection of transition-radiation photons with low energy from the electron in the TRT with the xenon-based gas enhances the identification efficiency.

Figure 3.4: End-cap cryostat system including the calorimeters, feed-throughs and front-end crates. An outer radius of the cryostat vessel is 2.25 m and a length of the cryostat is 3.17 m.

Figure 3.5: Schematic view of the silicon detector.

3.4.1 Pixel Detector

The pixel sensor is made of the oxygenated n-type wafer with a readout pixel on the n⁺ -implanted side of the detector. Its thickness and size are 250 µm and19×63mm², respectively.

Each sensor has 47232 pixel implants arranged in 144 columns and 328 rows. The size of 41984 implants is50×400µm and the size of the remaining pixel is50×600µm. A total number of the readout channels on the sensor is 46080. The pixel detectors are placed on concentric cylinders around the beam axis in a barrel region and are located on the disks perpendicular to the beam axis in the end-cap regions as each particle track passes through the three layers. There are a total of 1744 modules deployed into three cylindrical layers in the barrel region and 2×3 disks in the end-cap regions. The detectors cover the region within|η| < 2.5. The pixel layers are divided intoR-φandzsections for the barrel region orR-φandRsections for the end-cap regions. The intrinsic accuracy is shown in Table 3.2. A total number of readout channels of the pixel detectors is approximately 80.4 million. The barrel pixel modules have a tilt angle of 20^◦, which is the tangent to the support cylinder surface in the plane perpendicular to the cylinder axis.

Table 3.2: Main parameters of the pixel detector.

Detector Number of layer Position [mm] Intrinsic accuracy

(direction) [µm]

Pixel (barrel) 3 50.5< R <122.5,0<|z|<400.5 10 (R-φ), 115 (z) Pixel (end-cap) 2×3 88.8< R <149.6,495<|z|<650 10 (R-φ), 115 (z)

3.4.2 SemiConductor Strip Tracker

The SCT utilizes the single-sided p-in-n technology with AC-coupled readout strips. SCT is composed of two 6 cm long daisy chained sensors with a strip pitch of 80 µm and a thickness of 285 µm. There are a total of 768 active strips of 12 cm length per SCT module. A total number of the modules is 4088. The SCT modules are placed on concentric cylinders around the beam axis in the barrel region and are located on disks perpendicular to the beam axis in the end-cap regions as each particle track passes through the four strip layers, where the SCT module has a pair layer of the strip sensors. The detectors cover the region within|η| < 2.5. There are four cylindrical layers in the barrel region and2×9disks in the end-cap regions. In the barrel layer, one strip layer of the pair layer is parallel to the z axis, the other strip layer of the pair layer is crossed by 40 mrad to the z axis. This small angle crossing enable for the pair layers to measure theR-φcomponent.

In the end-cap layer, one strip layer of the pair layer is parallel to theφaxis, the other strip strip layer of the pair layer is crossed by 40 mrad to theφ axis. The intrinsic accuracy is shown in Table 3.3. A total number of readout channels of the SCT is approximately 6.3 million. The barrel SCT modules have a tilt angle of 11^◦, which is the tangent to the support cylinder surface in the plane perpendicular to the cylinder axis. The end-cap SCT disks have the tilt angle inφby 2.75 mm.

3.4.3 Transition Radiation Tracker

The TRT is composed of multiple layers of straw tubes. The straw detector is made of a cylindrical cathode of 4 mm diameter and a tungsten wire plated with gold anode of approximately 31 µm and a mixture gas of Xe/CO₂/O₂. The signal attenuation length is approximately 4 m

Table 3.3: Main parameters of the SCT.

Detector Number of layer Position [mm] Intrinsic accuracy

(direction) [µm]

SCT (barrel) 4 299< R <514,0<|z|<749 17 (R-φ), 580 (z) SCT (end-cap) 2×9 275< R <560,839<|z|<2735 17 (R-φ), 580 (z)

and the signal propagation time is approximately 4 ns/m. It covers the region within |η| < 2.0 and provides information of theR-φ component. The accuracy for the R-φcomponent is 130 µm per straw. There are 73 layers of straws in the barrel region, 563 < R < 1066 mm and 0 < |z| < 712 mm, and2 ×160layers of straws in the end-cap region, 644 < R < 1004 mm and848 < |z| < 2710mm. The TRT in the barrel region is 144 cm length and placed on concentric cylinders around the beam axis. The TRT in the end-cap regions is 37 cm length and placed radially in wheels. The TRT barrel module is made of a carbon-fibre laminate shell and an internal array of straws embedded in a matrix of 19 µm-diameter polypropylene fibres serving as a transition radiation material. The TRT end-cap module is composed of layers with 768 radially oriented straws of 37 cm length and polypropylene radiator foils of 15 µm thickness. The TRT has approximately 351,000 read out channels. The number of hits in TRT is typically 36 hit per track and this many hits can compensate low track accuracy.