3.2 The ATLAS Detector
3.2.3 Inner Detector
The inner detector (ID) lies within a cylindrical envelope of length±3.512 m and radius 1.150 m immersed in the 2 T magnetic field produced by the surrounding solenoid.
The inner detector consists of three independent but complimentary sub-detectors: the Pixel Detector, SCT and TRT. It is designed to provide momentum measurements of charged particles, primary and secondary vertex reconstruction within |η| < 2.5 and moderate charged particle identification. The layout of the various sub-detectors which make up the ID can be seen in figure 3.6.
As well as providing precision measurements, the design of the inner detector takes into account the harsh high-radiation environment within the LHC. The detector components are designed, unless otherwise stated, to last for ten years without being replaced. The design of the ID also considers the amount of material particles must traverse as several interaction effects can effect the accuracy of track measurements. Figure3.7 shows the cumulative amount of material in terms of radiation length as a function of |η|.
Chapter 3. The Large Hadron Collider and ATLAS Experiment 21
Figure 3.6: Schematic diagram of the ID [33]
| η | 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ) 0 Radiation length (X 0 0.5 1 1.5 2 2.5 | η | 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ) 0 Radiation length (X 0 0.5 1 1.5 2 2.5 Services TRT SCT Pixel Beam-pipe
Figure 3.7: Total amount of material in terms of radiative length vsη for the various components of the Inner Detector [35].
The key properties of the main components of the ID are given in table3.1, each com- ponent is described in more detail in the following sections.
3.2.3.1 Pixel Detector
The Pixel Detector is the nearest component of the ID to the beam pipe, it also offers the best resolution. A total of 1744 identical silicon pixel sensors make up the Pixel Detector and are arranged as three barrel layers; concentric cylinders around the beam pipe in the barrel region, and two sets of three end-caps located perpendicular to the
Chapter 3. The Large Hadron Collider and ATLAS Experiment 22
Component Position Resolution (µm) η Coverage Pixel Detector Inner barrel layer Rφ = 10, z = 155 ±2.5
2 outer barrel layers Rφ = 10, z = 155 ±1.7 End-caps Rφ= 10, R = 155 1.7 - 2.5 Silicon Tracker Barrel layer Rφ = 17, z = 580 ±1.4
End-caps Rφ= 17, R = 580 1.4 - 2.5 Transition Radiation Tracker Barrel straws 170 (per straw) ±0.7
End-cap straws 170 (per straw) 0.7 - 3.5
Table 3.1: Key properties of the ID components
beam. Each sensor consists of 47232 50 x 400 µm pixels resulting in a total of around 140 million silicon pixels.
In a typical event three of the pixel layers will be crossed by a charged particle; the interaction produces a series of electron-hole pairs which are separated by an electric field and read out by electronics giving a ’hit’ on the pixel. The intrinsic measurement accuracies for each of the barrel layers are 10µm in theR−φ plane and 155µm along thez-axis [33]; the end-caps also have an intrinsic accuracy of 10 µm in theR−φplane and 155 µm in R [33]. This accuracy combined with a close proximity to the beam pipe results in the Pixel Detector’s high resolution as outlined in table3.1and excellent vertexing capability.
3.2.3.2 Silicon Tracker (SCT)
The SCT utilises 4088 silicon strip modules in a similar configuration to the Pixel De- tector to provide additional tracking measurements. The barrel region is located at
255< R <549 mm and consists of four concentric cylindrical layers, comprising of 2112
of the 4088 modules. The remaining modules are combined to produce the two end-cap detectors, each consisting of nine disks.
Although the layout of the modules differs in the barrel and end-cap regions their struc- ture is similar. Each module consists of four wafers each containing 768 silicon strips, two on the top and two on the bottom, offset by a stereo angle of 40 mrad. The offset of the modules reduces noise in the detector and allows for 2D track reconstruction. The intrinsic measurement accuracies for the barrel layers are 17µm in theR−φplane and 580 µm along the z-axis; the end-caps also have an intrinsic accuracy of 17 µm in the
Chapter 3. The Large Hadron Collider and ATLAS Experiment 23
3.2.3.3 Transition Radiation Tracker (TRT)
The TRT is the outermost component of the ID and utilises gas filled straws to provide further tracking of charged particles through to the calorimeters. The different tech- nology also allows stand-alone electron identification. The TRT consists of 73 layers of straws laying parallel to the beam pipe at 554< R < 1082 mm and covers the region up to|η|<1. 224 end-cap layers split evenly between the two sides cover the remaining range of 1 < |η| < 2. In total there are 372,000 straws of 4 mm thickness [36]. The straws contained in the barrel region are 144 cm in length, the end-cap straws are 37 cm in length.
As a charged particle passes through the TRT it leaves a trail of ionisation electrons. Each straw contains an anode wire running from one end to the other, the ionisation elec- trons will drift to the anode. This drift time is used to give a hit location of the charged particle. Furthermore, as ultra-relativistic charged particles pass through boundaries to different media transition radiation photons are produced, these characteristic photons can be used to distinguish between pions and electrons.