The Inner Detector (ID) [29] is the precision tracker of ATLAS and it is a 6.2 m long cylinder with a radius of 1.15 m. A sketch of its layout is shown in figure2.19. The ID is the closest detector to the interaction point and, as mentioned in the previous section, its task is to reconstruct the trajectories of charged particles that are produced in the proton-proton collisions. It performs the pattern recognition, momentum and vertex measurements together with electron identification. The design of the Inner Detector provides pseudorapidity coverage up to
|η| < 2.5
. Pseudorapidity is a parameter commonly used due to the fact that only depends on the polar angle of the particle’s trajectory and not on the energy of the particle.Figure 2.19: A sketch of the ATLAS Inner Detector, showing the various subdetectors
Mechanically, the ID is divided in three parts: a central barrel region and two symmetric end-caps. The barrel extends over
±80
cm along the Z-axis. The components of the ID are summarized in the Table2.6. It combines high-resolution silicon detectors in the inner radii with continuous tracking detectors at outer radii. It is composed by three sub-systems: the Pixel Detector, the Semiconductor Tracker (SCT) and the Transition Radiation Tracker (TRT).Due to the large track density expected at the LHC (around 1500 charged particles crossing the ID every 25 ns), high precision measurements with fine-granularity detectors
Detector Distance from beamline Section Layers Area [
m
2] Channels [M] Table 2.6: Main parameters of the Inner Detectorneed to be performed to handle the particle fluxes and to reduce the influence of overlapping events. For this purpose the ID has 5832 individual silicon modules (with about 86 million of readout channels).The ID electronics and all the sensor elements must be fast enough and radiation hard.
Finally, figures2.20and2.21show the nominal positions of each barrel layer and end-cap disc in the radial and transverse plane, respectively.
•
Pixel Detector:The Pixel Detector occupies the radii between 5 and 15 cm from the interaction point and it is designed to provide a very high granularity (with 80.4 million channels) as well as high precision set of measurements as close as possible to the interaction point. This system is based on silicon pixel technology as a detection medium and it consists of one B-layer (for its importance in B-physics), two cylindrical barrel layers and two endcaps, with three discs on each side of the central barrel [30]. A 3D model of the Pixel detector can be seen in figure 2.22. The pixel modules (identical for all regions) are single silicon sensors of 6.08
×
1.64cm
2divided in 46.080 pixels and a size of50 µm ×
400µ
m resulting in an intrinsic resolution resolution of 10µ
m in the R (transversal) direction and 115µ
m in the Z (longitudinal) direction with a direct 2D readout. Each single silicon sensor has highly dopedn
+ implants on an
-type substrate. Thepn
junction is located on the back-side, with a multi-guard structure controlling the potencial drop towards the cutting-edges. These sensors have250 µm
of thickness and a sensitive area of 16.4×
60.8mm
2. There are 1456 modules in the barrel and 288 in the end-caps.2.3 The ATLAS Detector 33
Figure 2.20: Drawing showing the sensors and structural elements traversed by a charged track of 10 GeV pT in the barrel inner detector (η= 0.3). The track traverses successively the beryllium beam-pipe, the three cylindrical silicon-pixel layers with individual sensor elements of50× 400 µm2, the four cylindrical double layers (one axial and one with a stereo angle of 40 mrad) of barrel silicon-microstrip sensors (SCT) of pitch80 µm, and approximately 36 axial straws of 4 mm diameter contained in the barrel transition-radiation tracker modules within their support structure
Much more information about the pixel module components and its electronics can be found in reference [31].
•
Semiconductor Tracker (SCT):The SCT surrounds the pixel detector and consists of four barrel layers and two end-caps.
On each barrel, the modules are placed in rows parallel to the beam axis. There are 12 modules in each row with a total of 2112 modules [32]. A barrel module consists of two pairs of single-sided
p
+n
silicon detectors glued back-to-back at 40 mrad angle and separated by a heat transport plate. Each silicon wafer is6 × 6 cm
2,285 µm
thick, and has 768 readout strips with80 µm
pitch. On each side of the module, two wafers are wire-bonded together to form 12 cm long strips. Combining the measurements from both sides, a two-dimensionalFigure 2.21: Drawing showing the sensors and structural elements traversed by two charged tracks of 10 GeV pT in the end-cap inner detector (η = 1.4 and 2.2). The end-cap track at η = 1.4 traverses successively the beryllium beam-pipe, the three cylindrical silicon-pixel layers with individual sensor elements of50×400 µ m2, four of the disks with double layers (one radial and one with a stereo angle of 40 mrad) of end-cap silicon-microstrip sensors (SCT) of pitch∼ 80 µ m, and approximately 40 straws of 4 mm diameter contained in the end-cap transition radiation tracker wheels. In contrast, the end-cap track atη= 2.2 traverses successively the beryllium beam-pipe, only the first of the cylindrical silicon-pixel layers, two end-cap pixel disks and the last four disks of the end-cap SCT. The coverage of the end-cap TRT does not extend beyond|η|= 2
spacepoint is created. The readout is performed by means of 12 binary
ABCD
[33, 34] front-end chips and mounted above the detectors on ahybrid
. The readout chain consists of a front-end amplifier and discriminator, followed by a binary pipeline which stores the hits above threshold until the level-1 trigger decision.Each end-cap consists of 9 disks supported by a cylinder with modules arranged in rings within a disk. The disks are located at a 27.5
<
R<
56 cm from the beamline. A disk may have up to three rings, therefore three types of end-cap modules (namely inner, middle, and outer) are needed [35]. The end-cap modules are similar to the barrel modules in electronics and readout, except in their shape. The coverage of each disk is required to be fully hermetic for tracks above a transverse momentum of 1 GeV, except for the unavoidable dead area between the two sensors in each plane for outer and middle modules. Moreover, the layout allows sufficient overlapping active area between neighbouring modules2.3 The ATLAS Detector 35
Figure 2.22: A 3D model of the Pixel Detector and it’s framework.
for the module alignment parameters to be efficiently determined.
As a consequence, the shape of the modules is trapezoidal, resulting in a variable strip pitch. The effective strip length after bonding is around 12 cm for middle and outer modules, and half this value for inners (with only one sensor per side). The strip pitch varies from 55 to
95 µm
depending on the end-cap module type.With this performance the system fulfils the required intrinsic resolution of
17 µm(rφ)
and580 µm(z)
for the barrel and17 µm(rφ)
and580 µm(r)
for the disks. The SCT is constructed so that on average four space points are measured for particles up to a pseudorapidity of|η| < 2.5
as shown in figure2.23.The SCT has 4088 modules in total which means 61
m
2of silicon sensors with 6.3 million channels.•
Transition Radiation Tracker (TRT):The TRT is based on the use of straw detectors with the capability to generate and detect transition radiation in its outerpart.
It surrounds the other two subsystems and consists of about 300 000 gaseous straw tubes arranged in 73 layers in the barrel region and 2
×
160 straw planes in the end-cap regions. A picture of the barrel part can be seen in figure2.24.Figure 2.23: Schematic of the ATLAS inner detector
Figure 2.24: TRT barrel, just before SCT barrel insertion
2.3 The ATLAS Detector 37
One of the main reasons to build the TRT in straws filled with a gas mixture was to minimize the material used to build the ID. The pixel and strip detectors require a lot of material in the form of support structures and services (cables and cooling pipes). This material has a negative effect on the performance of the tracker [36].
An average number of 36 hits per track its provided (in the transverse plane to the beam pipe). The TRT gas mixture Xe/CF4/CO2 (70%/20%/10%) provides an efficient X-ray absorption, a fast charge collection and a stable operation over a sufficient high-voltage range even at high particle rates. The total number of channels that are read out is 420.000 and each channel provides a drift time measurement. Its technology allows to have an intrinsic resolution of