Análisis de la oferta

Figure 4.2 depicts the layout of the ATLAS ID. Each TRT Endcap consists of 80 “wheels” of straws, each one straw in thickness and consisting of 3072 straws each. Straws are assembled in groups of four wheels; this mechanical group is referred to as a “four-plane wheel” or occasionally “endcap ring.” There are two types of four-plane wheel—in which the straw wheels are separated by either 8 mm or 15mm. Four groups of four-plane wheels with 8mm spacing are referred to as “Type A-wheels” and positioned closer to the interaction point. Two groups of four-plane wheels with 15 mm spacing are referred to as “Type B-wheels,” farther from the interaction point. A TRT Endcap consists of six Type-A wheels and 8 Type-B wheels. Cryostat PPF1 Cryostat Solenoid coil z(mm) Beam-pipe Pixel support tube SCT (end-cap) TRT(end-cap) 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 Pixel 400.5 495 580 650 749 853.8 934 1091.5 1299.9 1399.7 1771.4 2115.2 2505 2720.2 0 0 R50.5 R88.5 R122.5 R299 R371 R443 R514 R563 R1066 R1150 R229 R560 R438.8 R408 R337.6 R275 R644 R1004 2710 848 712 _PPB1 Radius(mm) TRT(barrel) SCT(barrel) Pixel PP1 3512 ID end-plate R34.3

Figure 4.2: Schematic of the ATLAS inner detector. The first twelve divisions in the endcap correspond to the six Type A-wheels, and the last eight divisions correspond to the eight Type B-wheels [10].

4. Alignment of the TRT 44

A cross section of the TRT Barrel is depicted in Figure 4.3. Three types of module are

shown, which represent one of the 32 φ-sectors of the barrel. The barrel contains 96 modules

in total. Collectively there are 73 layers of straws in the TRT Barrel, arranged to maximize the number of hits along a particle track. Straws run along the entire length of the barrel (144 cm); an insulating glass wire joint is placed at the center of each wire to split the readout into two sides (A side and C side). Because of the expected high occupancy, the nine innermost layers of the barrel are split into three sections by glass joints, where the center section is inactive.

2008 JINST 3 P02014

Figure 3. The three types of modules are mounted in the Barrel Support System. The orientation with respect to the beam intersection area is shown to scale. The triangular sections on the space frame are radially symmetric.

Table 1.TRT Barrel Module parameters.

Module Inner Radius (m) |η|at Rmin Layers # Straws Mass (kg)

Type 1 0.56 1.06 19 329 2.97

Type 2 0.70 0.89 24 520 4.21

Type 3 0.86 0.75 30 793 6.53

Total for Barrel 73 52544 439

A triplet of modules comprising a stack in azimuthal angle (“phi”) is shown in figure 3. The three sizes of Barrel modules are sequentially mounted in 32 “phi” sectors. Each module is a quadralateral prism with front and back faces in a plane perpendicular to the local radial ray, and sides that follow the close packing array shape of straws, approximating a 30◦deviation with respect to a radial line. This design was choosen to minimize the amount of dead tracking area for high momentum particles. The resulting numbers of straws in each module are listed in table 1. The mass listed in the table are for modules only, with no electronics or external services connected. The total number of straws for all 32 sectors, and the total mass of the 96 modules is indicated in the bottom line of the table.

The straw diameter was chosen to be 4 mm as a reasonable compromise between speed of response, number of ionisation clusters, and mechanical and operational stability. The straw an- odes are 31µm-diameter gold-plated tungsten wires at ground potential and the straw cathodes are typically operated at a high voltage of 1530V, corresponding to a gas gain of 2.5×104_{for the gas}

mixture chosen, which contains 70% Xe, 27% CO2, and 3% O2. To accomodate the high occu-

pancy rate at the design luminosity, the sense wires are split in half by an insulating glass wire joint and instrumented with signal readout at both ends. The nine inner most layers are further divided into three sections with the middle section desensitized to further reduce the rate. The design and performance of the straw is described in detail in other documents [6, 21, 22], as is the evolution of the active gas mixture [14].

The dimensional specifications on the TRT were set by the requirements for the tracking preci- sion to be optimized for the drift tube straw intrinsic resolution of 130µm. Multiple measurements

– 5 –

Figure 4.3: End-view schematic of a singleφsector of the TRT Barrel [13].

The Level 2 alignment structures correspond to the 96 modules in the TRT Barrel and the 40+40 four-plane wheels in TRT Endcap A+C. Level 3 alignments occur at the individual straw level. Crucially, it is important to note that the TRT straw is not sensitive to the location of the hit along the length of the straw. Therefore, the Level 3 alignment omits translations and rotations along the straw axis. This is also the reason why Level 1 and 2 alignments of the TRT barrel modules omit the translation degree of freedom along the beam (z) axis.

Figure 4.4 displays images of the TRT barrel before being lowered into the ATLAS cavern and a four-plane wheel of the endcaps during assembly.

4. Alignment of the TRT 45

Figure 4.4: The TRT Barrel (left) and a single four-plane wheel of the TRT Endcap (right) [10].