Inner Detector

The layout of the Inner Detector combines high-resolution detectors at the inner radii with continuous tracking elements at the outer radii, all contained in the CS which provides a nominal magnetic field of 2 T.

The momentum and vertex resolution requirements from physics call for high-precision measurements to be made with fine-granularity detectors, given the very large track density expected at the LHC. Semiconductor tracking detectors, using silicon microstrip (SCT) and pixel technologies offer these features. The highest granularity is achieved around the vertex region using semi-conductor pixel detectors. The total number of precision layers must be limited because of the material they introduce, and because of their high cost. Typically, three pixel layers and eight strip layers (four space points) are crossed by each track. A large number of tracking points (typically 36 per track) is provided by the straw tube tracker (TRT), which provides continuous track-following with much less material per point and a lower cost. The combination of the two techniques gives very robust pattern recognition and high precision in both Φ and z coordinates. The straw hits at the outer radius contribute significantly to the momentum measurement, since the lower precision per point compared to the silicon is compensated by the large number of measurements and the higher average radius. The relative precision of the different measurements is well matched, so that no single measurement dominates the momentum resolution. This implies that the overall performance is robust. The high density of measurements in the outer part of the tracker is also valuable for the detection of photon conversions and of V⁰ decays. The latter are an important element in the signature of CP violation in the B system. In addition, the electron identification capabilities of the whole experiment are enhanced by the detection of transition-radiation photons in the xenon-based gas mixture of the straw tubes.

Pixel Detector

A Pixel sensor is a 16.4 x 60.8 mm wafer of silicon with 46,080 pixels, 50 x 400 microns each. The barrel part of the pixel detector consists of the 3 cylindrical layers with the radial positions of 50.5 mm, 88.5 mm and 122.5 mm respectively. These three barrel layers are made of identical staves inclined with azimuthal angle of 20 degrees. There are 22, 38 and 52 staves in each of these layers respectively. Each stave is composed of 13 pixel modules. There are three disks on each side of the forward regions. One disk is made of 8 sectors, with 6 modules in each sector. Disk modules are identical to the barrel modules, except the connecting cables.

Semiconductor Tracker (SCT)

The SCT system is designed to provide eight precision measurements per track in the intermediate radial range, contributing to the measurement of momentum, impact parameter and vertex position. In the barrel SCT eight layers of silicon microstrip detectors provide precision points in the r-phi and z coordinates, using small angle stereo to obtain the z-measurement. Each silicon detector is 6.36 x 6.40 cm² with 768 readout strips of 80 micron pitch. The barrel modules are mounted on carbon-fibre cylinders at radii of 30.0, 37.3, 44.7, and 52.0 cm. The end-cap modules are very similar in construction but use tapered strips with one set aligned radially. The SCT covers |η| < 2.5.

Transition Radiation Tracker (TRT)

The Transition radiation Tracker (TRT) is based on the use of straw detectors, which can operate at the expected high rates due to their small diameter and the isolation of the sense wires whitin individual gas volumes. Electron identification capability is added by employing Xenon gas to detect transition radiation photons created in a radiator between the straws. Each straw is 4 mm in diameter and equipped with a 30 µm diameter gold-plated W-Re wire. The maximum straw length is is 144 cm in the barrel, which contains about 50 000 straws, each divided in two at the center and read out at both end, to reduce the occupancy. The end-caps contain 320 000 radial straws, with the readout at the outer radius. The total number of channels that are read out is 420 000. Each channel provides a drift time measurement, giving a spatial resolution of 170 µm per straw, and two independent thresholds. These allow the detector to discriminate between tracking hits, which pass the lower threshold, and transition radiation hits, which pass the higher one. The nonflammable gas mixture is now Xe(70%) CO2(27%) O2(3%) (compared to 70% Xe, 20% CO2, and 10% CF4 originally). The barrel section is built of individual modules between 329 and 793 straws each, covering the radial range from 56 to 107 cm. The first six layers are inactive over the central 80 cm of their length to reduce their occupancy. Each end-cap consists of 18 wheels. The innermost 14 cover the radial range from 64 to 103 cm, while the last four extend to an inner radius of 48 cm. Wheels 7 to 14 have half as many straws per cm in z as the others, to avoid an unnecessary increase of crossed straws and material at medium rapidity.