In this section, a more detailed review of the technical specification of the Semicon-ductor Tracker (SCT) is outlined. As was briefly mentioned in the previous section, the SCT is the second closest sub-detector to the beam pipe in ATLAS, after the Pixel detector, and is based on a silicon micro-strip technology. A large fraction of the silicon modules, which constitute the building blocks of the SCT, were developed, produced and tested at University of Geneva before the sub-detector was assembled at CERN.
3.3.1 Description and design of the SCT
The requirements for the Inner Detector as a whole (described in chapter 3.2.2) trans-lates in the case of the SCT into the following layout conditions [49]:
• Leptons of pT >5 GeV must be reconstructed with a 95% efficiency within the limit of |η| < 2.5 and with a fake track rate of < 1%. Helices are therefore required to be reconstructed in 3D, which imposes at least six tracking layers in the combined SCT and pixel system to provide space point information.
• To be able to separate multiple pile-up events and to be able to reconstruct very high pT tracks impose a resolution of z <1 mm in the SCT. Achieving a granularity of (δpT/pT)<0.3 atpT = 500 GeV with a beam constraint requires anR-φ measurement accuracy of approximately 20µm at the SCT radius.
• In order to keep track losses in b-jets to a maximum of 5%, a high tracking resolution is required.
The Semiconductor Tracker is designed to provide accurate position measurements for the intermediate radii of the ID (30 to 52 cm from the beam pipe), from four precision space points per track [51]. The SCT has an active silicon area of 61 m2 with 6.3 million read-out channels. The SCT barrel consists of four cylindrical layers with a total of 2112 modules and a pseudorapidity range of |η| < 1.1 to |η| < 1.4 depending on the barrel layer. Each of the two SCT end-caps comprise nine disks with 988 modules arranged in up to three rings within the disks. The end-caps extend the range of the SCT up to |η|<2.5.
The silicon detectors of the ID must be particularly radiation hard due to the proximity to the beam pipe. The operational lifetime of the SCT modules is specified to be at least 10 years of running, assuming 3 years of LHC operation at a luminosity of 1033cm−2s−1 followed by 7 years at 1034cm−2s−1. Including a 50% uncertainty, this implies that the innermost barrel must be able to withstand a 1 MeV neutron equivalent flux of 2×1014n/cm2.
To improve the radiation hardness and reduce reverse annealing and leakage cur-rent the module sensors are cooled to around -7◦C using an evaporative cooling sys-tem. The system is shared with the pixels and uses C3F8 as an evaporative coolant.
The temperature of the coolant in the cooling pipes is kept around -25◦C in order to achieve the desired temperature in the modules. A major cooling problem occurred in May 2008 with the failure of three out of six compressors in the plant. By August 2008, the plant was repaired and upgraded and has since been running relatively stably. Only one cooling loop is turned off for the foreseeable future due to a leak, affecting 13 SCT modules.
3.3.2 SCT modules and front-end electronics
An SCT module consists of two pairs of identical single-sided micro-strip p-on-n sen-sors glued back-to-back at a stereo angle of 40 mrad to enable position identification [51]. The effective space point resolution provides 17 µm in the R-φ direction and 580 µm in the z (R) direction for a barrel (end-cap) module. The barrel modules are all identical while there are three different designs of end-cap modules depending on the ring within the disk (see Figure 3.12). The barrel module has a strip-pitch of 80 µm, while the pitch of the end-cap modules vary from 55 to 94 µm.
Each module sensor consists of 768 channels read out by 6 ABCD3T ASIC chips in the radiation hard technology DMILL [65]. Each chip provides binary readout of 128 detector channels. The signal from the silicon detector is amplified, shaped and compared to a programmable discriminator threshold of 1 f C. The hit information from each channel is stored in a 132-deep binary pipeline buffer. The chips also contain a charge injection circuitry used for calibration.
The SCT data acquisition system and read-out consists of several components (see
(a) (b)
Figure 3.12: Photograph of (a) an SCT barrel module and (b) the three models of end-cap modules (outer, middle and inner ring from left to right) [51].
Figure 3.13). The Read Out Driver (ROD) module performs the main control and data handling and is responsible for module configuration, trigger propagation and event data formatting, as well as module calibration and monitoring. A complemen-tary Back Of Crate (BOC) card interfaces the opto-signals with the electrical signals in the ROD. The BOCs contain two kinds of electro/optical converter plug-ins: an RX plug-in with an array of PIN diodes and a data receiver, and a TX plug-in with an array of VCSELs to return data from each side of one module. Each ROD/BOC pair deals with the control and data for 48 front-end modules.
Figure 3.13: A detailed schematic illustration of the SCT data acquisition and read-out architecture [66].
One of the issues encountered with the SCT is an elevated infant mortality of the TX plug-ins. Initial investigations suggested to that the VCSELs had been ex-posed to electro-static discharge (ESD) during manufacturing. Improved plug-ins
were produced and installed during summer 2009, but these TXs are also failing (a continuing issue during the 2010 data-taking period). Fortunately, failed TX units could be replaced during data-taking and redundancy is used, such that no data were lost due to this problem. A similar issue has been observed with the OTxs in the LAr calorimeter. Here, the failed components cannot be replaced during data-taking and a considerable fraction (a few %) of the calorimeter has become unusable. The dead OTxs in the calorimeter were replaced during the 2010-2011 winter shut-down.
The SCT is currently fully operational and meeting the design requirements. A detailed description of the SCT performance measured with cosmic ray data is pre-sented in section 4.2.2, while the current status of the sub-detector during collision data-taking is briefly discussed in section 4.2.3.