ATLAS's inner detector installed in the heart of the experiment

The ATLAS collaboration recently celebrated a major engineering milestone, namely the transport and installation of the central part of the inner detector (ID-barrel) into the ATLAS detector.


Right: Engineers and technicians work to carefully align and install the inner detector in the centre of ATLAS.


Left: The crane used in the carefully coordinated effort by the ATLAS collaboration to lower down the fragile inner detector 100 metres underground to its new home.

Many members of the collaboration gathered to witness this moment at Point 1. After years of design, construction and commissioning, the two outer detectors (TRT and SCT) of the inner detector barrel (ID-barrel) were moved from the SR1 cleanroom to the ATLAS cavern. The barrel was moved across the car park from Building 2175 to SX1. Although only a journey of about 100 metres, this required weeks of planning and some degree of luck as far as the weather was concerned. Special measures were in place to minimize shock and vibration during transportation. Accelerometers were fitted to the barrel to provide real-time monitoring and no values greater than 0.1 g were recorded, fully satisfying the transport specification for this extremely precise and fragile detector.

After arriving in SX1, it should have been straightforward to lower the barrel into the pit. Unfortunately, the crane that had been selected for this delicate task presented a technical problem. After a few worrying hours, the repair was completed and the detector was lowered into the heart of ATLAS. With only a few millimeters of clearance, the detector was successfully inserted in the liquid argon calorimeter cryostat.

The semiconductor tracker (SCT) and transition radiation tracker (TRT) are two of the three major parts of the ATLAS inner detector. The innermost layer (pixels) will be installed in 2007. The barrel part of the inner detector containing the two outer subsystems was successfully assembled in February 2006 and passed through complete characterization of its performance during standalone and combined tests this spring. Both detectors fully satisfy the design criteria of the technical design report. During the tests, particular attention was paid to making sure that the SCT did not generate noise in the TRT and vice versa. One eighth of the TRT and one quarter of the SCT were equipped with complete readout chains. Extensive standalone and combined tests were performed in order to detect any effect that could cause problems during LHC operation, particularly during the read-out cycle. The results were a triumph for those who designed both systems: mechanical engineers, electronics engineers and physicists - these two detectors are completely independent and able to operate at thresholds close to that defined by thermal noise.

The TRT barrel contains 52544 straws. To reduce straw occupancy, wires are split in two parts that are read out from both ends of the detector, doubling the number of read-out channels (see Bulletin No. 14/2005). Manufacturing precision guaranties wire position accuracy in the whole barrel of about 50 µm. It was validated in the test beam runs and cosmic runs during combined tests after integration with SCT (see Bulletin No. 09/2006). This precision allows the on-line drift-time accuracy of the individual straw, averaged over the whole detector, to be less than 160 µm for the Xe-based gas mixture and below 200 µm for the Ar-based gas mixture without any special alignment procedure. The total number of dead channels accumulated over mechanical and electronics sources of failure is about 1.5%.

The SCT barrel is composed of 2112 double-sided silicon detector modules, mounted on four nested carbon fibre cylinders. The modules are read out by custom, radiation-hard ASICs. In total there are about 3.2 million electronic channels, of which 99.8% are fully operational. During the combined TRT/SCT tests in the SR1 cleanroom, the average module noise was stable at 4.5 x 10-5, well below the design specification of 5 x 10-4. These measurements were made with about 500 modules being operated simultaneously.

The final installation and commissioning of the TRT barrel is scheduled to start in mid-September, shortly after all the required services are installed and commissioned. The tasks of completing the installation of the TRT barrel in 35 days and the SCT barrel in a further 77 days, with all the tests needed to sign off the detector for autonomous operation for 10 years without further access, is an absolutely daunting challenge.