ALICE takes its ITS to heart
In the study of heavy-ion events, the ALICE Inner Tracking System must use the most delicate materials. A hundred physicists and engineers from around the world witnessed its impressive journey to the centre of the ALICE experiment.
On 15 March, after 15 years of development, construction, commissioning and testing, the Inner Tracking System (ITS) finally reached its ultimate destination at the heart of ALICE. With almost five square meters of double-sided silicon strip detectors and over one square meter of silicon drift detectors, ALICE's ITS is the largest system built for either type of silicon detector.
In ALICE's search for heavy-ion events at the LHC, it is necessary for the ITS to be extremely lightweight and delicate. For this reason the ITS was designed and built using the smallest amounts of only the lightest materials, with the design team developing innovative construction and assembly systems. The team prepared in detail for the final transport from the final assembly room and test lab to the ALICE pit and even practised the transport at full scale with the actual tools and components beforehand. The last critical step before the descent-the integration of the strip and drift layers, which had been assembled in Utrecht and Torino respectively and had reached CERN fully constructed-took place on 22 February. For this task, and for the testing phase, a lab was set up as close as possible to the ALICE pit, in order to minimize the distance of the ITS' final trip.
The final transfer was a truly spectacular operation, like the launch of a satellite. Many of the over 100 physicists and engineers from Finland, France, Italy, Russia, the Czech Republic, the Netherlands, Ukraine and the USA, who have joined forces under the leadership of Lodovico Riccati of INFN Torino to construct the ITS drift and strip layers, gathered at CERN to watch apprehensively as the ITS was slid carefully into place. The path through the TPC was barely large enough to allow the ITS to pass through, so the ITS had to be moved very slowly, taking two hours to cover a few dozen of meters. With the ITS in place, it is the turn of the central section of the beam pipe to be installed at Point 2, after which the ITS will be connected to its final cables ready for cosmic ray data.
Six layers of silicon
The ITS consists of six layers of high-precision silicon detectors: double-sided silicon strips in the two outer layers, silicon drift detectors in the two middle layers and silicon pixels in the two inner layers. The four outermost layers of the ITS were installed on 15 March. The third and innermost component of the ITS, the silicon pixel detector, is scheduled to be installed later this summer.
