Artist's rendition of SiD. (Courtesy of SLAC)

In July, ILC NewsLine provided an overview of the Global Large Detector (GLD) concept. This week’s issue takes a closer look at the Silicon Detector (SiD) concept. ILC NewsLine will continue its series about the four ILC detector concepts in the weeks to come. Next: Large Detector Concept (LDC).

Standing at 12 metres tall, 12 metres wide and 12 metres deep, the Silicon Detector (SiD) is a compact International Linear Collider detector concept. As the only detector concept to use silicon technology exclusively for particle tracking and calorimetry, naming this detector concept SiD seemed very natural for the international collaboration of approximately 150 scientists. Silicon technology is an expensive commodity, however, making it important to keep the detector as small and cost effective as possible.

"We are trying to build in cost consideration from the very beginning," said one of the SiD Design Study Coordinators, John Jaros, a physicist at SLAC. "As you increase the physics performance, the cost also increases. We’re looking for the optimal balance. It is the integrated physics performance that matters and that pushes us hard to think about system integration from the very start."

Like the Global Large Detector (GLD) and Large Detector Concept (LDC), SiD uses particle flow calorimetry to precisely measure the energy of jets that emerge from the electron-positron collisions. The electromagnetic calorimeter uses dense tungsten absorber and silicon detectors to measure the total energy of photons and electrons. Consisting of a 30-layer sandwich of silicon and tungsten, the detector will require huge amounts of these materials. "Nothing on this scale has ever been built before with silicon and tungsten," Jaros said. "We will make it as small as the physics will allow to keep the detector compact and the costs down."

SiD compensates for its small size with a large magnetic field. A five Tesla magnet winds around the beam dispersing charged particles into circular arcs. The radius of the curvature and direction indicate the momentum and charge of the particle. The high field spreads the charged particles out before they hit the calorimeter and contains background particles made in the collisions close to the beamline. An iron flux return encloses the solenoid magnet to capture stray field lines, improve the field uniformity within the detector, and provide for muon detection. "If we see something go through all the iron, we will automatically know it is a muon," Jaros said. "They leave a great signature."

Unlike GLD, LDC and 4th detector concept, the SiD tracker does not utilise a Time Projection Chamber – a gaseous cylinder that creates three-dimensional images of particle tracks. Instead, the SiD concept uses silicon trackers to determine the trajectories of the charged particles. Their tracker consists of five large cylinders made out of carbon fiber that are rolled up and tiled with silicon sensors. These silicon microstrip detectors are 10 cm by 10 cm, with strips spread 25 microns apart, and measure with great precision – up to 5 microns – where a particle passes through the tracker.

"The ILC will make 3000 electron-positron collisions, stop, and then make them again," Jaros said. "The silicon tracker has the advantage of being fast enough to pick out an individual collision, minimising the effects of backgrounds. In conjunction with the vertex detector, with its powerful pattern recognition capability, the tracker has been shown to make highly efficient and extremely precise measurements of all the charged particles."

University and laboratory groups are collaborating on a number of SiD R&D projects to optimise the detector design and develop detector technologies Providing a simulation framework to make computation tools available to the entire community, perfecting particle flow algorithms, and performing physics benchmarking studies are all part of the effort. R&D work is also being done for such areas as the tracker system, hadronic calorimetry, muon system, solenoid magnet and beam calorimetry. While it may seem far off, one SiD group is already examining how the system would be installed in the experimental hall. "This is already coming up now," Jaros said. "We need to know how much space we will need to help in firming up the accelerator design." Plans for prototyping individual systems are also underway.

-- Elizabeth Clements