The Silicon Tracking System of the CBM Experiment at FAIR

Abstract

The Compressed Baryonic Matter experiment [1] at FAIR will explore the phase diagram of strongly interacting matter at highest net baryon densities and moderate temperatures. The CBM physics program will be started with beams delivered by the SIS 100 synchrotron, providing energies from 2 to 11 GeV/nucleon for heavy nuclei, up to 14 GeV/nucleon for light nuclei, and 29 GeV for protons. The highest net baryon densities will be explored with ion beams up to 45 GeV/nucleon energy delivered by SIS 300 in the next stage of FAIR. Collision rates up to 10 7 per second are required to produce very rare probes with unprecedented statistics in this energy range. Their signatures are complex. These conditions call for detector systems designed to meet the extreme requirements in terms of rate capability, momentum and spatial resolution, and a novel DAQ and trigger concept which is not limited by latency but by throughput. In the presentation we describe the concept and development status of CBM’s central detector, the Silicon Tracking System STS [2]. The STS realizes a large, highly granular and redundant detector system with fast read-out, and lays specific emphasis on low material budget in its physics aperture to achieve for charged particle tracks a momentum resolution of δp/p≈1% at p >1 GeV/c, at > 95% track reconstruction efficiency. The detector employs 1220 highly segmented double-sided silicon micro-strip sensors of 300 µm thickness, mounted into 896 modular structures of various types that are aggregated on 106 low-mass carbon fiber ladders of different sizes that build up the tracking stations. The read-out electronics with its supply and cooling infrastructure is arranged at the periphery of the ladders, and provides a total channel count of 1.8 million. The signal transmission from the silicon sensors to the electronics is realized through ultra-thin multi-line aluminum-Kapton cables of up to half a meter length. The electronics generates a free-streaming data flow of digitized time-stamped detector information that is sent via data aggregator boards to the first-level event selector, a computing farm for on-line event reconstruction. The power dissipated by the detector’s read-out electronics, amounting to 2 times 20 kW in two thin layers at the top and bottom of the detector, will be removed by a particularly efficient and space-saving bi-phase CO2 cooling. The system integration of the detector takes respect of operating the sensors in a thermal enclosure at -5oC, to limit leakage currents originating from radiation damage, and allows for maintenance to the detector components, in particular the sensors, if they should exceed an exposure to more than 10 14 1 MeV neq/cm 2 . The detector system is built by a CBM project team comprising institutes from Germany, Russia, Poland and Ukraine.