Performance of different silicon materials for the upgraded CMS tracker

The proposed LHC luminosity upgrade (HL-LHC) would push the operating conditions of the CMS tracking system beyond the design limits, both in terms of radiation dose and channel occupancy. A comparison of various silicon materials and processing types has been launched in the framework of the Compact Muon Solenoid (CMS) experiment to find the best suited material for a future strip tracker. Detectors made of Float Zone and Magnetic Czochralski silicon, both n-on-p and p-on-n types, have been produced. Two different isolation techniques, p-stop and p-spray, have been tested for the p-type sensors. Both 200\,um and 320\,um thick detectors have been irradiated with both protons and neutrons to the fluence foreseen at a radial position of 20\,cm in the upgraded CMS experiment. % The tested Multi-geometry Silicon Strip Detectors (MSSD) contain 12 regions, where the region pitches range from 70 to 240\,um, and strip width/pitch (w/p) ratios from 0.1 to 0.3. This article is based on test beam data for approximately 300 combinations of detector material, polarity, isolation type, thickness, pitch, and w/p. The test structures were measured using proton, muon, and pion beams, with beam energies between 15 and 300\,GeV. The detectors were mounted perpendicular to the beam inside a cold box. Reference planes fore and aft of Detectors Under Test (DUT) were providing high-quality tracks, with a positional uncertainty of 6\,um. % The results show smaller than expected differences between the performance of the material types. After irradiation, the performance of thin detectors was comparable to or even exceeded the performance of the thicker detectors.