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Development of Radiation Hard Tracking Detectors

Abstract: The Large Hadron Collider (LHC) at CERN has been designed to achieve the unprecedented luminosity of 10^34 cm^-2s^-1. Recently, a 10-fold luminosity upgrade has been proposed (SuperLHC). To exploit the physics potential of the upgraded LHC, an efficient tracking down to a few centimeters from the interaction point will be required, where fast hadron fluences above 10^16 cm^-2 will be accumulated after more than 5 years operation (3000 fb^-1). Present vertex detectors, relying on highly segmented silicon sensors, are designed to survive fast hadron fluences of about 10^15 cm^-2. Semiconductor detectors seem the best option for vertex sensors in the next generation of colliders as well, provided that their radiation hardness is significantly improved. The CERN RD50 collaboration "Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders" was founded in 2002 with the aim to develop a new reliable detector technology for the LHC upgrade or a future high luminosity hadron collider. Three main research lines have been identified: the understanding of the microscopic defects causing the degradation of the irradiated detectors, material engineering, with the aim of producing semiconductor material with increased radiation-hardness (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), and device engineering, i.e. variations in the sensor geometry, to develop a more radiation tolerant detector geometry. The main radiation damage phenomena to semiconductor detectors are: the increase of the leakage current with the irradiation fluence due to the creation of generation-recombination centers, the change in the effective doping concentration at heavy fluences leading to increased depletion voltage and possibly involving the inversion of the space charge sign, a shortening of the carrier lifetimes due to increased trapping at radiation-induced defects, responsible for a loss of charge and the detoriation of the surface condition influencing interstrip isolation and interstrip capacitance. All of these influence the parameters of the detectors and have to be taken into account in the selection of tracking detectors in high particle fluxes. The present plans for the ATLAS Upgrade are taken as an example how the work on radiation hardness is being applied.
Speaker: Hartmut F.-W. Sadrozinski - SCIPP, UC Santa Cruz
Speaker Bio: Hartmut F.-W. Sadrozinski has been working on Tracking Systems for Colliding Beam applications since getting his Ph.D. from MIT. This started at SPEAR with the Princeton group, where he built the polymeter, a 4pi tracker surrounding the beam pipe to identify event topologies. After moving to the University of California at Santa Cruz, he worked on prototyping and building the drift chamber for Mark II Upgrade at the SLC. As part of this work he started to be interested in radiation damage effects, in this case the aging of gases. With the rise of silicon detectors he applied them to the Leading Proton Spectrometer at HERA, which included the first radiation hard readout electronics. His interest in optimizing silicon sensors and readout electronics with special attention to radiation hardness continued with the silicon tracking system for the SSC, and was extended to the LHC, where he was involved in the development of many of the concepts used in the Semiconductor Tracker (SCT) in ATLAS. Radiation effects played also a role in GLAST, the first large-scale application of silicon sensors in space. With T. Ohsugi, he lead the procurement of the 11,500 silicon sensors for GLAST, jump-starting the use of 6 wafers in the commercial production of silicon detectors. In GLAST he also lead the radiation testing program for sensors and ASICs, including single event effect (SEE) testing in Legnaro and Texas A&M. The work on radiation effects led to application of silicon detectors in radiology. For the proton therapy synchrotron at Loma Linda University Medical Center, he built silicon strip detector tracking systems for Nanodosimetry and for the development of proton CT. The ultimate challenge in radiation hardness for semiconductor detectors will come with the proposed upgrade of the LHC, and he has responded to this challenge within the CERN collaboration RD50 for the last four years. Recently, ATLAS has organized the upgrade R&D work, and he is co-leading a collaboration to develop silicon strip sensors for the inner tracking detector.
Poster Link: Poster
Presentation: Presentation on 1/9/2008 (PDF)