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Lattice defect creation mechanism (top) and trapping/de-trapping of electons and holes at different temperatures (bottom) (*) When using semiconductor detectors in harsh radiation environments, defects begin to appear in the semiconductor crystal lattice as atoms become displaced because of the interaction with the high-energy traversing particles. These defects, in the form of both lattice vacancies and atoms at interstitial sites, have the effect of temporarily trapping the electrons and holes which are created when ionizing particles pass through the detector. Since it is these electrons and holes which drifting under an electric field produce a signal announcing the passage of a particle, when large amounts of defects are produced, detector signal can be strongly reduced leading to an unusable (dead) detector. However in 1997, Dr. Vittorio Palmieri and co-workers at the University of Berne (Switzerland) found out that at temperatures below 130 kelvins (about −143 degrees Celsius), dead detectors apparently come back to life. The explanation of this phenomenon, known as the Lazarus Effect, is related to the dynamics of the induced defects in the semiconductor bulk. At room temperature radiation damage induced defects temporarily trap electrons and holes resulting from ionization, which are then emitted back to the conduction band or valence band in a time that is typically longer than the read-out time of the connected electronics. Consequently the measured signal is smaller that it should be. This leads to low signal to noise ratios that in turn can prevent the detection of the traversing particle. At cryogenic temperatures, however, once an electron or hole, resulting from ionization or from detector leakage current, is trapped in a local defect, it remains trapped for a long time due to the very low thermal energy of the lattice. This leads to a large fraction of 'traps' becoming filled and therefore inactive. Trapping of electrons and holes generated by particles traversing the detector is then prevented and no or little signal is lost. References * “Evidence for charge collection efficiency recovery in heavily irradiated silicon detectors operated at cryogenic temperatures” V.G. Palmieri, K. Borer, S. Janos, C. Da Viá, L. Casagrande; Nucl. Instr. and Meth. in Phys. Res A, Vol. 413, pp. 475-478, (1998) Links * http://www.cern.ch/rd39 RD39 Collaboration Web Page Retrieved from "http://en.wikipedia.org/"  |
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