POSSIBLE LIMITATIONS ON THE OPERATING VOLTAGE OF SILICON DETECTORS INTHE INNER TRACKER

In working out a realistic long-term scenario for the operational cond itions for the innermost silicon layers, it is usually assumed that th e upper safe margin for the reverse-bias voltage is around 200 V. Life time of the detectors, and consequently total cost of the tracker are directly related to this value. Using pixel test structures, manufactu red by CSEM. in the same technology as DELPHI pixel detectors, we perf ormed a study of maximum tolerable bias voltages on nonirradiated dete ctors, and on detectors which have received different fluences of pion s, protons and neutrons. A special test was also done with detectors, continuously irradiated on a testbench with a Sr source, at a rate abo ut 13 krad/h.