Comparison between ATLAS forward microstrip detectors made on 6" <100> and 4" <111> crystal oriented silicon wafers

The noise seen by the front-end readout electronics of silicon microstrip detectors depends on the capacitive load on each channel and the shot noise due to the reverse current in the detector. Because of the short integration time of the LHC electronics (25 ns), the shot noise contribution is negligible as long as the current is kept to an acceptable level. However, any inter-strip capacitance increase after irradiation will degrade the noise performances of the detectors. A dependence of the capacitance on the crystal orientation of the silicon crystal used as a substrate for silicon detectors has been recently reported (Feld L., et al., Radiation induced changes in the Interstrip Capacitance of Silicon Microstrip Detectors, 4th ROSE Workshop on Radiation Hardening of Silicon Detectors, CERN 2-4 December 1998, Published in CERN/LEB 98-11 (697)). We present here the results of irradiation of silicon microstrip detectors made on 6'' (100) and 4'' <111> crystal oriented silicon wafers in term of inter-strip capacitance and noise, measured using LHC speed binary electronics (LBIC, Spencer E. et al., IEEE Trans. Nucl. Sci. NS-42 (1995) 796), as a function of bias. A strong dependence of the inter- strip capacitance on the bias applied to the detector has been found for the irradiated detectors made on <111> silicon, but this dependence decreases with the frequency of the measurement. At high frequencies, which are the only ones relevant for the noise behaviour of the detectors, we found no differences at high bias voltages between the values before and after irradiation for detectors made on silicon with different crystal orientations. The effect on the noise has been checked and at the nominal deration bias we found no measurable differences between detectors made on silicon with different crystal orientations. (5 refs).