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 d etectors depends on the capacitive load on each channel and the shot noise due to the reverse current in the detector. Because of the short integratio n time of the LHC electronics (25 ns), the shot noise contribution is negli gible as long as the current is kept to an acceptable level. However, any i nter-strip capacitance increase after irradiation will degrade the noise pe rformances of the detectors. A dependence of the capacitance on the crystal orientation of the silicon crystal used as a substrate for silicon detecto rs has been recently reported (Feld L. et al., Radiation induced changes in , the Interstrip Capacitance of Silicon Microstrip Detectors, 4th ROSE Work shop on Radiation Hardening of Silicon Detectors, CERN 2-4 December 1998, P ublished in CERN/LEB 98-11 (697)). We present here the results of irradiati on of silicon microstrip detectors made on 6 "(100) and 4 " (111) crystal o riented silicon wafers in term of inter-strip capacitance and noise; measur ed using LHC speed binary electronics (LBIC, Spencer E. et al., IEEE Traits . 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 depende nce decreases with the frequency of the measurement. At high frequencies, w hich are the only ones relevant for the noise behaviour of the detectors, w e found no differences at high bias voltages between the values before and after irradiation for detectors made on silicon with different crystal orie ntations. The effect on the noise has been checked and at the nominal opera tion bias we found no measurable differences between detectors made on sili con with different crystal orientations.