Noise behaviour of semi-insulating GaAs particle detectors before and after proton irradiation

Charge signal and noise were studied in non-irradiated and irradiated Schottky barrier, circular pad detectors of 100 mu m thick made on semi-insulating gallium arsenide at Alenia S.p.A., as a function of the reverse bias (V/sub a/), the shaping time ( tau ) and the fluence (f), for minimum ionising electrons yielded by a /sup 106/Ru source. The detectors have been irradiated with protons (energy 24 GeV) up to a fluence of about 2*10/sup 14/ p/cm/sup 2/. A charge signal degradation is observed for irradiated detectors. The charge signals for MIP's at 500 V are reduced from 12900 electrons before irradiation to 6600 electrons after about 2*10/sup 14/ p/cm/sup 2/ at a temperature of 7 degrees C and with a shaping time of 20 ns, typical of LHC inter-bunch crossing time (25 ns). The charge signal is found independent of tau for full depletion condition (V/sub a/>or =100 V). The measurement of the charge signal as a function of V/sub a/ shows that the full depletion voltage decreases from 100 V for non-irradiated detectors to about 70 V for irradiated ones. The equivalent noise charge extracted from the pedestal width is found to scale with square root (1/ tau ) according to the predictions when the series noise contribution dominates, while it does not scale with square root ( tau ) as it should be when the parallel noise contribution, due to the detector leakage current, becomes important. (15 refs).