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

Charge signal and noise were studied in non-irradiated and irradiated Schot tky barrier, circular pad detectors of 100 mu m thick made on semi-insulati ng Gallium Arsenide at Alenia S.p.A., as a function of the reverse bias (V- a), the shaping time (tau) and the fluence (f), for minimum ionising electr ons yielded by a Ru-106 source. The detectors have been irradiated with protons (energy 24 GeV) up to a flu ence of about 2 x 10(14) p/cm(2). A charge signal degradation is observed f or irradiated detectors. The charge signals for MIP's at 500 V are reduced from 12900 electrons before irradiation to 6600 electrons after about 2 x 1 0(14) p/cm(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-a greater than or equal to 100 V). The measurement of the charge signal as a function of V-a shows that the fu ll depletion voltage decreases from 100 V for non-irradiated detectors to a bout 70 V for irradiated ones. The equivalent noise charge extracted from t he pedestal width is found to scale with root 1/tau according to the predic tions when the series noise contribution dominates, while it does not scale with root tau as it should be when the parallel noise contribution, due to the detector leakage current, becomes important.