TIME STRUCTURE OF CHARGE SIGNALS AND NOISE STUDIES OF GAAS DETECTORS IRRADIATED BY NEUTRONS AND PROTONS

Semiconductor detectors processed in Aachen using SI GaAs from differe nt manufacturers have been irradiated with neutrons (peak energy simil ar to 1 MeV) up to 4.0 . 10(14) n/cm(2) and protons (energy 24 GeV) up to 8.2 . 10(13) p/cm(2). All detectors work well after the exposure. The leakage current density at 200 V of detectors made of AXT material increases by a factor of four after the highest neutron fluence and a factor of three after the maximal proton fluence. For the FEW-LC mate rial the leakage current decreases significantly after irradiation. No significant difference can be observed between biased and non-biased detectors during the exposure to neutrons. The equivalent noise charge (ENC) calculated from the noise density spectra agrees well with that extracted from the pedestal width for the different neutron and proto n fluences. Before irradiation the charge signals for minimum ionizing particles (MIPs) increase with the peaking time in the range of 40 ns to 2 mu s, while this dependence cannot be observed after the exposur e to neutrons or protons. The reason for this behaviour is the differe nt time structure of the charge signals before and after irradiation. For AXT material the charge signals for MIPs correspond to 7100 electr ons at 200 V before irradiation for a peaking time of 40 ns, while sig nals of 7200 electrons after 4.0 . 10(14) n/cm(2) and of 4300 electron s after 8.2 . 10(13) p/cm(2) are obtained. For FEW-LC material the sig nals at 200 V are reduced from 15700 electrons before irradiation to 6 000 electrons after 8.2 . 10(13) p/cm(2) for this peaking time.