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 different manufacturers have been irradiated with neutrons (peak energy ≈1 MeV) up to 4.0·1014 n/cm2 and protons (energy 24 GeV) up to 8.2·1013 p/cm2. 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 material 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 proton fluences. Before irradiation the charge signals for minimum ionizing particles (MIPs) increase with the peaking time in the range of 40 ns to 2 μs, while this dependence cannot be observed after the exposure to neutrons or protons. The reason for this behaviour is the different time structure of the charge signals before and after irradiation. For AXT material the charge signals for MIPs correspond to 7100 electrons at 200 V before irradiation for a peaking time of 40ns, while signals of 7200 electrons after 4.0·1014 n/cm2 and of 4300 electrons after 8.2·1013 p/cm2 are obtained. For FEW-LC material the signals at 200 V are reduced from 15 700 electrons before irradiation to 6000 electrons after 8.2·1013 p/cm2 for this peaking time.