Measurement of the current related damage rate at −50° C and consequences on macropixel detector operation in space experiments

An experiment was performed to measure the current related damage rate of silicon soon after a 10 MeV proton irradiation at −50° C, in a condition in which the effect of the leakage current annealing is negligible. This measurement is fundamental to predict the spectroscopic performance of the macropixel detectors which will be mounted on the Simbol-X and BepiColombo space missions. Macropixel detectors consist on matrices of Silicon Drift Detectors with an integrated DEPFET readout node on each pixel and offer an optimal solution when a large pixel area is needed but the noise should be kept at levels allowing X-ray spectroscopy. The most critical aspect of the operation of these detectors, in particular of the one which will be used in the BepiColombo mission, is whether the leakage current increase due to the proton irradiation would still allow the required energy resolution. This leakage current increase cannot be predicted with the available models because, during the whole mission, the sensor will be kept at temperatures below −40° C, and the existing empirical parameterizations are valid only at higher annealing temperatures. The irradiation was performed with diodes at the tandem accelerator of the Meier-Leibnitz Laboratorium in Garching with 10-MeV protons and fluences below 10¹¹ protons/cm², the interesting range for the missions. The diodes were cooled at a temperature of −50° C during the experiment and biased and read out with a charge sensitive preamplifier to perform a dosimetry based on proton counting. The leakage current was measured at the end of every exposure, before warming up and replacing the samples. Its time evolution after several steps of annealing at 60° C was then studied in the laboratory to check the agreement with the NIEL hypothesis predictions and thus, to validate the experiment. The experimental setup, the measurements of current induced damage rate at- - −50° C and its annealing are discussed in detail. The consequences on the Simbol-X and BepiColombo experiments are also examined.