Effects on hard x-ray response of a double-sided Si strip detector caused by interstrip surface charge

We studied a surface effect of Double-sided Si Strip Detectors (DSSDs) in order to apply it for imaging spectroscopy of X-ray photons down to 5 keV for the first time. The Japanese cosmic X-ray satellite Hitomi, launched in February 2016, is equipped with the Hard X-ray Imager (HXI), which employs the DSSDs in 5-80 keV. In such a low energy band, the surface effect is non-negligible. When interstrip regions of p-side are irradiated, the DSSD sometimes show signals with negative pulse heights, presumably caused by positive surface charges between Si and SiO² layers.1{5 The effect modifies the X-ray response of the HXI towards its low-energy end, below ∼ 10 keV. By irradiating the DSSD with uncollimated mono-energetic X-rays of different energies, we measured the fraction of the negative events to be 2% at 26.4 keV and 30% at 6.0 keV. Using an 8 keV colli- mated X-ray beam, we directly verified that the negative events originated from the interstrip gaps on the p-side where the SiO² layers exist. The measured energy- and position- dependences can be modeled by assuming that the negative events are produced in approximately 25 μm deep and 120 μm wide interstrip regions. When the bias voltage are halved (from 350 V to 180 V), fraction of the negative events increased by a factor of ∼ 1:7, qualitatively consistent with this picture.