Radiation tolerance test and performance verification of pnCCD at high temperatures for future satellite mission HiZ-GUNDAM

A wide-field X-ray survey in the soft X-ray band is crucial for future satellite missions in the astronomical observations. HiZ-GUNDAM, currently under development, is a proposed satellite designed to observe soft X-ray transients including gamma-ray bursts. This satellite employs wide-field X-ray monitors consisting of lobster-eye optics and focal-plane pixel sensors in the soft X-ray band of 0.4–4 keV. A pnCCD is a candidate for focal-plane Si pixel detectors, featuring a back-illuminated X-ray CCD, large pixel size (70–100 µm), and a large active image area of approximately 55 × 55 mm2 for the flight model. Here, we investigated the basic characteristics and performance of the small-size pnCCD with 128 × 256 pixels, a pixel size of 75 µm, and a detector thickness of 450 µm. High-energy cosmic rays such as protons can degrade the performance of pnCCDs by increasing dark current and charge transfer inefficiency due to ionizing and displacement damage. These factors may affect soft X-ray observations, potentially causing the degradation of lower-detectable energy thresholds and an increase in the number of hot pixels. Therefore, we conducted a radiation tolerance test at room temperature using a proton beam. After irradiating the pnCCD with 10-MeV protons equivalent to three years of nominal operation for HiZ-GUNDAM, we found that the operation temperatures of the pnCCD should be lower than -35 °C. This requirement will be incorporated into the design of the mission operation system.