Developing the detector of x-ray imaging spectrometer for GEO-X mission

We report development status of an X-ray imaging spectrometer for scientific micro satellite mission GEO-X that aims for imaging of Earth's magnetosphere from the vicinity of the Moon (∼40 R_E). The planned direction for the observations includes proximity of the day-side Earth. Therefore the primary requirement for the detector is the fast frame rate to decrease the visible light background. In this regard we will apply complementary MOS (CMOS) sensor that is originally fabricated for the visible light and/or infrared spectroscopy. Faster readout speed improves time resolution and decrease the contribution from visible light compared with the conventional CCD detectors. We evaluate imaging and spectroscopic performances of backside illumination type scientific CMOS sensors with low noise performance. Most of the signals produced by X-rays distributes within 2 by 2 pixels. Spectra of monochromatic X-rays exhibit significant difference of pulse height between the event within single pixel and that spreads across multiple pixels, which indicates that a part of the signal charges are lost around the pixel edges. Then we adopt another type of the sensor that have been updated in terms of the incident surface treatment. We found that the amount of the lost charges are substantially decreased with the new sensor. Another measure to improve the spectroscopic performance is the dark level determination. Gradual or discontinuous change of the dark level in orbit might it difficult to evaluate the appropriate dark level especially for the high frame rate and the limited resources of onboard computer. Then we take the average of pulse heights for the outermost pixels in a event (5 by 5 pixels) and correct the pulse height of all pixels with the average value. With these measures the energy resolution improved successfully.