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 RE). 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.
We developed an X-ray imaging spectrometer with a high timing resolution. CMOS image sensors, widely used in optical imaging, have an excellent time resolution with considerably low readout noise. We performed experiments using a Zyla CMOS image sensor manufactured by Andor Technology pic. (UK). It employs a front illuminated(FI) scientific CMOS (sCMOS) with each pixel size of 6.5×6.5μm2. We found that nearly all X-ray events spread more than 5×5 pixels and obtained good energy resolution of 260 eV at 5.9 keV at room temperature. The spectrum shows no degradation at lower energy, which imply that there is negligibly small charge loss. We report photon counting performance for a broad range of X-rays from 3-25keV at room temperature.
HiZ-GUNDAM is a future satellite mission which will lead the time-domain astronomy and the multi-messenger astronomy through observations of high-energy transient phenomena. A mission concept of HiZ-GUNDAM was approved by ISAS/JAXA, and it is one of the future satellite candidates of JAXA’s medium-class mission. We are in pre-phase A (before pre-project) and elaborating the mission concept, mission/system requirements for the launch in the late 2020s. The main themes of HiZ-GUNDAM mission are (1) exploration of the early universe with high-redshift gamma-ray bursts, and (2) contribution to the multi-messenger astronomy. HiZ-GUNDAM has two kinds of mission payload. The wide field X-ray monitors consist of Lobster Eye optics array and focal imaging sensor, and monitor ~1 steradian field of view in 0.5 – 4 keV energy range. The near infrared telescope has an aperture size 30 cm in diameter, and simultaneously observes four wavelength bands between 0.5 – 2.5 μm. In this paper, we introduce the mission overview of HiZ-GUNDAM.
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