We have been developing an optical blocking filter for GEO-X which will perform soft X-ray imaging spectroscopy of Earth’s magnetosphere. GEO-X will provide an unprecedented view through X-ray observations in terms of the interaction between solar wind and magnetosphere. GEO-X consists of MEMS X-ray mirrors and a focal plane CMOS detector which is sensitive to visible photons necessitating a thin-film optical blocking filter to attenuate noise from out-of-band photons while providing high transmittance for in-band soft X-ray photons. The OBF design for GEO-X spans an aluminum 40 mm x 40 mm aperture with 200 nm polyimide and 150 nm aluminum. A square-celled stainless steel mesh with a grid size of 3 mm and a width of 200 um provides mechanical support against launch stresses. The material combination provides high transmittance across the GEO-X bandpass and tolerances for severe launch and space environments at the same time. We conducted performance tests for the EM coupons. We confirmed the pinhole frequency of the aluminum film is very small (visible transmittance < 1e-8). It is found that film uniformity is very good, varying by 1% in polyimide and 2% in aluminum thicknesses over the 20 mm x 20 mm scan region at 277, 380, 522, and 776 eV. Before and after the UV irradiation test (50 ESD), no significant differences in terms of the pinhole frequency are found. Additionally, there are no significant descrepancies in the size of the pinholes for all of the pinholes we investigated before and after the high-speed atomic oxygen irradiation test with a fluence value of 1019 atoms cm−2 even though additional pinholes were detected after the test possibly due to accelerated metal outgassing contaminants remaining the vacuum chamber.
High-speed atomic oxygen (AO) irradiation tests were performed for free-standing graphene membranes to investigate the degradation in terms of mechanical intensity for astronomical applications. The high-speed AO was irradiated with a typical velocity of 5 km/s and fluence values of 3×1015, 1×1016, 3×1016, and 1×1017 atoms/cm2. Two kinds of samples were prepared using single- and bi-layer graphene sheets with through-hole patterns with diameters of 10 μm and 10-200 μm, respectively. Consequently, the yield rate is found to be approximately 90% and 100% for the single- and bi-layer freestanding graphene samples, respectively. Almost all ruptured events occurred after the minimum fluence of 3E+15 atoms/cm2. The results suggest that the lifetime of the free-standing graphene structure can be longer by increasing the number of layers or any other causes such as accelerated metal outgassing contaminants remaining the vacuum chamber and detonation waves are not negligible and almost no damages are seen through the high-speed AO irradiation up to the fluence value of 1×1017 atoms/cm2. To improve the sensitivity, our original transfer method was introduced and we fabricated 300 / 900 μm diameter free-standing single- / bi-layer graphene structures successfully. Additionally, we confirmed that our technique can be applied to ground-based applications such as high-sensitivity graphene Transmission Electron Microscopy (TEM) support films.
The first results of high-speed atomic oxygen (AO) irradiation tests for atomically thin single-layer graphene sheets are presented as space environmental tolerance evaluation tests toward application in astronomy. The single-layer graphene sample was prepared without a metal coating, and high-speed AO irradiation tests were conducted with an averaged velocity of ∼6 km/s using a laser-detonation AO beam source assuming a low Earth orbit (LEO) case. The Raman spectral features were examined before and after the tests with fluence values of 2×1015, 2×1016, 2×1017, 2×1018, and 2×1019 atoms/cm2. It was found that there is no significant change in the observed D/G ratios for fluence up to 2×1017 atoms/cm2. In contrast, the D/G ratios changed from 0.04±0.03 to 0.8±0.4 for 2×1018 atoms/cm2 drastically in both the averaged value and 1-sigma range. Furthermore, the D/G ratio could not be measured beyond 2×1019atoms/cm2 because no peaks were observed in both the G and D bands, which suggests that the degradation occurs between 2×1017 and 2×1018 atoms/cm2 and no graphene sheets exist after the 2×1019 atoms/cm2 irradiation. Scanning electron microscopy images also support this conclusion in terms of the observed image contrast. Consequently, to protect the single-layer graphene sheets from erosion, a special treatment such as coating is needed to survive in an LEO for ≳ a day.
GEOspace X-ray imager (GEO-X) is a small satellite mission aiming at visualization of the Earth’s magnetosphere by X-rays and revealing dynamic couplings between solar wind and the magnetosphere. In-situ spacecraft have revealed various phenomena in the magnetosphere. X-ray astronomy satellite observations recently discovered soft X-ray emissions originating from the magnetosphere. We are developing GEO-X by integrating innovative technologies of a wide field of view (FOV) X-ray instrument and a small satellite for deep space exploration. The satellite combines a Cubesat and a hybrid kick motor, which can produce a large delta v to increase the altitude of the orbit to about 30 to 60 RE from a relatively low-altitude (e.g., geo transfer orbit) piggyback launch. GEO-X carries a wide FOV (5 × 5 deg) and a good spatial resolution (10 arcmin) X-ray (0.3 to 2 keV) imaging spectrometer using a micro-machined X-ray telescope and a CMOS detector system combined with an optical blocking filter. We aim to launch the satellite around the solar maximum of solar cycle 25.
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