KEYWORDS: Charge-coupled devices, X-rays, Temperature metrology, X-ray imaging, Solar processes, Silicon, X-ray fluorescence spectroscopy, Solar energy, Signal detection, Sensors
The SMILE mission, a collaborative effort between the European Space Agency and the Chinese Academy of Sciences, seeks to enhance our comprehension of the interplay between solar phenomena and the Earth's magnetosphere-ionosphere system on a global scale. Among its instrumental arsenal is the Soft X-ray Imager (SXI), designed to capture photons generated within the 200eV to 2000eV energy spectrum through the solar wind charge exchange process. This imaging tool employs two large CCD370s, each with 4510 x 4510 18μm pitch pixels, as its focal plane. SMILE will orbit Earth in an elliptical trajectory, traversing the radiation belts approximately every 52 hours. Over the course of its anticipated 3-year mission, the CCDs onboard will endure progressive deterioration from the persistent presence of trapped and solar protons. To gauge the extent of this damage and its effect on the devices' functionality, a sequence of proton radiation campaigns is underway. The final cryogenic irradiation campaign has now been completed using a fully functioning engineering model of the SXI CCD370s that will be used in flight and irradiating up to the expected end of life total non-ionising dose. The results show that the measured parallel charge transfer inefficiency (pCTI) varies with temperature both before and after irradiation, however the trend changes from decreasing with temperature to increasing. This is thought to be due to a change in the dominant effective trap species. The impact of multiple charge injection lines and 6x6 binned frame transfer is also assessed and shows that between -130 to -100°C the pCTI, when both measures are utilized, is independent of temperature. This suggests potential for more flexible thermal controls in future missions that use similar devices.
Space observatories utilizing micro pore optics (MPOs) have been used and are planned for several future X-ray astronomy space missions. The optical systems are designed to facilitate the focusing of incoming photons onto the focal plane of telescopes. Unfortunately, as well as having a small solid angle “open” to the sky, MPOs also have the unintentional effect of focusing high-energy particles from the space radiation environment. This causes additional radiation damage to mission-critical imaging sensors with solar energetic particles being particularly focusable. Typically, processes such as sectoral analysis are used to estimate the predicted dose to components, which is a ray tracing approach, and does not include focusing effects. We investigated focused dose estimation techniques for MPOs using Monte Carlo (MC) simulations. The focused dose contribution was compared with the unfocused contribution for the Solar wind Magnetosphere Ionosphere Link Explorer mission. The unfocused dose estimates were calculated using a traditional sectoral shielding analysis. The Monte Carlo-focused dose simulations enabled dose mapping over the image sensor to be analyzed. This revealed a relatively uniform dose across the device with some focusing artifacts present. The simulations also showed that the total ionizing dose and total non-ionizing dose decreased with depth into the sensor from the entrance window. This is key when considering that charge is often stored at varying depths in imaging devices across different technologies, for example, in front or back illuminated devices.
A major source of background for x-ray focal plane detectors in space instrumentation aboard missions, such as Extended Roentgen Survey with an Imaging Telescope Array and Athena Wide Field Imager, is the space radiation environment. High-energy radiations from the environment interact with the spacecraft structure leading to large productions of secondary particles with energies that are detectable in the science region of interest for instrumentation. Reducing the background from these events is vital for the success of many missions. Graded-Z shielding is a common solution to help reduce the instrument background. Layers of materials with decreasing atomic numbers near detectors help reduce the background. Much of the design is determined through iterative simulations to find an optimal solution that meets the requirements for the scientific operation of the instrument. Recent results have indicated an underestimate in the instrument background from the simulations. One hypothesis has been that the simulations do not typically include the impurities in the shielding materials. The work presented investigates the association of impurities in the graded-Z materials and the instrument background spectra. Typically, impurities are not included in material definitions as they can significantly increase computational time. The impurities, percentage loading, and distribution have all been explored and evaluated for an Al-Mo-Be graded-Z shield.
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