Soft x-ray imaging with a newly designed large-area CCD (Conference Presentation)

SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) is a joint venture between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS). The mission will study the dynamic interaction between the solar wind and the Earth’s magnetosphere. Two of the instruments, namely the Soft X-ray Imager (SXI) and the Ultra-Violet Imager (UVI), will observe northern aurora and the boundary of the magnetosphere simultaneously, a first for space-missions. To complement these remote observations, in-situ measurements of the solar wind ion distribution as well as measurements of the strength of magnetic fields will be attained via the Light Ion Analyser (LIA) and the Magnetometer (MAG) respectively. Together, these four instruments will provide a complete picture of the interactions between the solar wind and the response of the Earth’s magnetosphere, which is the main driver of space-weather. The SXI will be used to observe and image Solar Wind Charge eXchange (SWCX) that occurs at the interface between the solar wind and the Earth’s magnetosphere. At this location, ions in the solar wind interact with neutrals in the Earth’s exosphere leading to the production of soft X-rays with key emission lines at energies between 0.1-2 keV. The SXI will use a wide angle silicon micro pore optic to focus the incoming X-rays onto a focal plane of two large area Charge-Coupled Devices (CCDs) from Teledyne-e2v. The CCD for SXI is designated the CCD370, with 4510x4510 pixels of 18 µm, which will be read out with 6x6 on-chip binning to give an effective pixel size of 108 µm square. The binning improves charge transfer efficiency and energy resolution, and allows the pixel area to be divided into asymmetric frame and store regions of the device for frame-transfer operation. The CCD370 design includes a range of modifications from its predecessor (the CCD270 from the PLATO mission), with the goal of optimising it for imaging soft X-rays; a supplementary buried channel in the parallel region, a narrowed serial channel width, and increased output amplifier responsivity will improve the low signal sensitivity and charge transfer efficiency. Here, the CCD370 performance in the SXI telescope will be presented, predicted from the first measurements using the laboratory SXI CCD characterisation camera and CCD270s. The improvements expected from design changes that optimise the SXI CCDs for soft X-ray detection, and plans for pre-flight calibration and radiation damage testing will be described.