The CCD instrument background of the SMILE SXI

The ESA and CAS SMILE mission orbit is highly elliptical and will pass through multiple radiation environments. The Soft X-ray Imager (SXI) instrument aboard has a radiation shutter door designed to close when the surrounding radiation flux is high. The shutter door will close when passing below an altitude threshold to protect against trapped particles in the Earth’s Van Allen Belts. Therefore, two radiation environments can be approximated based on the shutter door position: open and closed. The instrument background for the CCDs (Charge-Coupled Devices) that form the focal plane array of the SXI were evaluated for the two environments. Due to the correlation of the space environment with the solar cycle, the solar minima and maxima, the background was also evaluated at these two extremes. The results demonstrated that the highest instrument background will occur during solar minima due to the main contributing source being Galactic Cosmic Rays (GCRs). It was also found that the open background was highest for solar minima and that the closed background was highest during solar maxima. This is due to the radiation shutter door acting as a scattering centre and the changes in the energy flux distribution of the GCRs between the two solar extremes.