Effect of particle contamination on the WFI filters of the Athena mission in a launch vibration environment

Abstract

Many astrophysical missions rely on radiation observation, necessitating the use of filters to shield detectors from unwanted radiation, serve as radiofrequency shields, and protect from contaminants, including molecules and particles. These filters must maintain high transparency to the radiation of interest, often requiring thicknesses at the sub-micrometer scale. For the upcoming Athena mission, launch expected in second half of 2030s, the designated filters will be ultra-thin polyimide coated with a few tens of nanometers of aluminum. The focal plane of the Athena mission contains two main instruments: the Wide Field Image (WFI) and the X-ray Integral Field Unit (X-IFU). Both will be equipped with filters to shield the detectors from out-of-band radiation. The baseline of the filter design of the large array detector of the WFI consists of a large square-shaped membrane of polyimide 150 nm thick coated with 30 nm of aluminum. Adhering to the ECSS-Q-ST-70-01C standards of the European Space Agency, a cleanliness and contamination control plan must be established for all Athena components, including filters, to monitor manufacturing, assembly and integration phases. Furthemore, contaminant particles might be dangerous for the intactness of thin filters when they are subject to vibro-acoustic loads during launch. In fact, such small particles, if present on the filter surface or in the close environment, can act as bullets, with high risk of puncture or cut the ultra-thin filters that protect the detectors. To assess potential damage caused by particle contaminants, we performed a Contamination Vibration Test (CVT) of sample filters artificially exposed to different types and size of contaminants. Our results highlight that no major damage occurred with aluminum shaves contaminants up to 125 //, m size while some minor defects can be identified by the use of optical digital microscopy.