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The Wide Field Imager (WFI) is one of the two focal plane instruments foreseen on board the future Advanced Telescope for High ENergy Astrophysics (ATHENA) planned by ESA. It is a DEPFET-based detector working in the energy range between 0.2 and 12 keV and covering a field-of-view of 40 arcmin diameter. A major advantage of a DEPFET active sensor is that each pixel has integrated its own amplifier, so that no charge transfer between pixels is needed. Therefore, it could be speculated that a DEPFET is less susceptible to radiation damage than a CCD, as lattice defects from nuclear scattering by particles in the space environment, mainly protons, would degrade the detector performance decreasing the charge transfer efficiency. However, any other degradation of the performance that might be caused by solar and cosmic protons must be quantified by dedicated irradiation tests in laboratory. For the non-X-ray-background (NXB) assessment conducted so far, we assumed that a bulk shielding providing a proton cut-off at ∼100 MeV was appropriate. The reasoning behind that is simply that below ∼100 MeV the non-ionising energy loss (NIEL) of protons in silicon increases with decreasing energy, while it tends to flatten above ∼100 MeV, as shown in Fig. 1. The drawback of a shielding surrounding the detector is the production of secondary emission in its environment from the interaction of omnidirectional energetic cosmic particles. This secondary emission is indeed mostly responsible for the generation of NXB. Therefore, it is crucial to conduct a trade-off analysis aimed to identify materials and geometries able to guarantee an adequate proton stopping power and, at the same time, minimize the induced NXB. In this paper, we review the assessment study conducted for the WFI in the former phase A. The new assessment has recently started after mission reformulation, and the results previously achieved in the framework of the WFI Background Working Group constitute the basis to continue the investigation, aimed at improving further the camera design and the methods of NXB reduction.
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