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X-ray telescopes opened up a new window into the high-energy universe.However, the last generation of these telescopes encountered an unexpectedproblem: their optics focused not only X-rays but low-energy (so called soft)protons as well. These protons are very hard to model and can not bedistinguished from X-rays. For example, 40\% of XMM-Newton observations issignificantly contaminated by soft proton induced background flares. In orderto minimize the background from such low-energy protons the Advanced Telescopefor High ENergy Astrophysics (ATHENA) satellite introduced a novel concept, theso called Charged Particle Diverter (CPD). It is an array of magnets in aHallbach design, which deflects protons below 76 keV before they would hit theWide Field Imager (WFI) detector. In this work, we investigate the effect ofscattering of the deflected protons with the CPD walls and the inner surfacesof the WFI detector assembly. Such scattered protons can loose energy, changedirection and still hit the WFI. In order to adopt the most realisticinstrument model, we imported the CAD model of both the CPD and the WFI focalplane assembly. Soft protons corresponding to $\approx$2.5 hours of exposure tothe L1 solar wind are simulated in this work. The inhomogeneous magnetic fieldof the CPD is included in the simulation. We present a preliminary estimate ofthe WFI residual background induced by soft proton secondary scattering, in thecase of the optical blocking filter present in the field of view. A firstinvestigation of the volumes responsible for scattering the protons back intothe field of view is reported.

Geant4 simulation of the residual background in the ATHENA Wide Field Imager from protons deflected by the Charged Particle Diverter