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Low energy (< 200 keV) protons entering the field of view of the XMM-Newton telescope and scattering with the mirror surface are observed in the form of a sudden increase in the background level. Such flaring events, a effecting about 30-40% of XMM-Newton observing time, can hardly be disentangled from true X-ray events and cannot be rejected on board. A response matrix for protons would allow a better understanding of the proton radiation environment, with the aim of modeling the in-flight non X-ray background of current (e.g. XMM-Newton, eROSITA) and future (e.g. ATHENA) X-ray focusing telescopes. Thanks to the latest validation studies on the physics models describing the reflection process of protons at grazing angles, we propose to build a prototype XMM-Newton EPIC proton response matrix describing the effective area and energy redistribution of protons entering the mirror aperture. The simulation pipeline comprises two independent simulation frameworks for the X-ray optics reflectivity, based on ray-tracing and Geant4, and a Geant4 simulation for the proton transmission efficiency caused by the combination of optical filters, on-chip electrodes and the detection depletion regions, requiring a detailed mass model of the MOS focal plane assembly. We present here the pipeline design, the characterization and verification of the proton transmission efficiency, and the algorithms for the effective area and energy redistribution computation. After the verification and validation activity, an opportune data formatting of the tool and its interface with widely-used analysis software (e.g. XSPEC) will allow the distribution of the proton response matrix to the scientific community.
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