Athena Charged Particle Diverter (CPD) scientific assessment: preliminary results on the WFI residual background

Date Issued
2021
Author(s)
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Zlamal, Jakub
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Galgoczi, Gabor
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Breuer, Jean-Paul
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Werner, Norbert
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Perinati, Emanuele
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Boudin, Nathalie
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Ferreira, Ivo
Abstract
Soft protons deflected by the CPD can potentially be scattered at lower energy and still reach the detection plane. Proton scattering measurements with samples of the Athena CPD surface, WFI baffle, and other compositions with different levels of roughness are currently being finalized, together with the validation of proton scattering simulations, to study the impact of secondary proton scattering. Preliminary results indicate that protons are indeed scattered with the CPD surface, with a level of mean roughness > tens of nm, with an efficiency almost uniform in scattering angle and large energy losses (>90% at 100 keV). The presence of heavier elements increases the scattering efficiency, while the impact of the roughness is still under investigation. SRIM and Geant4 simulations modelling the surface roughness are currently ongoing, and preliminary SRIM simulations are in general consistent with the measured distributions, but discrepancies in the energy losses are still under study. Preliminary results on the WFI CPD simulation aimed to confirm its compliance with the residual background requirement and to estimate the impact of secondary proton scattering were obtained with a combined simulation of the Athena SPO and the proton interaction with the WFI optical filters and detection plane, assuming no roughness on the exposed surfaces. The statistical uncertainty of the present results, with few protons reaching the WFI, is not yet enough to perform a final CPD scientific assessment and characterization of the secondary proton scattering. We can however conclude with current SRIM simulations that the WFI residual background level), if the CPD is present, is well within the requirement of 5 ✕ 10-4 counts cm-2 s-1 keV-1 in the 2 - 7 keV energy range, with a maximum level of (3.4±2.0) ✕ 10-5 counts cm-2 s-1 keV-1 obtained with only the OBF filter. Most of the protons generating background counts scatter with the filter wheel structure as the last scattering interaction. Current results indicate that no corrective measures are required to minimise the secondary proton scattering or increase the CPD efficiency, but the adoption of vanes or coatings in the filter wheel structure could help reduce the secondary proton flux at the WFI.
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