Magnetic shielding of soft protons in future X-ray telescopes: the case of the ATHENA Wide Field Imager

TL;DR

This study uses detailed Geant4 simulations to evaluate magnetic shielding effectiveness against soft protons in future X-ray telescopes like ATHENA, demonstrating that magnetic diverters are essential for reducing proton-induced background to acceptable levels.

Contribution

First end-to-end Geant4 simulation of proton scattering and magnetic shielding effectiveness for the ATHENA Wide Field Imager.

Findings

Protons are funneled towards the focal plane without shielding.

Magnetic diverters can reduce residual background to acceptable levels.

Proton shielding is mandatory for high-sensitivity X-ray telescopes.

Abstract

Both the interplanetary space and the Earth magnetosphere are populated by low energy ($\leq300$ keV) protons that are potentially able to scatter on the reflecting surface of Wolter-I optics of X-ray focusing telescopes and reach the focal plane. This phenomenon, depending on the X-ray instrumentation, can dramatically increase the background level, reducing the sensitivity or, in the most extreme cases, compromising the observation itself. The use of a magnetic diverter, deflecting protons away from the field of view, requires a detailed characterization of their angular and energy distribution when exiting the mirror. We present the first end-to-end Geant4 simulation of proton scattering by X-ray optics and the consequent interaction with the diverter field and the X-ray detector assembly, selecting the ATHENA Wide Field Imager as a case study for the evaluation of the residual soft proton induced background. We obtain that, in absence of a magnetic diverter, protons are indeed funneled towards the focal plane, with a focused Non X-ray Background well above the level required by ATHENA science objectives ($5\times10^{-4}$ counts cm$^{-2}$ s$^{-1}$ keV$^{-1}$), for all the plasma regimes encountered in both L1 and L2 orbits. These results set the proton diverter as a mandatory shielding system on board the ATHENA mission and all high throughput X-ray telescopes operating in the interplanetary space. For a magnetic field computed to deflect 99\% of the protons that would otherwise reach the WFI, Geant4 simulations show that this configuration, in the assumption of a uniform field, would efficiently shield the focal plane, yielding a residual background level of the order or below the requirement.