Magnetic Shielding of Soft Protons in Future X-Ray Telescopes: The Case of the ATHENA Wide Field Imager
Abstract
Both interplanetary space and Earth’s magnetosphere are populated by low-energy (≤300 keV) protons that are potentially able to scatter on the reflecting surface of the 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 find that in the absence of a magnetic diverter, protons are indeed funneled toward the focal plane, with a focused non-X-ray background well above the level required by ATHENA science objectives (5 × 10-4 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2018
- DOI:
- 10.3847/1538-4357/aade99
- arXiv:
- arXiv:1808.09431
- Bibcode:
- 2018ApJ...867....9F
- Keywords:
-
- instrumentation: miscellaneous;
- telescopes;
- Astrophysics - Instrumentation and Methods for Astrophysics
- E-Print:
- Accepted for publication on The Astrophysical Journal on 28/08/2018