Effectiveness of a dual solenoid magnetic shield at reducing x-ray-like background in silicon-based x-ray detectors

Abstract

.Magnetic shielding has been used during past and current x-ray astronomy missions to shield detectors from electrons entering through telescope optics, and soft proton diverters have also been planned for future missions. However, simulations performed throughout the past decade have discovered that a significant proportion of x-ray-like background originates from secondary electrons produced in spacecraft shielding surrounding x-ray detectors, which hit detectors isotropically from all directions. Here, the results from Geant4 simulations of a simple dual solenoid magnetic field surrounding a 450 μm thick detector with several on-chip layers based on the structure of preliminary designs for the ATHENA Wide Field Imager are presented. We found that for a magnetic field strength at the middle between the two coils of equal to or greater than ∼12 mT, a dual solenoid magnetic field is extremely effective at preventing secondary electrons depositing between 2 and 7 keV and induced by galactic cosmic protons from reaching the detector. While the exact level of background reduction would depend on specific spacecraft and detector design, this magnetic shielding method could remove almost all background associated with backscattering and absorbed electrons, which are expected to account for approximately two thirds of the expected off-axis background in silicon-based x-ray detectors of several hundred microns in thickness and would likely be even more effective for thinner detectors where x-ray-like background is even more dominated by electrons. The magnetic field structure necessary for doing this could be produced using a set of solenoids or neodymium magnets providing that power requirements can be sufficiently optimized or neodymium fluorescence lines can be sufficiently attenuated, respectively. Testing would also have to be done to ensure that detectors it is used on are sufficiently magnetically hard; for context, the current limit for magnetic field strength around the ATHENA Wide Field Imager is 1 mT although CCD97s, another type of space-based x-ray detector, are magnetically hard at least above 6.4 mT.