Predicting the particle-induced background for future X-ray astronomy missions: the importance of experimental validation for GEANT4 simulations

摘要

Particle-induced background, or "instrument background", produced from the interaction of background photons and charged particles with a detector, either as primaries or through the generation of secondary photons or particles, is one of the major sources of background for the focal plane sensors in X-ray astronomy missions. In previous studies for the European Space Agency (ESA) X-ray Multi Mirror (XMM-Newton) mission, the dominant source of background was found to be caused by the knock-on electrons generated as high-energy protons pass through the shielding materials surrounding the detector. From XMM-Newton, the contribution of Compton and other photon-generated background was small in comparison to the knock-on electron component. However, for the Wide Field Imager (WFI) on board the ESA Advanced Telescope for High-ENergy Astrophysics (ATHENA) mission Athena, which houses much thicker silicon in the depleted p-channel field effect transistor (DEPFET) active pixel sensors of the focal plane when compared to the Charge Coupled Devices (CCDs) used in the XMM-Newton EPIC MOS cameras, this photon component may no longer be expected to have such a minimal impact and therefore both the photon and proton-induced components must be considered in more detail. In order to minimise the background, studies have been conducted on the use of a graded-Z shield in addition to an aluminium proton shield (employed for radiation damage minimization). For thin detectors, a low-Z component alone may suffice, reducing the fluorescence components of the background. However, with thicker detectors a high-Z component may give added benefit through the combination of the high-Z component to reduce the photon-induced effects and a low-Z component to reduce the fluorescence components from the shielding's inner-surfaces, thus creating an "aluminium sandwich". In all cases, careful optimization of the shielding configuration is required to balance each component of background specific to the design of the instrument involved. The optimization of any shielding relies heavily upon a validated and verified simulation toolkit. Here we present the latest progress on our ongoing validation and verification studies of the GEANT4 simulations used for such an optimization process through a series of experimental test campaigns.