Impact of impurities in shielding material on simulations of instrument background in space

Abstract. A major source of background for x-ray focal plane detectors in space instrumentation aboard missions, such as Extended Roentgen Survey with an Imaging Telescope Array and Athena Wide Field Imager, is the space radiation environment. High-energy radiations from the environment interact with the spacecraft structure leading to large productions of secondary particles with energies that are detectable in the science region of interest for instrumentation. Reducing the background from these events is vital for the success of many missions. Graded-Z shielding is a common solution to help reduce the instrument background. Layers of materials with decreasing atomic numbers near detectors help reduce the background. Much of the design is determined through iterative simulations to find an optimal solution that meets the requirements for the scientific operation of the instrument. Recent results have indicated an underestimate in the instrument background from the simulations. One hypothesis has been that the simulations do not typically include the impurities in the shielding materials. The work presented investigates the association of impurities in the graded-Z materials and the instrument background spectra. Typically, impurities are not included in material definitions as they can significantly increase computational time. The impurities, percentage loading, and distribution have all been explored and evaluated for an Al-Mo-Be graded-Z shield.