Last update: Apr 1, 2000
Speaker: Petteri NieminenX-ray astronomy in space relies on the focussing of X-ray photons by low-angle scattering from shaped "shells". In most cases the "optics" consist of two sets of nested concentric shells with shapes near to sections of cones. Two grazing-incidence scatters result in focussing of the X-rays on the shell axis. ESA's XMM mission has three mirror modules of outer diameter 70 cm, each consisting of 58 nested shells which focus the X-rays onto CCD detectors some 7 m from the mirrors. The shells' configuration precludes any direct line-of-sight of space through the mirror system.
CCD detectors are known to be radiation sensitive and much attention is given to shielding them against radiation penetrating spacecraft structural materials. However, it has been found that protons of energies in the range of hundreds of keV to a few MeV can scatter at low angles through the mirror shells. These protons, because of their low energy can produce a high non-ionising dose in unshielded CCDs and are therefore a potential threat. This paper discusses the efforts made in the context of the XMM mission to simulate the processes involved in the transport of protons and generation of non-ionising dose in CCD's.
The tools used for this analysis included the Geant4 Monte-Carlo toolkit and the TRIM code. Geant4 was used for fully three-dimensional calculations while TRIM was used to examine details of the low angle scattering.
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