Detectors for X-ray astronomy operated in space are exposed to the whole spectrum of cosmic rays. The background rate exceeds by far the X-ray event rate of the majority of cosmic X-ray sources. A sophisticated event selection logic is mandatory to distinguish real X-ray events from charged particle events or fluorescent X-rays from surrounding materials. One possibility is to limit the energy band of accepted events. The depth of the detector cell must be chosen large enough so that minimum ionizing particles deposit more energy than the most energetic accepted X-ray event. In this way, minimum ionizing particles can be easily discriminated with an upper event threshold. Another possibility to distinguish particle events from X-ray events is the geometric shape of the related ionization cloud. Particle events leave an ionized track, whereas X-ray events leave a more point-like ionization cloud resulting in different rise times of the detector signal. A further background reduction method is to surround the actual X-ray detector with anticoincidence detectors on three to five sides. Coincident signals in an anticoincidence counter and the X-ray detector indicate with a high probability a non X-ray interaction and the event should be rejected. To eliminate X-rays generated by cosmic rays in the detector housing, the gas column density of the anticoincidence counter should be large enough to absorb X-rays efficiently up to the upper threshold of the accepted energy band. Large area X-ray detectors achieve by these methods background rejection efficiencies of 99.6% .
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