29 September 2004 Shielding of cosmic-ray-induced background in CCD detectors for x-ray astronomy
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An active anticoincidence detector system for background reduction cannot be integrated in CCD detectors for X-ray astronomy. The background rate within an integration-readout interval would result in an unacceptable dead time of about 50% or more. Events of minimum ionizing particles can be discriminated in CCD detectors due to their high energy deposit and their image pattern. Events of X-rays or charged particles within the accepted energy band originating from cosmic ray interaction with the material surrounding the CCD cannot be distinguished from valid cosmic X-ray events and therefore contribute to the background noise. Graded-Z shielding is an efficient method to shift the energy of the locally produced X-rays to low energies. At low energies low-Z shielding material can be used, which rather produces Auger electrons than fluorescent X-rays. Low energy electrons can be stopped in the passivation layer of the CCD. Due to the low operating temperature of the CCD (~170 K) the shielding material has to have a similar thermal expansion coefficient as silicon. With regard to future X-ray missions the properties of several shielding materials like aluminium oxide, aluminium nitride, silicon nitride and boron carbide were investigated in more detail. The results are presented.
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Elmar Pfeffermann, Peter Friedrich, Michael Freyberg, Guenther Kettenring, Ludwig Kraemer, Norbert Meidinger, Peter Predehl, Lothar Strueder, "Shielding of cosmic-ray-induced background in CCD detectors for x-ray astronomy", Proc. SPIE 5501, High-Energy Detectors in Astronomy, (29 September 2004); doi: 10.1117/12.551262; https://doi.org/10.1117/12.551262

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