And the very thin radiation entrance windows of only

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and the very thin radiation entrance windows of only a few tens of partially insensitive atomic layers of silicon and native SiO 2 , which can be penetrated by (even soft) X-rays. For a good quantum efficiency at higher X-ray energies, only the depleted thickness of silicon (charge collection depth) is of relevance. At 500 µ m sensitive detector thickness, e.g., a fraction of 25% of 25 keV X-rays is converted in electron–hole pairs and can be collected and detected. For two-dimensional silicon detectors with high position and energy resolution, the fabrication by a planar process – comparable to the fabrication in state- of-the-art microelectronics – is obligatory. Depletion thicknesses of 1,000 µ m are a practical limit for detector fabrication. State-of-the-art imaging silicon detector systems are an ideal instrument for direct detection in the energy band between 0.1 keV and 30 keV with high quantum efficiency, position and energy resolution. 1 Cryogenic detectors are able to perform single photon counting in the near infrared, visible and soft X-ray domain [67]. The band gap for this kind of detectors is in the millielectron volt range as compared to 1.1 eV for Si. But cooling down to the order of 100 mK is required.
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264 L. Str¨uder et al. The astrophysical requirements have driven the development of the high resolution X-ray detectors from 0.1 keV to 10 keV in the last 10 years. The X-ray multi mirror (XMM) mission of the European Space Agency (ESA) was successfully launched in a highly eccentric orbit with three large X-ray telescopes and reflecting grating spectrometers all having specially designed X-ray charge coupled devices (CCDs) in their focal planes [68, 69]. The energy dispersive gratings are read out with the more conventional backside illuminated 30 µ m deep depleted MOS CCDs for energies up to 4 keV [70]. The pn-CCD detectors, formed only by rectifying pn-junctions, deliver excellent position (in the order of tens of microns), energy (about 140 eV FWHM at 6 keV), and time resolution (below 100 µ s in dedicated operating modes) with high quantum efficiency (above 90%) at soft and medium energy X-rays from 0.5 keV to 10 keV. All experimental results shown here are from devices which have been designed, fabricated and tested at the MPI Halbleiterlabor. 4.3.2 Fully Depleted Backside Illuminated pn-CCDs Conceptually, the pn-CCD is a derivative of the silicon drift detector [30]. The development of the pn-CCDs started in 1985. In the following years, the basic concept was simulated, modified, and designed in detail [71]. N-channel JFET electronics was integrated in 1992 [40, 72] and the first reasonably fine working devices were produced in 1993. Up to then, all presented devices were “small” devices, i.e., 3 cm 2 in sensitive area [68]. The flight-type large area detectors were produced from 1995 to 1997, with a sufficiently high yield to equip the X-ray satellite missions ABRIXAS and XMM [73–75] with defect free focal plane pn-CCDs.
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  • Spring '14
  • MichaelDudley

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