[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

They have been developed for astrophysics ex

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20 keV are described in detail. They have been developed for astrophysics ex- periments in space, for material analysis, and for experiments at synchrotron radiation facilities. The functional principles of the silicon devices are derived from basic solid state device physics. The spatial resolution, the spectroscopic performance of the systems, the long-term stability, and the limitations of the detectors are described in detail. The various fields of application show the unique utility of silicon radiation detectors. Imaging of photons is best known in the visible domain, ranging from a wavelength of 3,500 ˚ A up to 6,000 ˚ A. Optics and detectors are equally well de- veloped for those applications. But all these imaging systems do not count the incoming photons individually to measure their position, energy, and arrival time. The photon information is either integrated in the grains of a photographic film that is developed chemically afterwards or the photons are
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X-Ray Detectors and XRF Detection Channels 263 collected in individual picture cells (pixels) and sequentially read out after a given time. The photonic or electronic content of each grain or pixel is then “counted” to measure the intensity of the incident photon flux. Traditionally, the energy of the photons is determined by an arrangement of various filters, transparent only for a narrow, well-defined bandwidth of the incoming pho- tons. In this sense, the image is a static, integrated reconstruction of a local photon intensity distribution. Single “optical” photons cannot be counted up to now in a practical man- ner, i.e., with large arrays. The energy of the photons is too small to detect them individually with non-cryogenic detectors: it is a fraction of an elec- tron volt in the near infrared and up to 4 eV for the violet part of the visible spectrum. 1 In gas detectors, more than 20 eV are needed for the ionization of a detector gas atom, and room temperature silicon detectors need around 1 eV for the generation of an electron–hole pair in the optical range and 3.7 eV on average for ionizing particles with sufficiently large energy. For a proper electronic extraction of the very weak signal of one optical photon, readout electronics should operate below 0.1e equivalent noise charge (ENC). This is by far not reached today in the state-of-the-art silicon sensor systems. From approximately 11,000 ˚ A to 3,000 ˚ A only one electron–hole pair per photon is generated due to the ionization process and its statistics in silicon. In this sense, direct spectroscopic information in the optical region is physically not available from silicon detectors. The X-ray imaging detector systems that are described below record si- multaneously the energy, position, and arrival time of each individual X-ray photon without using selective absorbers. The physical reasons for being able to make truly energy-dispersive X-ray detectors are the low average electron– hole pair creation energy of about 3.7 eV in silicon at room temperature
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