20 218 a longoni and c fiorini are converted into

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measured with a Gas Proportional Scintillation Counter (figure from Ref. [20])
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218 A. Longoni and C. Fiorini are converted into electrons by means of a photodetector, typically a photo- multiplier tube. In comparison to the gas detectors, the scintillation detectors are based on a higher Z material with a higher density and often they have a larger thickness. This means that they have a high probability of detecting photons in a wider X-ray energy range. However, the energy resolution offered by this detector is the poorest among X-ray detectors, especially in the low energy range. When separated from the noise, a 5.9 keV peak can be measured with a resolution of the order of 30%. They are mostly used for low- resolution applications in an energy range from a few tens of keV up to hundreds of keV or a few MeVs. In semiconductor detectors , electron–hole pairs are generated by direct interaction of the photon inside the detector material, similar to the primary electron–ion pairs generation in a gas proportional counter. In contrast, the output signal is generated by the collection of this primary charge without any multiplication process (except in the silicon avalanche photodiode). With respect to gas detectors, semiconductor detectors have higher density and Z . Moreover, in a semiconductor material the average energy required to pro- duce a charge–carrier pair is of few electron-volts (3.62 eV for Si, 2.96 eV for Ge) while for gases this quantity is about 30 eV and for a scintillator- photodetector system could be, at best, of the order of 25 eV. According to these energy/charge conversion factors, the number of charge carriers gener- ated for a given energy is higher for semiconductor detectors than for gases, leading to a much smaller statistical broadening of the peaks produced in the X-ray spectrum. For this reason, semiconductor detectors are nowadays the most preferred detectors in X-ray spectroscopy, especially when energy resolution is of primary concern. Energy resolutions much better than the ones achievable even by the best nitrogen-cooled semiconductor detectors can be provided by cryogenic detectors . 4.2.4 Semiconductor Detectors In this section, we will briefly discuss the main characteristics and perfor- mances of semiconductor detectors, with special focus on Si and Ge detectors. Within the scope of this presentation, we cannot discuss in depth the funda- mental solid-state physics of these detectors. Interested readers could refer to specialized books, such as those of Knoll [19] and Lutz [21]. In a single crystal of semiconductor material such as silicon or germanium, the sharply defined atomic electron states are broadened into bands of energy states. The outer electrons are kept in the valence band while the next higher states lie in the conduction band, separated from the valence band by an energy gap (Fig. 4.12). The band gaps are 1.12 eV and 0.74 eV, respectively, in silicon and germanium. In pure semiconductors without impurities the gap contains forbidden states. An electron can be promoted from the valence band to the conduction band if it receives an energy at least equal to that of the
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  • Spring '14
  • MichaelDudley

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