However the dark current value can be highly reduced

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However, the dark current value can be highly reduced by suitably cooling the detector. The contribution of the various parameters of the detector to the overall electronics noise of the detector–amplifier system will be discussed in detail in Section 4.2.6 (pp. 235–249) The depletion depth that can be achieved by reverse biasing a conventional silicon pn detector is usually limited to 0.3–1 mm. Thicker depletion depths (5–10 mm) can be reached by means of the lithium drifting process. In a lithium-drifted silicon detector or Si(Li), lithium ions, which act as donors, are driven through a large volume of a high purity silicon crystal, which tends to be p-type, in order to obtain an “intrinsic”-like bulk material by means of the compensation of the donors and acceptors impurities concentrations. The excess lithium on the surface of deposition on the crystal results in a highly doped n+ layer which acts as an electrical contact, while the uncompensated p region on the opposite is contacted by either a metallic contact or a thin p+ layer. In a Si(Li) detector, the lithium continues to drift significantly at room temperature. Therefore, in order to prevent an undesired redistribution of the lithium dopants, the detector must always be kept cold (usually at liquid nitrogen temperature), even when not operated. The Si(Li) detector is currently the most popular X-ray detector in the energy range from a few hundreds eV up to about 40 keV. The typical energy resolution of commercial detectors is of the order of 135 eV at 6 keV. An energy resolution of 128 eV has been measured by using a Pentafet front-end transistor [23].
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X-Ray Detectors and XRF Detection Channels 221 Nitrogen-cooled high purity germanium HPGe detectors can be also used for high-resolution X-ray detection. Unlike Si(Li) detectors, Ge detectors need not be cooled permanently, but only when they are operated. With respect to Si, Ge is characterized by a better detection efficiency for higher energy photons, because of both higher Z (32 for Ge, 14 for Si) and higher density (2.33 g cm 3 for Ge, 5.46 g cm 3 for Si). For a given thickness, the better efficiency of a Ge detector with respect to a Si detector is shown in Fig. 4.3. HPGe detectors exhibit superior statistical energy resolution (∆ E statistical in (4.6)) compared with Si(Li) detectors of the same geometry. The two factors which contribute to the superior statistical contribution to the energy resolu- tion are the lower values respectively of mean energy required for an electron– hole pair generation (2.96 eV with respect to 3.62 eV for Si) and of Fano factor (0.08 with respect to 0.11 for Si). The statistical term in the energy resolution, typically dominating at high energies ( > 10 keV) with respect to the electron- ics noise contribution, is then about 25% better for a Ge detector than for a Si detector. Moreover, the possibility of fabricating thicker HPGe detectors, compared with Si(Li), translates directly into a smaller output capacitance for the former ones with a correspondingly smaller electronics noise and higher energy resolution.
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  • Spring '14
  • MichaelDudley

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