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

However other factors limit the spectroscopic

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energy resolution. However, other factors limit the spectroscopic performances of HPGe detectors with respct to Si(Li) especially in X-ray detection lower than about 20 keV. In the low energy range ( < 3 keV), the X-ray spectrum measured with a Ge detector can be affected by distortions and higher background resulting from the thicker entrance window with respect to Si(Li) detectors. In the energy region above the Ge K-absorption edge at 11.1 keV, a complex X-ray spectrum can be significantly disturbed by the presence of escape peaks which are much more intense in Ge detectors, compared to Si detectors. The higher escape/photopeak ratio for Ge than for Si (shown in Fig. 4.4) is due to the smaller average penetration distance of the incident radiation in the detector, the higher fluorescence yield, and the higher escape probability of the more energetic Ge-K fluorescence lines (9.9 and 11 keV). Escape peaks in spectra measured with Si(Li) detectors are observed at lower energies ( > 1 . 84 keV, Si K-absorption edge) with respect to Ge. However, they are much less in- tense and therefore overlap only very weak photopeaks. The presence of the escape peaks on a 241 Am spectrum measured with HPGe detector is shown in Fig. 4.14, where, for comparison, the spectrum of the same source mea- sured with a Si(Li) detector is also reported. A more detailed comparison between Ge and Si(Li) detectors in the 2–20 keV energy range is presented by Rossington et al. [24]. In some applications, like portable X-ray spectrometers, the difficulties arising from the use of a liquid nitrogen cryostat (large size of the cryostat, nitrogen refill) can be overcome by the use of a Peltier-cooled silicon detector. Si-PIN diodes from AMPTEK (25 mm 2 active area, 500 µ m thickness) [25],
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222 A. Longoni and C. Fiorini 0 10 10 2 10 3 10 4 10 5 33.2 59.5 - Ge k b escape 59.5 - Ge k a escape 59.5 20.8 17.8 14.0 20.8 Ge k a escape 17.8-Ge k a escape 16.9 - Ge k a escape 14.0 - Ge k a escape Counts 10 20 30 40 Energy (keV) Energy (keV) 50 60 70 0 10 20 30 40 50 60 70 10 10 2 10 3 10 4 10 5 14.0 17.8 20.8 33.2 59.5 Inelastic scattering from 59.5 8.1 Fig. 4.14. 241 Am spectrum measured with an HPGe detector (left) and a Si(Li) detector (right) (figure from [24]) cooled at about 30 , offer an energy resolution of the order of 250 eV at 6 keV at 20 µ s shaping time. Smaller devices (5 mm 2 , 500 µ m thickness) have recently reached energy resolutions as good as 158 eV FWHM at 6 keV at 20 µ s shaping time. Also Peltier-cooled SDDs, described in Section 4.2.5, have reached resolutions not far from the best ones of Si(Li) and Ge detectors, with a shaping time of the order of 1 µ s. Room temperature or Peltier-cooled high- Z semiconductor detectors like HgI 2 [26], CdTe [27], CdZnTe (CZT) [28], GaAs [29] can also be used for X-ray detection without the need for liquid nitrogen cryostat. Due to the high mean atomic number, a thickness of a few mm is sufficient for these detectors to provide a good efficiency in a wide X-ray energy range, up to a few hundred keV.
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