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Photon Detector Categories Energy Resolution The energy resolution is a measure of the detector’s ability to distinguish closely-spaced lines in the spectrum. The method used to measure the energy resolution is also described in ANSI/IEEE 325–1996.Energy Resolution as a Function of Energy For the energy range up to 1.5 MeV, the following approximate (and not dimensionally correct) expression is useful for predicting the resolution of a Germanium detector:R = (N2 + 2E)1/2 where R is the energy resolution (FWHM) at the energy of interest, N is the noise line width, and E is the energy of interest, with all quantities expressed in eV (not in keV). For the range from 1.5 MeV to 10 MeV (as shown in Ref. 4), the expected resolution (FWHM) is approximately 0.08% to 0.1% of the energy of the line of interest. At the higher energies the measured resolution can be worse than this due to even minor trapping. The actual measured values depend on the quality of the Ge crystal used to manufacture the detector element, the depth of the hole in the center of the crystal, extent of shaping of the crystal’s front "corners," and other manufacturing details. All Ge detectors are not created equal!The Energy Resolution Advantage Most ORTEC detectors have measured energy resolution substantially better than the warranted resolution.Energy Resolution as a Function of Temperature Most Germanium detectors begin to show increasing leakage current and electronic noise at temperatures above ~110 K. Due to the different cooling capabilities of various cryostats, Germanium (HPGe) detectors normally operate at temperatures in the range from 85 to 100 K. A stable operating temperature is essential. Because E, the average energy necessary to create an electron hole-pair (see Table 1), varies with temperature at a rate of 2.53 X 10–4 per degree K (Ref. 6), temperature variations during a measurement result in a peak shift that degrades the energy resolution. Temperatures below 40 K may result in deterioration in energy resolution due to trapping effects.There are several references7,8 useful for those planning to use germanium detectors at temperatures higher or lower than the customary temperature. Because the FET that is in the first stage of the preamplifier is inside the cryostat and yet must be held at ~115 K, the use of germanium detectors at unusual operating temperatures may result in increased first-stage preamplifier noise. Si(Li) detectors do not operate well at temperatures below 77 K.10 |
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