Performance of a Silicon Drift Detector
Figure 2: Spectra of the Mn standard as a function of time constant (TC) at 20 kV and ~22-percent DT. The Mn Ka peaks are normalized to 4× full scale. Net counts are given for the Mn Ka peak for 100 clock second acquisition.
is associated with the pulses (X-rays) that are successfully processed by the detector electronics and placed in the acquired spectrum.
Results Te SDD provides ~3.5× the geometric collection of the
previous conventional 10-mm2 Si(Li) detector. As the SDD has a 3× larger active area relative to that of the nominal 10-mm2 Si(Li) detector, the observed ~3.5× increase may be due to small differences in the actual active areas of the two detectors or to slightly different detector-to-specimen distances. Te fast time constants that are permissible because of the low capacitance of the SDD allow spectra to be collected at high input count rates (as high as ~500 kcps). Spectra acquired for the Mn standard at ~22 percent dead time (DT) for six different time constants (0.5–12.8 ms) are compared in Figure 2. In addition to the Mn-Ka, b, and Mn-L peaks, there are spectral artifacts that appear at high count rates and short time constants, such as peak shiſts, peak broadening associated with a loss of energy resolution, high-energy tailing (for example, above the Mn–Ka peak), sum peaks (for example, the 2× peaks near 12 keV), and other pulse pile-up effects. It should be noted that the sum peaks become progressively non-Gaussian in shape for shorter time constants. Tere is a decreased intensity of low-energy X-rays below the Mn L peak, especially at the two fastest time constants. Because the spectra are normalized (Mn Ka to 4× full scale), there is a slight increase in the background at higher input count rates associated with a loss of Mn Ka intensity due to pulse pile-up.
42 Input count rate capability, throughput counts, energy
resolution/stability, energy stability, and spectral artifacts (sum peaks and peak broadening) all impact the quality and quantification of X-ray microanalysis. As such, performance testing is required to identify the operational space for both the detector and the processing electronics that yield spectra of sufficient quality for valid analyses. Te throughput for the Mn standard as a function of
indicated input count rate and time constant is shown in Fig- ure 3. It should be noted that the maximum throughput occurs at progressively lower dead times for faster time constants (higher input count rates). Te maximum throughputs observed occur at 61, 61, 61, 57, 53, and 49 percent dead time with progressively shorter time constants (that is, 12.8, 6.4, 3.2, 1.6, 0.8, and 0.5 ms, respectively). If the detector and processing electronics are processing pulses in an optimized fashion, the maximum throughput should occur at a calculated dead time of ~65 percent [4]. It is presumed that these lower dead time values for maximum throughput are related to either the increased fraction of detector live time lost to detector reset events or to processing electronics problems associated with the high input count rates. A second problem noted in Figure 3 is the saturation
of input count rate for the 0.5-ms time constant at ~500 kcps, even though the last data point was taken with ~24 percent higher probe current than the penultimate data point. Te detection of this effect required the high incident probe currents (~100 nA at 20 kV) that the JEOL 6500F SEM was able
www.microscopy-today.com • 2011 May
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