Performance of a Silicon Drift Detector
Figure 7: Spectrum for Mn collected with the 3.2-ms time constant at 40-percent DT and pulse pile-up corrected spectrum calculated by correction routine.
point for the indicated input count rate depended on the average X-ray energy being processed. In contrast to manganese at 20 kV, a 50-percent higher saturation value for the count rate was measured for aluminum at 10 keV, indicating that the problem may be related to processing larger (and therefore longer) current pulses from the SDD.
To demonstrate the efficacy of the SDD in X-ray micro-
analysis, elemental mapping of the fine-scale microstructure of a CMSX4 superalloy was performed by spectrum imaging at both 15 and 5 kV (top and bottom rows in Figure 9, respectively). Te BSE images and corresponding X-ray maps shown in Figure 9 (100 × 128 pixels) reveal the primary and secondary g’ phase (appears dark in BSE images) surrounded by g phase (appears bright in BSE images). Te X-ray maps reveal the relative elemental distribution of Al, Cr, and Re for the two phases. Te maps were acquired at either condition (1) 15 kV, 8 nA, 183-kc/s input, in ~9.6 min at 1.6 ms TC, 50-percent DT or condition (2) 5 kV, 6.8 nA, 60-kc/s input, in 8.6 min at 1.6 ms TC, 19-percent DT. It should be pointed out that for the 5-kV Cr elemental map, the Cr L peak, rather than the Cr K peak, was used. As would be expected, the spatial resolution of the maps is significantly improved for the 5-kV accelerating voltage as a result of the decrease in the excited volume. Tis is evident when compared with the increased “blurring” observed in the 15-kV elemental maps of the finely intermixed g′ + g areas (for example, Al, Cr, and Re maps). It is evident that the combination of the high-brightness
Figure 8: Input count rate for Mn standard at 20 kV as a function of incident probe current. Indicated and calculated values are input count rate displayed during acquisition and that calculated from a ratemeter function after acquisition, respectively.
2011 May •
www.microscopy-today.com the
FEG of the JEOL 6500F SEM, together with the high collection efficiency and high count rate capability of the SDD permits elemental mapping to be performed at speeds where instrumental problems such as specimen image driſt do not degrade the spatial resolution, even at lower accelerating voltages where the reduced excited volume gives improved
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