Performance of a Silicon Drift Detector
DT at 0.5-ms TC versus ~61-percent DT at 3.2- to 12.8-ms TC). Te indicated input count rate saturates at high probe currents (~80 nA) and saturation depends on the average detected X-ray energy (for example, ~500 kc/s for Mn at 20 kV versus >750 kc/s for Al at 10 kV). Additionally, the indicated input count rate is not proportional to the probe current at high input count rates. It appears that the throughput and indicated input count rate problems are related to problems in the detector and/or processing electronics. Tough the manufacturer is aware of these problems and is currently working on their next generation of signal processing electronics, a prototype was not available for testing at the time of writing. Tough the manufacturer has incor-
Figure 9: BSE image and elemental maps (100 × 128 pixels) for Al, Cr, and Re from CMSX4 superalloy specimen at 15 and 5 kV (top and bottom row, respectively). See text for experimental details.
spatial resolution (the estimated lateral spatial resolution for the elemental maps is ~100 nm at 5 kV).
Discussion Te silicon driſt detector (SDD) tested here provides
both high geometric collection efficiency and high count rate capability for X-ray microanalysis and elemental mapping (spectrum imaging). Testing of this SDD on the JEOL 6500F SEM indicated input count rates ≥500 kc/s for a pure manganese standard. Pulse pile-up effects increased with increasing input count rate and decreasing time constant (which allowed higher count rates). However, it should be noted that the use of a pure Mn standard (or likewise another pure element specimen) is a stringent test for pulse pile-up effects as ~60 percent of the X-rays detected are in the Mn K and Mn L peaks (46 percent K, L 12 percent L). Te intensity of a sum peak is proportional to the count rate for the two peaks being summed. For a given input count rate and specimens with multiple elements present, the total counts are divided among more peaks. Tis reduces the count rate for each of the peaks, thereby reducing the intensity of the sum peaks. Of course, there will be additional sum peaks from the multiple elements. Both the energy resolution and peak energy increased
slightly with input count rate. However, if the analyst uses optimized counting conditions (that is, input counts rates giving less than or equal to the maximum throughput), the increase in FWHM or peak position will be ≤10 eV, respectively. With the current generation of detector/processing electronics, there is a measurable degradation in the energy resolution as the time constant is decreased. It appears that the larger SDD chip is more prone to such effects, but no detailed testing of other SDDs with smaller active areas was undertaken as part of this work. Te throughput depends on time constant; a maximum
throughput of ~215 kc/s is achieved at 0.5-ms TC. Te DT at maximum throughput decreases at short TCs (~49-percent
46
porated a routine for correcting pulse pile-up effects into their latest version of soſtware, that routine does not appear to be correctly optimized for this large SDD (best case removal was ~60 percent of
the counts in the 2× sum peak). Te remaining sum peak was not symmetric. Te current routine simply deletes the counts arising from pulse pile-up effects. Ideally the pile-up counts should be returned to their correct (original) energy.
Conclusion From the results presented above, it is clear that there
are numerous advantages of silicon driſt detectors (SDDs) for SEM-based microanalysis and elemental mapping. Te larger geometric collection effiency, high count rate, and high throughput capabilities and operation without liquid nitrogen offer benefits to the analyst. Peak shiſt with increasing input is minimal for usable input count rates (that is, <70-percent DT). As the detector using an older, large diameter SDD chip, the energy resolution does not match more current SDDs that can resolve <130 eV under optimum conditions.
Acknowledgments Research was supported by the Oak Ridge National
Laboratory’s SHaRE User Facility, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
References [1] DE Newbury, Scanning 27 (2005) 227–39. [2] EA Kenik, Microsc Microanal 14 (Supp. 2) (2008) 1172CD.
[3] EA Kenik, Microsc Microanal 15 (Supp. 2), (2009) 560CD. [4] Private communication, R Anderhalt. [5] TS Elam, R Anderhalt, A Sandborg, J Nicolosi, and D Redfern, Microsc Microanal 14 (Supp. 2) (2008) 1260CD.
[6] RP Gardner and L Wielopolski, Nuclear Instruments and Methods 140 (1977) 289–96.
[7] PR Statham, X-Ray Spectrom 6(2) (1977) 94–103. [8] Q Bristow and RG Harrison, Nucl Geophys 5(No.1/2) (1991) 141–86.
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