Effect of the Silicon Drift Detector
a benefit to the PeBaZAF method. Te test presented here shows that for 269 mea- surements of different multi- element specimens, 95% of the results were within ±13.7% of the known composition. Tis result is better than some pub- lished results for standardless EDS methods in [9,17]. Further development of this method is expected to improve the P/B- based standardless analysis to about ±10% relative deviation with 95% of the results [14]. Te major uncertain-
Figure 4: Effect of count rate. Internally calculated errors Err% are marked with dark blue boxes (considering system- atic+random errors). The top two analyses are for low-count measurements (30 seconds, 2 kcps): (a) eZAF method and (b) PeBaZAF method. Lower two analyses show results obtained with 100 times higher counts (200 kcps): (c) PeBaZAF and (d) eZAF. Both methods have similar total error for low counts. The PeBaZAF (P/B) method improves in precision and total error with higher counts. The eZAF method is hampered by the same high correction factors that limited accuracy at lower counts, and the total error improves only marginally.
the method is good (Figure 4a). Te PeBaZAF method has better accuracy, but repeated values are more widely scattered, causing worse precision (Figure 4b). Under these conditions, the calcu- lated total error (Err%) is similar for both methods. When the same sample is analyzed 100 times longer or
with a count rate 100 times greater (for example, 200 kcps with an SDD versus 2 kcps with an Si(Li) detector), the precision improves for the PeBaZAF method (Figure 4c). Tis causes repeat analyses to be closer to the correct value. For the eZAF method the calculated error does not improve much, if at all, with higher count rates because the systematic errors in the correction factors still dominate (Figure 4d). Figures 5 and 6 show the results of PeBaZAF standardless
EDS analyses on 269 multi-element specimens containing various amounts of elements with atomic numbers from Z= 6 to Z = 83. For this group of test specimens, the relative deviation from the correct values was smaller than±13.7% for 95% of the results. Tus, it is possible to improve the error factor for the PeBa-
ZAF method with more counts in the spectrum, which leads to more counts in the background and better overall precision. Since the use of an SDD spectrometer makes spectrum acquisi- tion at high count rates routine, the PeBaZAF method now has a significant advantage.
Discussion Te original PeBaZAF disadvantage, that it required higher
than usual acquisition times, was reduced with the introduction of SDD detectors, where the count rate can be more than 2 orders of magnitude higher compared to the old Si(Li) detectors. Tis ability to offer more counts in the same collection time is clearly
38
Figure 5: Results for 269 PeBaZAF standardless EDS analyses with an SDD on specimens containing various amounts of elements with atomic numbers from Z=6 to Z=83. The statistical evaluation of these results indicates that the rela- tive deviation from correct values was better than±13.7% for 95% of results [14].
www.microscopy-today.com • 2020 March
ties with the net-count-based eZAF approach are systematic errors in the ZAF correction calculations and errors related to the parameters used (for example, MACs). Te example presented here shows larger corrections, especially for the absorption correction, for the eZAF method compared to the PeBaZAF method. It is possible to improve the uncer- tainties with the eZAF method
by employing a measured standards database (this standardless method uses “remote standards” [18]), which makes this method behave in a manner closer to a true standards-based quantitative analysis.
Conclusion Two standardless methods were compared in light of the
high count rate capabilities of the silicon driſt detector (SDD). Te disadvantage in precision typical of the PeBaZAF method is mitigated when using the high count rates available with the SDD. Te test presented here shows that for 269 measurements
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76
