Improvements in SDD Effi ciency
Figure 8 : Overlay of normalized spectra collected from a Ti-6Al-4V alloy for 10 seconds at 20 kV and 20,000 cps incoming count rate with a processing times of 3.84 µs (red solid curve) and 0.24 μ s (cyan outline) under identical microscope conditions. The Ti Kβ –V Kα peak overlap was separated with an iterative Bayesian deconvolution.
to roughly 1M cps on parts of the sample. With a short 0.12 μ s amp time, the average dead time was 37%. For this acquisition a pixel dwell time of 1 ms was used, which is roughly equivalent to a slow SEM image scan. Even phase maps [ 8 ] can be generated with these short dwell and processing times ( Figure 7a ), while still maintaining sensitivity to low-energy C X-rays. Obviously fast processing times are needed when collecting maps at high count rates, but most of the time, data acquisition is carried out at lower count rates and beam currents, oſt en because of sample limitations or imaging needs. T is is where improved detector effi ciency can be helpful. For a fi xed-input count rate, a better detector effi ciency would yield a higher number of counts in the spectrum.
Ti-6Al-4V alloy . The precision and accuracy of quantitative analysis and the software’s ability to deal with potential spectral artifacts are concerns with going to higher count rates and faster processing times. To address these concerns, a titanium alloy known as Ti-6Al-4V, a common aerospace alloy known for its superior tensile strength and high-temperature toughness, was used to demonstrate
quantitative analysis accuracy at high speeds. This alloy has a Ti Kβ –V Kα peak overlap ( Figure 8 ), making quantitative analysis of the minor concentration of vanadium a challenge. The overlapped peak separation method in this analysis is based on an iterative Bayesian deconvolution. For more details see [ 9 ]. T e analysis was performed at traditional collection rates: 20 kV, ICPS=20,000 cps, and 30 sec live time. Using an amplifi er processing time of 3.84 µs resulted in a 124 eV energy resolution, and the standard- less quantifi cation of the spectrum resulted in weight percent concen- trations of 89.39% Ni, 6.17% Al,
and 4.44% V ( Table 1 ). We will consider these the baseline or “target” quantitative values for subsequent faster collections. T ree additional measurements were taken with the same ICPS: one with 10 live seconds collection time at the same processing time and two at 0.24 µs processing time with 10 s and 1s collection times, respectively. T e quantitative data remained good even for the diffi cult V overlap. T is shows that the small loss in detector resolution at the short processing times did not aff ect the quantifi cation reliability, and good quantifi cation is possible even with limited statistics at short collection times. However, to reap the benefi ts of the faster processing times, one should increase the count rates signifi cantly. Table 1 shows that even at 100,000 ICPS the quantifi cation numbers show no signifi cant changes. An ICPS of 100,000 cps was chosen because this is a value that can be obtained on most microscopes and samples with a reasonable beam current. Carbon in steel . A second application focuses on the accurate
quantifi cation of a known trace amount of carbon in a tool steel standard. Low-energy X-ray analysis of carbon is challenging because of X-ray absorption within the sample and in the various
Table 1 : Results of standard-less quantitative analysis of a Ti-6Al-4V alloy. The Ti Kβ –V Kα peak overlap was separated with an iterative Bayesian deconvolution. Spectrum acquisition was with an Octane Elite Super detector at 20 kV.
Ti
3.84 μ s, 124 eV, 20 kcps, 30 sec
3.84 μ s, 124 eV, 20 kcps, 10 sec
0.24 μ s, 133 eV, 20 kcps, 10 sec
0.24 μ s, 133 eV, 20 kcps, 1 sec
0.24 μ s, 133 eV, 100 kcps, 2 sec
89.39 89.09 89.11 89.06 89.22 2017 March •
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Weight % Al
6.17 6.19 6.23 6.05 6.16 Total V 4.44 4.72 4.66 4.89 4.62 collection time 36.14 12.05 10.20 1.02 2.12 Total spectrum counts 723 K 241 K 204 K 20 K 213 K 51
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