Abstract:In a specimen which is transmission thin to 100 kV electrons - a sample in which X-ray absorption is so insignificant that it can be neglected and where fluorescence effects can generally be ignored (1,2) - a ratio of characteristic X-ray intensities, I1/I2 can be converted into a weight fraction ratio, C1/C2, using the equationwhere k12 is, at a given voltage, a constant independent of composition or thickness, k12 values can be determined experimentally from thin standards (3) or calculated (4,6). Both exper… Show more
“…To analyse the smaller η-phase GB precipitates formed during ageing, particles were selected on HAGBs from PFIB-prepared and electropolished samples that had no visual signs of chemical attack and analysed following the same procedure described above as close to their centre as possible. At least 12 A-GBPs were again measured for each material and the data plotted against Al content, to deconvolute the background matrix signal following the method of Cliff et al [56]. This allowed their average Al concentration to be determined by extrapolation of the fitted data to the assumed ideal Mg stoichiometry of 33 at.%.…”
Section: Stem-eds Analyzesmentioning
confidence: 99%
“…Due to the relatively small size of the A-GBPs, the Cliff-Lorimer extrapolation procedure [56] was employed to deconvolute their Al content from the surrounding matrix. Typical STEM-EDS chemical maps from the A-GBPs are shown in Figure 9, which again indicate they have relatively uniform compositions.…”
“…To analyse the smaller η-phase GB precipitates formed during ageing, particles were selected on HAGBs from PFIB-prepared and electropolished samples that had no visual signs of chemical attack and analysed following the same procedure described above as close to their centre as possible. At least 12 A-GBPs were again measured for each material and the data plotted against Al content, to deconvolute the background matrix signal following the method of Cliff et al [56]. This allowed their average Al concentration to be determined by extrapolation of the fitted data to the assumed ideal Mg stoichiometry of 33 at.%.…”
Section: Stem-eds Analyzesmentioning
confidence: 99%
“…Due to the relatively small size of the A-GBPs, the Cliff-Lorimer extrapolation procedure [56] was employed to deconvolute their Al content from the surrounding matrix. Typical STEM-EDS chemical maps from the A-GBPs are shown in Figure 9, which again indicate they have relatively uniform compositions.…”
“…In the absence of absorption, fluorescence, and electron channeling effects (Goldstein, 1979; Zaluzec, 1979; D’Alfonso et al, 2010; Allen et al, 2015), the characteristic α th-shell X-ray emission from a thin solid film of atomic number Z and thickness t , is essentially isotropic, and can be written approximately aswhere I α is the measured X-ray intensity (counts) per unit area, σ α ( E o , Z ) the α th-shell ionization cross-section (Inokuti, 1971, 1978; Zaluzec, 1984) excited by electrons of incident energy ( E o ), ω α the α th-shell fluorescence yield, Γ α the α th-shell radiative partition function, W Z and C Z the atomic weight composition (in at%) of the element Z , N o Avogadro’s number, ρ the local specimen density, ξ o = η o × τ the incident electron flux, ε α the detector efficiency, and Ω α the detector collection solid angle. As there is no angular dependence in this equation, the emission can be described as spherically uniform over 4 π steradians, a plane section through the specimen and incident beam results in an angularly symmetric and circular intensity profile as illustrated in Figure 1.Figure 1Illustration of the angular distribution of characteristic and bremsstrahlung radiation relative to the incident electron beam from a thin specimen.…”
Section: Theoretical Calculationsmentioning
confidence: 99%
“…Since the initial inception of interfacing an X-ray energydispersive spectrometer (XEDS) to the column of a scanning/transmission or analytical electron microscope (AEM), numerous predictions have been made concerning the optimum configuration of the detector/specimen relative to the incident electron beam (Cliff et al, 1978;Zaluzec, 1978). There are four criteria, which can be considered to be major parameters, that contribute to this optimum:…”
Modern analytical electron microscopes equipped with silicon drift detectors now allow for a wide range of geometrical configurations capable of performing quantitative X-ray spectrometry. Recent work has improved the collection solid angles of these detectors, however, the impact of increasing the solid angle on detection sensitivity as measured by the peak/background ratio has not been addressed. This work compares theoretical and experimental peak/background ratios for incident electron energies in the range of 20-200 keV, with X-ray detectors in both conventional orientations (on the electron entrance surface) as well as new geometries (the electron exit surface). The implications of these parameters on detectability limits for the next generations of "Lab-in-the-Gap" analytical microscope are also considered. It was found that theoretical calculations of the angular distribution of bremsstrahlung and their effects on the peak/background ratio match well with experimental measurements, and indicate that new geometries which can result in large solid angles provided an added benefit in addition to increased characteristic signal, namely increased sensitivity for the analyst.
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