Manipulating Spectra
Tis dialog box has a zoom button, which immediately reduces the energy axis range to the specified values. Vertical scale changes. Te generic spectrum display panel
menu also has menu items to change the vertical (ordinate) scale mode from the default linear axis to logarithmic or square root axes. Te linear vertical axis is the most common display option, but the logarithmic axis is oſten better for viewing spectra with major, minor, and trace elements. Te square root axis takes into account the Poisson (count statistics limited) nature of EDS spectra to display the spectra such that the relative magnitude of the bin-to-bin or channel-to-channel variation looks similar for both high and low count features. DTSA-II recalls the previous setting of the axis scale mode between program invocations. Relative vertical display. Te generic spectrum display
panel menu also has menu items to specify how the spectra are scaled relative to each other. Te most common display mode is to plot the spectra on the “same scale.” In this case, the labels on the vertical axis represent the number of x-ray event counts. Sometimes it is convenient to scale several spectra relative to some characteristic they hold in common. Tere are various different options. One option is to plot all spectra relative to the tallest peak in the spectrum (ignoring any peaks in the range of channels identified by the relevant detector’s zero strobe peak). Because there is no consistent mapping between the vertical axis and counts, the vertical axis is unlabeled. Another option is to scale the spectra relative to an electron dose of 60 nA·s. In this case, the vertical axis is labeled with the x-ray counts normalized to a relative dose of 60 nA·s. (For this operation, spectra for which these quantities are not available are assumed to have a probe current of 1 nA and a live time of 60 s.) Alternatively, the spectra can be scaled relative to the
number of counts integrated over a range of energies. Tere are two alternatives. Either you can specify one or more ranges of channels over which to integrate, by dragging-and-clicking to select ranges of energies, or you can integrate over all energies. Te “scale to region integral” employs the user-specified range(s). Te “scale to equal integral” scales relative to the integral of all counts in the spectra.
Characteristic Line Labels DTSA-II contains a database of characteristic line energies
and approximate line weights for all elements up to Z = 95. Characteristic line labels are oſten called “KLM labels” because the characteristic x-ray lines are associated with the K, L, and M atomic shells. Te K lines are the most energetic x-rays, and the M lines are the least energetic x-rays for a given element. Assuming the electron beam can excite them, K lines are typically visible for all but the lowest Z (<Be) and the highest Z elements. L lines are visible for elements above about Ca and M lines for elements above Ba. Te K, L, and M series are families of characteristic x-ray
lines, each line uniquely identified with a transition between two bound levels in an atom. Te most intense K line is nominally the K-L3 line (representing a transition from the L3 shell to the K shell), and the line is designated the Kα1. Te K-L3 notation is more modern and is the preferred notation according to the IUPAC [3]. Te archaic Kα1 notation (Siegbahn notation) is preferred by most long-term practitioners. DTSA-II supports both. Te default spectrum display pop-up menu allows you to select between “IUPAC,” “element,” “large font” (element only), “Siegbahn,” or “no labels.” KLM lines panel. Characteristic lines are selected using
the KLM lines panel on the Spectrum tab (see Figures 1 and 6). Tis panel provides controls to select the full range of elements, and, for each element, select either full families of lines (K, L, and/or M), common subsets of lines (Kα, Kβ, Lα, etc.), or individual lines (K-L3, L3-M5, M5-N7, etc.). Element selection. Tere are a number of ways to select
an element: (a) You may enter the full name or the standard abbreviation or the atomic number into the element edit box, (b) You may use the scroll bar located below the element edit box, (c) You may locate the mouse cursor within the KLM panel (except the tree view) and rotate the mouse wheel. When the element changes, the list of available transitions
will update in the KLM panel tree view. In addition to KLM lines, the spectrum display panel can display markers at the energies associated with atomic shell edges (absorption edges) and at the positions of the Si escape peaks. Te Si escape peak
Figure 6: An example of a high-fidelity spectrum exported using “save → as displayed” to “scalable vector graphics” format. The elements in NIST K309 glass have been annotated with IUPAC labels.
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www.microscopy-today.com • 2011 May
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