Conceptual model for explaining how x-rays form under an electron beam (4.9 Mb original image)

The diagram above is conceptual, of course.  Electrons don't orbit atomic nuclei like planets, nor are beam electrons exactly like cue balls in billiards.  The reality of what happens is incompressible, which is true of most of the world, but this simplified model is useful and predicts observations well.

① When beam electrons strike an inner-shell electron in an atom in the sample, 

② the inner-shell electron can be ejected, creating an empty "vacancy" in that inner shell 

③ the beam electron continues after being deflected becoming an "inelasically-scattered" electron that goes off to do more damage elsewhere

④ An electron from a higher-energy outer shell drops down into the lower-energy vacancy in the inner shell.  

⑤ Because energy is conserved, the change in energy levels (ΔE) releases an x-ray photon of exactly that amount of energy.

Like the rainbow of colors of visible light, x-rays also come in a spectrum of "colors" that are invisible to us, but differ just as red from blue for our eyes.  The EDS (Energy Dispersive Spectrometer) detects two things:

1. which energy wavelengths ("colors") of x-ray are glowing from the sample, and

2. how much of each wavelength of x-ray glows from the sample.

The "which energy wavelengths" data tells us which chemical elements are present in the sample because each element on the periodic table glows a characteristic color of x-ray when beamed with energy (either in the form of an electron beam for EDS or an x-ray beam for XRF).

The "how much" data tells us how much of each chemical element is present at each point in the sample because if the sample has lots of an element, then proportionately lots of x-rays of that element's characteristic color will glow.

The EDS measures the chemical composition at each pixel in the image.  This is only a 2k image, so 2048 × 1557 pixels = 3,188,736 individual chemical analyses.  The SEM/EDS can do those 3 million analyses in less than 10 minutes.  The EDS reports those data as maps of individual elements (see below) or the microscopist can overlay selected maps to highlight different crystals in the sample.

(4.9 Mb original image)