= Absorption edge
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An <absorption edge> is an abrupt change in <photon absorption> as incident <photon> <energy> crosses an electronic excitation threshold. For inner-shell <photoionization>, a new vacancy-production channel opens near the shell's <binding energy>. This threshold concerns incident <energy>; a <Characteristic X-ray> concerns a subsequent difference of atomic <energies>. They are different observations. Barkla's lecture explicitly treats characteristic emission as requiring a critical excitation condition, while discussing characteristic radiation separately from scattering.<Nobel Lectures, Physics 1901–1921/Characteristic Röntgen Radiation by Charles Glover Barkla>[Nobel Lectures, Physics 1901–1921; Barkla, Characteristic Röntgen radiation, printed p. 395, PDF p. 135: shorter-wavelength excitation and critical condition]{ref}
In the stipulated <X-ray fluorescence> model, the K threshold is $10.0\,\mathrm{keV}$, whereas the K lines are $8.5$ and $9.7\,\mathrm{keV}$. An absorber made of a different material could have its own <absorption edge> between those line <energies>, preferentially removing the upper line. Actual near-edge structure and attenuation values require material data; the following deliberately chosen coefficients are not such data.
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