Continum X-Rays Incident electron beam Ejected Electron White radiation (Continuum) Characteristic X-ray
Change in background slope due to increased charge during acquisition. Bkg for a Conductor Actual Bkg keV 8 12 The blue dashed background is for an insulator with an equilibrium charge. The red background shows a varying amount of charge.
Depth of Excitation • Electrons lose energy in steps as they go deeper in the sample. • The electron energy may drop below the critical ionization energy of the element in the sample. • The ratio of the primary beam energy to the excitation of the element is referred to as the overvoltage.
X-Ray Absorption primary beam sample surface Area of absorption x-rays The ratio of absorbed to emitted x-rays increases with the accelerating voltage.
X-Ray Fluorescence What if there was Mn in this sample? The Mn Kab is 6.53. Fe K can not excite MnK but Ni K would be able to fluoresce MnK
Comparison of window material X-ray transmission A Be detector will not show any characteristic X-rays below about 0.75 keV. UTW and SUTW detectors will show peaks at these low energies but at a diminished peak height.
Peak Broadening Peak Distortion /Asymmetry Escape Peaks Absorption Edges Silicon Internal Fluorescence Peak Sum Peaks Stray Radiation A Warming Detector X-Ray Artifacts
Peak Broadening • Peaks broaden as you increase in energy. • Characteristic asymmetry of peak- high end of peak is sharp (the absorption edge) while low end of peak tails due to possible incomplete charge collection.
Escape Peaks 1.74 1.95 Si @ 1.74 SiLi crystal Peaks with energies greater than 1.84 keV will create escape peaks. Higher energy peaks deposit energy deeper in the crystal and have a lesser chance of creating an escape peak because the Si X ray tends not to travel such a distance. Ca @ 3.69
Warming Detector • As the detector warms the noise peak widens and may appear in the spectrum as a low-end noise peak. • All peaks will broaden and may shift in energy
Ni-Cu K & L Series Peaks Note the better separation of adjacent elements at higher energies. In manual ID, you should start with the higher energy peaks for this reason.
K-Series Peak with Sum and Escape Peaks The Kb peak is about 1/8 the size of the Ka. The Kb becomes smaller and closer to the Ka at lower energies.
Tin- L-Series Peaks Typical L-series peaks. A small Ln peak exists between the Ll and La. At lower energies, the peaks move closer together and are eventually not resolved.
Osmium- M Series Peaks M-series peaks are similar to the L-series peaks at lower energies. Better separation of the peaks at higher energies is not achieved because the periodic table does not contain sufficiently high Z elements.
Stainless Steel Deconvolution The peak fit is shown with the HPD function, the misfit of the Cr Kb results from the small presence of Mn Ka.
Resolution-Time Constant Relation At faster time constants, the throughput is increased but the resolution broadens. Fast time constants are commonly used for mapping but not for the collection of spectra with subtle overlaps.