Meaning
Signals generated by backscattered electrons vary in intensity based on the average weight of the nucleus in the targeted sample volume to create distinct variations in visual brightness. Higher atomic number regions appear brighter in an image because they deflect more primary electrons back toward the detector. Inside the chamber of a scanning electron microscope, atomic number contrast provides immediate information about local composition without waiting for chemical analysis.
This mechanism works through elastic scattering of the incident beam by the electrical field of the atoms. Heavier elements like tungsten return a high signal while lighter elements such as lithium appear as dark patches.
Imaging Mechanism
Efficiency of electron backscattering increases monotonically with the target number in the periodic table. When observing a multi phase alloy, aisi d2 steels show clear differences between the iron matrix and the chromium carbides through atomic number contrast alone. Detection occurs through a dedicated solid state diode or a scintillator mounted near the pole piece.
Surface topography often masks these results if the detector is not configured correctly. High energy beams penetrate deeper which averages the signals from several layers of atoms below the surface.
Composition Mapping
Distinguishing between oxides and pure metals becomes possible when the change in average weight exceeds the sensitivity of the sensor. Variations in atomic number contrast allow for the identification of contaminants in battery powder mixtures. Analysis becomes faster when researchers use this visual cue to guide their spots for spectroscopic measurements.
Image processing software uses these gray levels to measure the distribution of specific components in a cross section. No additional staining or preparation is needed if the surface is flat.
Analytical Limits
Spatial resolution in these images is typically lower than in images formed by secondary electrons. Since the interaction volume for backscattered signals is quite large, edge details often appear blurred or indistinct. Samples must be flat to ensure that differences in intensity reflect only chemistry rather than physical height changes.
Thin coatings can hide the true signal from the material underneath. Calibration involves using known standards of pure silver or copper to set the brightness levels correctly.
