Scanning electron microscopy (SEM) is a technique for imaging the surfaces of samples which when coupled with energy dispersive analysis (EDA) is a very powerful tool for imaging and elemental analysis.

SEM Technique

An SEM consists of a filament from which electrons are emitted, an applied accelerating voltage of typically up to 30kV to target the sample. A series of electron lens focuses and rasters the beam across the sample surface from which an image can be viewed. Magnifications from 20x to 50000x are possible using this technique.

Two sorts of electrons are produced from the interaction of the electron beam with the sample surface, secondary electrons and backscatter electrons. Secondary electrons relate to topographic features. The backscatter image derived shows the contrasts in chemical composition.

EDA Technique

A further interaction that takes place between the primary electron beam and the sample is that electrons are knocked out of orbits within the atom. Vacancies are filled by electrons from higher energy outer electron shells with a consequent loss of energy. The energy photon is emitted as an X-ray. The energy (or wavelength) of the X-ray is characteristic of both the element and the energy level transition and provides a direct means of elemental identification at the point of electron beam interaction.

EDA uses a solid-state cooled detector system to detect and measure individual X-ray pulses from the point of interaction. A single spectrum will detect elements from carbon upwards in the periodic table.

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