Figure 1 - Electron–matter interactions.

Electron microscopes are highly versatile instruments capable of providing various types of information tailored to the user’s specific needs. This case study will explore the different types of electrons produced in a Scanning Electron Microscope (SEM), how they are detected, and the unique insights they offer for geological Samples.

As the name implies, electron microscopes utilize an electron beam for imaging. Figure 1 illustrates the diverse range of signals generated from the interaction between electrons and the sample. Each type of signal contains valuable information about the sample, and the choice of which signal to capture is at the discretion of the microscope operator.

Most recent Scanning Electron Microscopes (SEM) are typically are fitted to detect three types of signals: backscattered electrons (BSE), secondary electrons (SE) and X-rays.

Figure 1 - Electron–matter interactions.

Secondary Electrons Imaging

Secondary electrons originate from the surface or near-surface regions of the sample. They result from inelastic interactions between the primary electron beam and the sample, and they have lower energy than backscattered electrons. Secondary electrons are particularly useful for inspecting the topography of the sample’s surface.

The Everhart-Thornley detector is the most commonly used device for detecting secondary electrons. It consists of a scintillator inside a positively charged Faraday cage, which attracts the secondary electrons. The scintillator accelerates the electrons and converts them into light before they reach a photomultiplier for amplification. This detector is positioned at the side of the electron chamber at an angle to enhance the efficiency of secondary electron detection. The below Fig. 2 shows secondary electron image of the geological sample shows the surface morphology of relatively flat polished geological sample.

Backscattered Electron Imaging

Backscattered electrons (BSE) originate from a broad region within the interaction volume of the sample. They result from elastic collisions between electrons and atoms, which alter the electrons' trajectories. This can be visualized using the "billiard-ball" model, where small particles (electrons) collide with larger particles (atoms). Larger atoms scatter electrons more effectively than lighter atoms, producing a higher signal. The number of backscattered electrons reaching the detector is proportional to the atomic number (Z). This correlation helps differentiate between different phases in the sample, providing information on its composition. BSE images can also reveal crystallography, topography, and magnetic fields.

For example, in an SEM image below in Fig. 3 shows difference in atomic number between the bulk and the corner crystals. higher Z (bulk)scatter more electrons back towards the detector than low Z (bulk)atoms, appearing brighter in the image.

The most common BSE detectors are solid-state detectors with p-n junctions. These detectors generate electron-hole pairs when backscattered electrons are absorbed. The amount of these pairs, and the resulting electrical current, depends on the energy of the backscattered electrons. The detectors are placed above the sample, concentric with the electron beam, in a "doughnut" arrangement to maximize electron collection. They consist of symmetrically divided parts. When all parts are enabled, the image contrast depicts the atomic number Z of the elements. By enabling specific quadrants of the detector, topographical information can also be obtained.

EDS Mapping and Elemental Characterization

X-rays generated during the high energy electron-matter interaction can be analyzed using a Energy-dispersive X-ray spectroscopy (EDS, EDX, or EDXA) which provides the elemental composition of a sample. EDS operates on the principle that high-energy electron can eject core electrons from an atom which forces the high energy electron drop down to fill the low energy vacancy, the energy difference radiates out as X-ray with a unique corresponding to the element. The unique X-ray energy signature allows EDS to identify the elements present and their proportions. Figure 4 , 5 and 6 Showing the element’s spatial distributed and its quantification. This is very useful for identification of the phases and confirming growth theory of the crystals.