A scanning electron microscope (SEM) uses an electron beam and detectors to view small areas. The beam is directed at a sample, and the resulting electrons are detected through backscattering, secondary, or X-ray imaging. SEMs use a raster pattern to produce 3D images and are commonly used in forensic science.
An SEM microscope is a type of microscope that uses an electron beam combined with detectors to view very small areas. The device is usually referred to as SEM, as the letters are an acronym for the microscope’s correct name: scanning electron microscope. This type of microscope is extremely powerful and has an average useful resolution between 7 nm and 3 nm, although lower resolutions have been achieved.
SEMs work by interpreting data from detectors when an electron beam is directed at a sample. The electron beam is generated by a filament inside the SEM’s electron gun, then travels down the column towards the sample. While in the column, the electron path is shifted, condensed, blocked and/or altered from various parts in order to improve the image. The column opens into the sample chamber, where the electron beam strikes the sample. The electrons released or reflected from the sample will then strike the detectors located in the sample chamber. The results of the shots are then used to create highly magnified images of the specimen.
The electrons released from a sample in an SEM can be detected in many different ways; the three most common, however, are through backscattering, secondary, and X-ray imaging. Backscattering electrons (BSEs) tend to penetrate deep into the surface of a sample, and images produced through their detection can show more easily the contrast in the materials within the substance. Secondary electrons are used to produce images of the surface of a sample and can produce stunning 3D representations. X-ray detectors can tell which elements make up a specific part of a sample and are often used in forensics. There are other sensing methods as well and they include cathodoluminescence and auger sensing.
The “S” in SEM stands for Scanning, an aspect that differentiates the SEM from other types of electron microscopes. Instead of using a fixed beam of electrons, SEM uses a beam that travels over the desired area in what is known as a raster pattern (rastering). Rastering offers many benefits and is one of the reasons why the images produced by the secondary detector have an almost three-dimensional quality.
SEMs are used in many different research areas, but are probably most famous for the roles they play in forensic science. One method of testing for gun residue involves swabbing the back of a suspect’s thumb, sling, and trigger finger; the swab is then analyzed using backscatter detection, with areas of interest examined with x-ray detection to determine what they are made of. Backscatter detection can also be used to examine the surface composition of an object, and abnormal areas can be tested using X-ray detection to find unwanted materials such as lead.
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