Electron Microscopes
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Electron Microscopes

Photos Courtesy of LEO Scanning Electron Microscopes

An electron microscope is a microscope which uses electron to gain data and view objects at extremely high magnifications.  A Transmission Electron Microscope, the type which we were able to use, is somewhat similar to a light microscope, but instead of shining light through a specimen, electrons are propelled through the specimen, allow much better resolution, and a much higher magnification. 

 An electron microscope uses electrons to view a very small slice of an object, usually a cell.  These electrons are generated by the electron source, located at the top of the microscope.  These electrons are then accelerated towards an anode.  The anode is positively charged, causing the negatively charged electrons to accelerate towards it.  The more highly charged the anode is, the faster the electrons travel, and so a thicker specimen can be viewed.  (The electron microscope at UCSD is a 400,000-volt electron microscope, meaning the anode is charged with four hundred thousand volts) 

As the electrons are accelerated towards the anode, the stream of electrons are confined and focused by using a series of electromagnets and two condenser lenses.  The first condenser lens focuses the general size range of the electron stream, and the second lens changes the size of the actual spot where the electrons hit the specimen.  The electrons are also confined by using a condenser aperture, which makes sure that all the electrons are focused on the same point, so as to produce a clearer image.

The electrons reach and pass through the sample.  After they have passed through the sample, they are focussed by an objective lens, and then pass through a column of lenses, each enlarging the image further.  Once the beam of electrons has passed through the column of enlarging lenses, it hits a phosphor image screen, where light is generated.  The more electrons that hit a single point, the more light is generated, causing that section of the image to be lighter, and the denser sections of the specimen, which permit less electrons through, results in less emitted light, and therefore a darker section of the image.  A camera is placed beneath this screen, and takes the images and displays them on a screen for the user to view. 

Another factor helps determine the brightness of each part of the image.  This factor is the atomic number of the atoms making up this section of the specimen.  The higher the atomic number, the more electrons are scattered.

One downside to a transmission electron microscope is that electrons have very little penetrating power, so a slice must be very thin to allow the electrons to pass through.  The thickness of the sample depends on how strong the microscope is, and so it varies with each one. 

A scanning electron microscope is slightly different from a transmission.  Instead of registering how many electrons pass through the image, the scanning microscope uses a secondary electron collector to collect the electrons which are reflected by the substance, so called because these electrons are called secondary electrons.  This allows the image produced to be very accurate in three dimensions, since it can be calculated where each electron came from and thus the surface at that point; however, it means that only the surface can be viewed.  This type of electron microscope has other disadvantages as well, including the fact that the specimen must be treated with a reflective substance for it to work, and this will kill any living specimen, such as a blood cell, and is possible to damage the specimen if not careful. 

Also, both these microscopes are large, expensive, and not easy to learn or use.  However, they are very powerful and thus they are necessary in many research projects.


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