Scanning Electron Microscope

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Preparing Samples
Radiation Safety
SEM Safety Guidelines

What is a SEM?

SEM stands for scanning electron microscope. The SEM is a microscope that uses electrons instead of light to form an image. Since their development in the early 1950s, scanning electron microscopes have developed new areas of study in the medical and physical science communities. The SEM has allowed researchers to examine a much bigger variety of specimens.

The SEM has a large depth of field, which allows more of a specimen to be in focus at one time. The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels. Because the SEM uses electromagnets rather than lenses, the researcher has much more control in the degree of magnification. All of these advantages, as well as the actual strikingly clear images, make the scanning electron microscope one of the most useful instruments in research today.

How is a sample prepared?

Because the SEM utilizes vacuum conditions and uses electrons to form an image, special preparations must be done to the sample. All water must be removed from the samples because the water would vaporize in the vacuum. All metals are conductive and require no preparation before being used. All non-metals need to be made conductive by covering the sample with a thin layer of conductive material. This is done by using a device called a “sputter coater.”

The sputter coater uses an electric field and argon gas. The sample is placed in a small chamber that is at a vacuum. Argon gas and an electric field cause an electron to be removed from the argon, making the atoms positively charged. The argon ions then become attracted to a negatively charged gold foil. The argon ions knock gold atoms from the surface of the gold foil. These gold atoms fall and settle onto the surface of the sample producing a thin gold coating.

What are the radiation safety concerns?

The radiation safety concerns are related to the electrons that are backscattered from the sample, as well as X-rays produced in the process. Most SEMs are extremely well shielded and do not produce exposure rates greater than background. However, scanning electron microscopes are radiation-generating devices and should be at least inventoried. The Indiana State Department of Health requires that the machines be registered with their office thus it’s required to inform Environmental Health and Safety of any SEM purchases of transfer, Radiation Machine Registration Application. It is also important that the integrity of the shielding is maintained, that all existing interlocks are functioning, and that workers are aware of radiation safety considerations.

The main reasons for developing a SEM safety plan are:

To keep accurate inventory of all SEMs on campus (manufacturer/model, serial number, location, contact person and phone number).
To warn workers of the risk of interfering with any safety devices (investigator needs to have permission to override any interlocks or warning devices).
To make sure shielding is not compromised (exposure rate not greater than 0.5 mrem/hr at 5 cm from any surface of machine).
To let workers know who to contact in an emergency or if they have any questions.