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 1950's, 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 scanning electron microscope has
many advantages over traditional microscopes. 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.
Diagram courtesy of Iowa State University
The SEM is an instrument that
produces a largely magnified image by using electrons instead of light to form
an image. A beam of electrons is produced at the top of the microscope by an
electron gun. The electron beam follows a vertical path through the microscope,
which is held within a vacuum. The beam travels through electromagnetic fields
and lenses, which focus the beam down toward the sample. Once the beam hits the
sample, electrons and X-rays are ejected from the sample.
Detectors collect these X-rays,
backscattered electrons, and secondary electrons and convert them into a signal
that is sent to a screen similar to a television screen. This produces the
final image.
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.
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 using State Form 16866, 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 SEM's 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
- Safety evaluations will be performed initially when machine is purchased and after machine has been moved.
- Each machine should be key controlled when not in use. Interlocks, if present, must remain operational unless approved by the RSO.
- Shielding must be sufficient to maintain exposure rates less than 0.5 mrem/hr at 5 cm.
- The Radiation Safety Office will keep inventory and survey information on file in their offices. The SEM user should keep logbook of any maintenance done on machine. RSO must be notified if any modifications are made to the interlocks or any other safety devices. The SEM user should also keep a copy of operating and emergency procedures at the accelerator panel.
- No survey meters or personnel dosimetry are required.
- Encyclopedia.Com
- Iowa State SEM Homepage
- Lawrence Livermore Radiation Safety Regulation, App. B, Summary of Radiation Generating Devices, Radiation Safety Requirements
- Virginia Tech Radiation Safety Pages
Many thanks also to the many
responses and suggestions from the members of the RADSAFE list server.
