Electron Source and Beam
The electron beam is generated from a thermionic electron emission source consisting of a cathode of a single crystal of lanthanum hexaboride (LaB6), a Wehnelt cylinder, and an anode. The first lens in the TEM is created by establishing a negative bias between the cathode and the Wehnelt cylinder. The bias of the cathode causes electrons to be pulled off of the tip, then accelerated through the anode and down the column To create the high voltages necessary to operate the TEM, an array of 12 electrodes is located beneath the anode. These electrodes accelerate the beam to voltages as high as 400 kV. Because of the very high accelerating voltages used in the TEM, even the most stable samples will suffer electron beam damage.
The TEM utilizes four electromagnetic lenses: condenser lenses, objective lens, intermediate lens and projector lens. The condenser lens is responsible for controlling a) the electron beam size and b) the spread or amount of illumination on the specimen. The objective lens is used to focus the image. The intermediate lens is used for diffraction, and the projector lens is used to magnify and project the beam onto the fluorescent screen for image viewing.
As the beam electrons interact with the specimen, they are scattered elastically (changing their trajectory but carrying almost the same kinetic energy) and inelastically (losing energy but remaining at almost the same trajectory). During inelastic scattering, loosely bound outer shell electrons are ejected from target atoms by incident beam electrons. These ejected electrons are known as secondary electrons (SE) and are useful in imaging surface topography.
The image is then projected onto either a fluorescent screen or one of two cameras. The film camera uses a special type of film that is highly sensitive to low intensity photons. The film also has a fine grain for increased resolution. The TEM is also equipped with a 2048 pixel by 2048 pixel, water cooled Gatan digital camera for collecting digital images and video.
