high-voltage electron microscope

device

Also known as: HVEM

Written by Savile Bradbury, Brian J. Ford•All

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high-voltage electron microscope, type of electron microscope that has been constructed to operate at accelerating voltages in excess of the 200–300 kV normally used in the conventional transmission electron microscope. High-voltage microscopes now in commercial production are designed for accelerating voltages of up to 1,500 kV.

These instruments have several advantages over the conventional transmission electron microscope: (1) the theoretical resolving power of the instrument increases as the accelerating voltage increases; (2) thick specimens are easily penetrated by the beam; (3) chromatic aberration caused by energy losses and scattering in the specimen is decreased; and (4) specimen heating and irradiation damage of organic samples are decreased. Many metals and semiconductors suffer increased damage at these high energies, however, because the incident electrons may knock an atom from its correct position in the crystal lattice.

Although the gain in resolution at 1,000 kV is theoretically about two and a half times that of a 100-kV instrument, such increases have not been fully achieved in practice. It is primarily for the increased penetrating power and consequent reduction in chromatic aberration that the high-voltage instruments are used, as in the study of metals and alloys, for example. Some biological systems, such as bacteria and whole cells contained within a quartz chamber, may also be examined at high voltage. However, specimen damage due to the energetic electron beam is considerable.

Savile Bradbury David C. Joy Brian J. Ford

electron microscopy

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Key People:: Richard Henderson; Jacques Dubochet; Joachim Frank

Related Topics:: microscope; electron microscope; cryo-electron microscopy

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electron microscopy, Technique that allows examination of samples too small to be seen with a light microscope. Electron beams have much smaller wavelengths than visible light and hence higher resolving power. To make them more observable, samples may be coated with metal atoms. Because electrons cannot travel very far in air, the electron beam and the sample must be kept in a vacuum. Two different instruments are used. In the scanning electron microscope, a moving beam of electrons is scanned across a sample; electrons scattered by the object are focused by magnetic “lenses” to produce an image of the object’s surface similar to an image on a television screen. The images appear three-dimensional; they may be of small organisms or their parts, of molecules such as DNA, or even of large individual atoms (e.g., uranium, thorium). In the transmission electron microscope, the electron beam passes through a very thin, carefully prepared sample and is focused onto a screen or photographic plate to visualize the interior structure of such specimens as cells and tissues.

This article was most recently revised and updated by Emily Rodriguez.