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ion microscope[′ī‚än ′mī·krə‚skōp]
a device in which a beam of ions from a thermionic or gas-discharge ion source is used to produce images. The principle of operation of the ion microscope is similar to that of the electron microscope. As the ion beam passes through the object and is scattered and absorbed in various sections of it, it is focused by a system of electrostatic or magnetic lenses and produces a magnified image of the object on a screen or photosensitive film.
Only a few experimental models of the ion microscope have been made. Studies on its improvement are being stimulated by the fact that it should have higher resolving power than the electron microscope. The de Broglie wavelength for ions is considerably shorter than that of electrons (at equal accelerating voltage); as a result the effects of diffraction, which limit the resolving power of electron microscopes, are very small in an ion microscope. Other advantages of the ion microscope include decreased influence of a change in mass of the ions at high accelerating voltages and better image contrast. For example, calculations show that, for organic films 50 angstroms thick, the image contrast caused by scattering of protons should exceed by several times the contrast produced by electron scattering.
Among the disadvantages of the ion microscope are the considerable loss in ion energy even during passage through very thin objects, which causes their destruction; large chromatic aberration; damage to the fluorescent screen by the ions; and poor photographic action. As a result of these shortcomings the ion microscope, despite its advantages as compared to the electron microscope, has not yet found practical application. A lens-less ion microscope—a field-ion microscope—has proved more efficient.
REFERENCEThe Proceedings of the 3d International Conference on Electron Microscopy. London, 1956. Pages 220–99.
IU. M. KUSHNIR