Ion Tracks in Solids: From Science to Technology to Diverse Applications
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Visualization of Tracks
Fast ions create linear trails of intense atomic disorder in many solids. The partide tracks are in themselves scientifically interesting because they consist of unique, localized radiation damage. They also are noteworthy for their diverse practical uses, which range from improved high field superconductors to mineral exploration and bird altimetry. The two areas—what tracks are and what they do practically—are the subjects of this introduction and the following three articles. Although the mechanism for producing tracks in insulators is semiquantitatively well-established, there is a distinct mystery as to the formation mechanism in superconductors, intermetallics, and metals. This mystery is the subject of the next two articles written by discoverers of tracks in these materials. We will not discuss in detail the multitude of scientific uses for these tracks as particle-track detectors. Uses range from nuclear, elementary-particle, and cosmic-ray physics to geochronology, geochemistry, and geophysics; chemistry; and radiobiology. The interested reader can learn more on the subject through a book, part of which surveys scientific applications of particle tracks in solids.1 The key to these uses—and most of the practical uses—is that, in materials where tracks can be observed, either directly or by a widely applicable trick to be described, each detector sample is a nuclear particle-track chamber—the solid-state equivalent of the well-known gaseous and liquid detectors (i.e., cloud chambers and bubble chambers). The major distinction is that tracks in solids are longlasting rather than transient features.
How are tracks in solids observed? Mostly they are seen by one of two generally useful methods, examples 23 of which are pictured in Figure 1. Direct imaging in the transmission electron microscope (TEM) (Figure la) is possible via the diffraction contrast caused by the lattice planes being severely bent near the disordered material of the tracks. Although in principle this method should be almost universally applicable, high track densities are needed in order for tracks to be located and reviewed conveniently. In some cases the electron beam of the microscope heals the damage4 so that it is only briefly observable. The other method of seeing tracks is far more convenient and has been widely used for insulating solids. Preferential chemical attack5 depends on the fact that the damaged material is in a higher energy state than the surrounding, undamaged material so that a moderately corrosive reagent will dissolve the track material more rapidly than the matrix (Figure lb). Etching replaces the radiation damage by holes that remain as permanent features in the solid. Recipes for etching tracks in dozens of materials are known,1 including in amorphous solids such as glasses and plastics, whose disorder makes TEM ineffective. By continued etching (as in Figure lb), etched tracks can be enlarged for simple viewing in optical microscopes and even by the naked eye.
MRS BULLETIN/DECEMBER 1995
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