Production and use of nuclear tracks: imprinting structure on solids

Latent nuclear tracks are damaged zones, created along the paths of rapidly moving ions in solids. They are stored indefinitely in many insulators and can be used to initiate a phase separation process by one of several procedures that remove, collect, or transform material along the latent track. The resulting two-step technique is based on separate inscription and development stages, similar to the lithographic techniques using visible light, uv, x rays, or electrons. In contrast to these other techniques, however, it is applicable even to mechanically very stable, chemically inert, radiation-resistant dielectric materials. The most common development procedure is track etching, which acts as a chemical "amplifier" that dissolves the damaged zone of the latent track preferentially, creating etch pits or channels that can be extremely fine. Etched track diameters start around 10 nm and increase linearly with the etching time. The most outstanding feature of the technique is that one single particle is sufficient to create a developable damage. This unique combination of recording and revealing properties makes possible a single-particle structuring tool with a very wide array of applications. With the advent of high-energy, heavy-ion accelerators, with ion ranges between micrometers and centimeters, large-scale application of the technique to the imprinting of structure onto solids has become possible. At present, most frequently random track arrays are employed to induce global property changes of the solid volume or surface. In the future, scanning ion microbeam technology will make possible local generation of oriented track structures with high precision. This "ultimate" microtool should provide the versatility of a computer-controlled mechanical lathe. This review describes the processes involved in track generation. It outlines applications that are on the verge of being commercialized at present and considers future possibilities.