Etching of recoil tracks in solids

Low-energy (keV/atomic mass) ions in solids displace atoms, producing damage sites that are often described as tracks. Common natural sources of such tracks are recoiling residual nuclei from natural alpha-particle decay. Different models of such tracks are noted and compared with existing experimental results. First, the drastically lower rates of etching along such tracks in muscovite mica, compared to those for continuous ionization-produced tracks (such as result from fission), imply that recoil tracks are discontinuous, with clumped damage that is interrupted by gaps that either are undamaged, or lightly damaged. Second, based on the size of recoil-tracks and the numbers that bring about total disorder in crystals such as zircon, the tracks are diffuse regions–containing only about 2% of misplaced atoms. A simple model describes the profound effect of the gaps in recoil tracks on etching efficiency, removal of recoil nuclei by leaching, track revelation, and their vastly different etching behavior from that of high-energy tracks.