Atomic force microscope study of etched tracks of low-energy heavy ions in mica

Atomic force microscopy has been used to study the surface of lepidolite and muscovite mica before and after etching in hydrofluoric acid. Before etching, scans with atomic resolution show that alternating oxygen atoms in the hexagonal rings differ in height by 0.015 to 0.040 nm. Whether all six or only the three highest oxygen atoms in a ring are imaged varies from region to region. After etching, we have measured the surface structure of etch pits at tracks of two types of low-energy heavy ions: recoil daughter nuclei emitted in alpha decay of uranium and thorium impurity atoms in the mica over geological time, and 400 keV cesium ions produced at an accelerator. The etch pit structure evolves with etching time by rapid increase in depth of a central core followed by spreading of steps laterally from the core. The rate of growth of etch pit depth is faster for alpha-recoils than for cesium ions, which suggests a method of rough identification of low-energy heavy ions. As measured by steepness of etch pit wall slope, lepidolite is more sensitive to low-energy particles than is muscovite.