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 lepidoli te and muscovite mica before and after etching in hydrofluoric acid. B efore etching, scans with atomic resolution show that alternating oxyg en 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 t he 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 k eV cesium ions produced at an accelerator. The etch pit structure evol ves with etching time by rapid increase in depth of a central core fol lowed by spreading of steps laterally from the core. The rate of growt h of etch pit depth is faster for alpha-recoils. than for cesium ions, which suggests a method of rough identification of low-energy heavy i ons. As measured by steepness of etch pit wall slope, lepidolite -is. more sensitive to low-energy particles than is muscovite.