ARTIFICIALLY-PRODUCED HEAVY ION TRACKS IN MICA STUDIED BY ATOMIC FORCE MICROSCOPY

Trace concentrations of uranium and thorium are often found in apatite, zircon, and mica. The alpha-decay and spontaneous fission of ²³⁸U, ²³⁵U, and ²³²Th displaces atoms in the structure, leaving damage zones. The two types of radiation result in different damage zone dimensions: i.) fission tracks (FT, cylindrical zones of damage with diameter: ~10 nm and length: ~15 µm) and alpha-recoil tracks (ART, a nearly spherical zone, 5 - 10 nm in diameter). While FT’s are widely used in geochronology to date rocks and to infer geological processes in Earth’s crust, ART-dating is less developed due to nanoscale dimension of the track. ARTs are, however, important for dating younger rocks because they are ~2×10⁶ times more abundant than FTs. Atomic force microscopy (AFM) provides a new approach for ART dating because of its atomic-scale resolution. In order to study general aspects of track formation and assess the utility of AFM for the routine characterization of radiation damage zones, a set of previously-annealed biotite, muscovite, and phlogopite samples have been irradiated with energetic gold ions (2.2 GeV) at high and low fluences (1×10¹⁰ ion/cm² and 5×10⁸ ions/cm², respectively). Radiation-induced hillocks were identified on the surfaces of biotite, muscovite, and phlogopite samples. Characterization of high-fluence samples shows a similar hillock diameter (26.4 to 28.4 nm) for all mica types. To study the etching behavior of ion tracks, phlogopite was etched with 3% HF in 10 second intervals followed by AFM characterization after each interval. After the first interval, hillocks disappear and are replaced by etch pits with the same areal density as hillocks (in agreement with the low irradiation fluence). By measuring diameter and depth of 25 etch pits for each etching step, we obtained a horizontal etch rate of 0.92 nm/s and a vertical (track-) etch velocity of 0.38 nm/s. The morphology of the etch pit transitioned from an initial triangular shape to one that becomes circular with continued etching. Etch-pit morphology and etch velocity are related to the energetics of track formation and the structure of the mica.