Etching behaviour of alpha-recoil tracks in natural dark mica studied via artificial ion tracks M. Lang a, * , U.A. Glasmacher b , R. Neumann a , G.A. Wagner b a Gesellschaft f€ ur Schwerionenforschung (GSI), Planckstr. 1, D-64291 Darmstadt, Germany b Forschungsstelle Arch€ aometrie der Heidelberger Akademie der Wissenschaften am Max-Planck-Institut f€ ur Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany Abstract Alpha-recoil tracks (ARTs) created by the a-decay of U, Th, and their daughter nuclei, are used by a new dating method to determine the formation age of dark mica bearing Quaternary and Neogene volcanic rocks and the cooling age of plutonic and metamorphic rocks [Chem. Geol. 166 (2000) 127, Science 155 (1967) 1103]. The age equation combines the volumetric density of ARTs with the U and Th contents. Etching latent ARTs (diameter 30–100 nm) in the mica mineral phlogopite by HF and measuring the areal density of triangular etch pits by optical and scanning force microscopy (SFM) leads to a linear growth of ART areal density versus etching time. The ART volume density is a function of the slope of the areal density and the etching rate (v eff ). Therefore, the determination of v eff is essential for the calculation of an age value. To determine the etching parameters such as etching efficiency and v eff , phlogopite samples were irradiated with 80 keV Au ions. Irradiated surfaces were etched with 4% HF at 23  2 °C during successive time intervals and after each interval studied with SFM. The etching rate v eff was determined by different techniques. To evaluate the threshold of etchability, the energy losses of the Au ions and a-recoil nuclei in phlogopite were calculated with the SRIM00 code. The etching efficiency of the Au ion tracks was then used to predict the corresponding etching efficiency of the natural radioactive nuclei. Ó 2003 Elsevier B.V. All rights reserved. PACS: 07.79.Ch; 81.40.Wx; 81.05.Jc; 81.65.Cf Keywords: Artificial ion tracks; Alpha-recoil track dating; Scanning force microscopy; Natural radiation damage 1. Introduction Natural phlogopite, a dark mica mineral, con- tains uranium and thorium as trace elements. Their isotopes 238 U, 235 U and 232 Th as well as their daughter nuclei undergo radioactive decay by emitting a series of alpha particles. Due to mo- mentum and energy conservation (several MeV per a-particle), the aÕs carry away most of the re- leased energy. By each a-emission, the daughter nucleus transfers a recoil energy of 10 2 keV to the crystal lattice of the mineral by nuclear colli- sions forming a damage zone with 10 3 to 10 4 atomic displacements and with a diameter of about 30 nm [3]. This so-called alpha-recoil track * Corresponding author. Tel.: +49-6159-71-2173; fax: +49- 6159-71-2179. E-mail address: m.lang@gsi.de (M. Lang). 0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)00514-7 Nuclear Instruments and Methods in Physics Research B 209 (2003) 357–361 www.elsevier.com/locate/nimb