Burchard, M.

Dedera, S.

Glasmacher, U. A.

Trautmann, C.

GSI Repository

GSITemplate2007 Optimized etching of swift heavy ion tracks in calcite* Sebastian Dedera1, Ulrich A. Glasmacher1#, Michael Burchard1, Christina Trautmann2,31Institute of Earth Sciences, University of Heidelberg, 2GSI Darmstadt, 3Technische Universität Darmstadt Carbonate minerals are of interest for thermochronology (fission-track dating). By irradiating calcite (CaCO3) with swift heavy ions, we perform systematic track etching studies to develop and optimize the track etching process. For optimal etching several properties need to be ful-filled: (i) Each ion track should lead to a single etch pit, i.e., the etching efficiency should be 100%. (ii) The etch-ing solution should be selective, i.e., the track etch rate should be larger than the bulk etch rate. (iii) The overall etching process should be slow enough to handle the samples in a controllable manner. (iv) The etch pits should be as homogeneous allowing automated analysis by optical microscopy (standard method in geology). Earlier investigations revealed etched tracks in calcite crystals using HNO3 [2, 3] or a 1:1 mixture of ethyl-enediaminetetraacetic acid (EDTA) and 0-5% acetic acid as etchant [1], but the results were far from being optimal.  Natural calcite crystals were irradiated with 11.1 MeV/u 197Au ions at the UNILAC accelerator applying a fluence of 106 ions/cm2. During irradiation, the crystals were cov-ered with a hexagonal mask to provide neighboring irra-diated and non-irradiated surfaces.Knowing that calcite crystals are very sensitive to nitric acid, etching series were performed at two temperatures (15 and 21°C) in highly diluted HNO3 of concentrations of 0.91, 0.091, and 0.0091%. Regarding shape and size of the etch pits, best results are obtained for 4 s etching in 0.091% HNO3 at 15°C. As expected [3], the etch pit ge-ometry depends on the solution: tracks etched in EDTA + 5% acetic acid have an elongated hexagonal shape while 0.091% HNO3 produces pentagon-shaped pits (Fig.1).  Fig.1: Optical images of etched tracks in calcite crystals at different magnifications: (top) 20 s etching in EDTA + 5% acetic acid; (bottom) 4 s etching in 0.091% HNO3.Note the different shape of the etch pits. Figure 2 compares the size of the etch pits as a function of etching time for HNO3 and EDTA. Compared to EDTA, the etch rate of HNO3 is by a factor of about five higher. Besides that, the development of the etch pits regarding length and width is the same, both show linear growth with length > width. Fig. 2: Length of etch pits as a function of etching time for two etching solutions at 21°C: (open symbol) EDTA + 5% acetic acid and (full symbol) 0.091% HNO3Another promising aspect of HNO3 as etching solution is given by the fact that the contrast of the etch pits in calcite under the optical microscope is excellent. This allows automated counting by image analyzing software (Fig. 3), an important issue for fission track dating. Fig.3: Optical image of HNO3-etched tracks recorded in transmission mode. The excellent contrast allows auto-mated counting indicated by the white detection marks (right image) produced by the analyzing software. In conclusion, HNO3 is a very promising option for etching heavy ion tracks in calcite crystals. The solution is easier to produce and it fulfils the above mentioned re-quirements for thermochronological track application. [1] S. Dedera et al., “Etching of heavy ion tracks in cal-cite”, GSI-Report 2011 (2012) PNI-MR-23  [2] R. E. Keith & J.J. Gilman, „Dislocation Etch Pits and Plastic Deformation in Calcite“, Acta Metallurgica, Vol. 8, 1960 [3] A. Boos, “Ätzversuche an Calcit, Rhodochrosit und Siderit”, N. Jahrbuch f. Mineralogie, 1943 20μm20μm 5μm5μm20μm 20μm* Work supported by BMBF Verbundprojekt 05K10VH1 # ulrich.a.glasmacher@geow.uni-heidelberg.dePNI-INHOUSE-EXP-31 GSI SCIENTIFIC REPORT 2012400

Optimized etching of swift heavy ion tracks in calcite

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Optimized etching of swift heavy ion tracks in calcite

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