Preview

PDF, English Download (5MB) | Terms of use

Abstract

The present study focuses on the visualization and counting of ion-induced linear tracks in dolomite and siderite crystals by using etching and spectroscopic techniques to answer the question whether dolomite (CaMg[CO3]2) and siderite (FeCO3) are usable as fission-track dating archives. Since the uranium content in natural samples can be highly variable, the areal density and distribution of fission tracks created by spontaneous fission of 235U in natural samples might be very variable too. Therefore, to conduct etching experiments, artificial linear tracks had to be generated. These artificial ion tracks were created on natural samples by swift heavy ion irradiation with 197Au (1.10-2.18 GeV, fluence 1*106 ions/cm2 ) at the Universal Linear Accelerator (UNILAC), GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. This method produced artificial ion tracks that were used for the etching experiments described in this study. Induced fission tracks were created on natural samples by covering them with a uranium source (CN5 glass and zircon) and performing thermal neutron irradiation (1*1015 -1*1016 neutrons/cm2 ) at the FRM II reactor in Munich. This method causes fission of 235U, generating fission fragments which induce fission tracks upon hitting the mineral samples. These induced fission tracks can be assumed to be very similar in nature to spontaneous fission tracks occurring on natural samples as a result of their 235U content. The primary research objective was to visualize induced heavy ion tracks on dolomite and siderite, respectively, by using chemical etching techniques, and quantifying them with an optical microscope. For this purpose, the quality of mineral samples was very important; when possible, samples with very smooth surfaces were used, but for siderite in particular where the available samples were too coarse, a preparation method had to be established in order to obtain high-quality, polished surfaces. First, the established mineral etching techniques published in literature were tested for their applicability and ability to visualize induced or natural tracks on the mineral samples. In order to establish the optimal etching recipe, these etching methods had to be slightly modified with regard to the etchant concentration, etching time, and etching temperature, which also involved figuring out methods to conduct etching at elevated temperatures (50-70 °C), safely. Ideally the etching methods should be as safe as possible, with non-toxic etching solutions that are easy to handle III and don’t require special safety precautions. For the visualization of ion-induced tracks in dolomite, one suitable etching solution was found: 0.001 mol/l HCl at a temperature of 20 °C, which produces roughly trigonal etch pits after 60-120 minutes of etching. Two etchants proved capable of revealing heavy ion tracks on siderite: 10% HCl at a temperature of 50 °C results in triangular etch pits after 10 minutes of etching, while 10% H2SO4 at a temperature of 50 °C produces hexagonal etch pits after an etching time of 30-60 minutes. These etching methods are all optimized for the areal density of tracks that an ion fluence of about 1*106 ions/cm2 will generate. The observation of different shapes of etch pits on siderite depending on the etchant used matches the descriptions of etching techniques found in literature, including a couple of very old publications, which also showed various etch pit shapes depending on which etchant is used. After the etching methods for visualizing heavy ion tracks had been established, they were tested on neutron-irradiated samples with induced fission tracks. For both dolomite and siderite, the established etching solutions also proved capable of etching induced fission tracks generated by irradiation with fission fragments of a kinetic energy of about 170 MeV. The results showed that the etching conditions that were used for visualization of heavy ion tracks work basically the same for visualizing induced fission tracks. This means that dolomite and siderite are minerals that can be used as an archive for fission-track dating, as long as they have a sufficiently high uranium content. For the purpose of conducting these irradiation and etching experiments, it was important to develop suitable preparation methods for mineral samples so they can be used in heavy ion irradiation or thermal neutron irradiation experiments. This was done by establishing a grinding and polishing procedure that results in very smooth mineral surfaces, minimizing the negative effects that a natural crystal topography would otherwise have on the sample’s ability to display etched tracks. The secondary objective of this work was to test the thermal stability of induced ion tracks in dolomite and siderite, as well as studying their annealing behaviour. In the case of dolomite, it was found that tracks generated by the heavy ion irradiation method will anneal when the sample is heated to about 500 °C for 100 h, while natural dislocations in the samples will persist at this temperature. Therefore, this method can be used to separate natural dislocations from artificial ion tracks. Siderite, on the other hand, decomposes when exposed to a temperature of 400 °C for 5 h, leaving only a IV black powdery residue made up of decomposition products which are expected to either be wüstite (FeO) or graphite (C). Exposing siderite to 300 °C for 4.5 h results in a thin layer of black powder on the surface, and the heavy ion tracks present on the sample could still be etched normally, meaning that in the case of siderite the annealing temperature for ion tracks lies above the decomposition temperature. Furthermore, online Raman measurements were conducted to study the behaviour of siderite samples during irradiation, observing changes in the sample’s structure and possible also in the chemical composition. The results showed that all three siderite bands (180, 282 and 1084 cm-1 ) decrease in intensity as the fluence of ions is increased, culminating in their complete disappearance at a fluence of about 1*1012 ions/cm2 . After the experiment was finished, the siderite sample was found to have transformed into the same black residue seen in the annealing experiments, indicating that siderite will also decompose when exposed to high amounts of irradiation.