A new approach to crystallographic orientation measurement for apatite fission track analysis: Effects of crystal morphology and implications for automation

Apatite fission track analysis and in particular, computer-assisted fission track recognition require the determination of crystallographic orientation, crystal morphology, cracks, fractures and inclusions. The selection of a crystal for fission track analysis has largely been based on its surface and track etching characteristics that, in conventional fission track dating, is performed by the analyst. This requires manual scanning of the entire mount for suitable grains, which is a very time-intensive process during manual fission track counting. Therefore, a computer-assisted grain characterisation system is of interest to allow for an automated pre-selection of suitable crystals. With the use of a fully automated Fabric Analyser, c-axis orientations can be measured inside an apatite crystal for each pixel with a spatial resolution of 5 μm. The results are represented in geometric quality and retardation maps consisting of the c-axis orientations for each pixel and two quality values evaluating each orientation. These maps allow the determination of the crystallography of the apatite grain as well as its outline. In addition, most of the crystal morphologies, fractures and impurities, which can influence automated fission track recognition techniques based on image analysis, are determinable using the quality maps. On the other hand, the method is not sensitive to smaller crystal cracks, high fission track densities or coating. The quantification of the crystal geometric properties by this method is a step forward to develop a fully automated fission track analysis process.