APPLICATIONS OF NUCLEAR TRACKS IN GEOSCIENCE

Geoscience is one of the application fields of nuclear tracks. In the Laboratory of Nuclear Micro-Analysis, applications in geochronology, micromapping and radon dosimetry are studied. For these purposes, theoretical models are developed and applied to mineral and polymeric detectors. First, uranium fission tracks (FT) are analysed in order to trace the thermal history of a rock. The apatite/fission track system is particularly well suited to such studies due to its low closing temperature. Then, several different models are developed to understand, and to use, the partial annealing phenomena. With this aim in mind, we propose a model allowing simulation of the projected track length distributions. According to this model, track distributions are transferred onto a time scale graph (the ''age function''). This highlights the type of thermal history of the mineral and corrects the apparent FT age. Using the traditional law for the production of tracks, and experimental results for annealing kinetics, an analytical model based on a reaction situation associated with one or several activation energies has been developed. The aim of this approach is to obtain a single solution for both direct and inverse problems linking thermal history and track length data. The fission track etching process in apatite is also studied in order to model the evolution of track characteristics. A computer program has been developed, taking into account track segmentation during annealing and crystallographic parameters of apatite. Secondly, the track etching process is studied in polymeric detectors. A geometrical approach based on two distinct etching velocities is used. For light ions such as alpha particles with nonrelativistic energies, etching times are short, so the bulk velocity can be considered constant. The track velocity, however, is variable. The critical angle of registration can then be determined as a function of incident energy and etching conditions. Such a model can be applied in alpha microcartography, for example, in order to determine the uranium and thorium content of geological materials. Finally, in the context of this work, a mathematical model to determine the solid state nuclear track detector (SSNTD) detection efficiency for an alpha particle emitter has been developed, This work is specially intended to measure radon using a cylindrical cell. This theoretical work makes it possible to choose the size of the cell for which the SSNTD response does not depend on the radon daughters (Po-218 and Po-214) behaviour, This size guarantees the reproducibility of the results. Such a device can be used to study radon emanation over wide areas in radioprotection and earth science applications (thermal prospecting, volcanology, etc....).