Modellierung des Quarz-Lumineszenzsignals im Kontext von Datierung und Dosimetrie
Zusammenfassung der Projektergebnisse
Quartz is a well-suited natural luminescent dosimeter for geological and archeological dating, not only because of its abundance in the Earth’s continental crust. Numerical simulations, especially the solution of a set of coupled differential equations, can help to understand the complex system of charge carrier transport in the quartz crystal lattice, leading to the emission of luminescence. Existing quartz models succeed in reproducing known thermoluminescence (TL) and optically stimulated luminescence (OSL) phenomena, but fail so far for radiofluorescence (RF), the signal emitted during ionizing irradiation. Radiofluorescence, however, offers some key advantages, e.g., direct and real-time observation of temperature- and dose-driven effects on luminescence production. This project presents fundamental experimental UV-RF investigations and the qualitatively successful simulation of RF and other luminescence signals and phenomena. Published quartz models and parameters were gathered in the open-source R package 'RLumModel'. The software was designed for simple use without the need for deep knowledge of programming. Stepwise annealing of natural quartz samples to different temperatures before UV RF measurement showed maximum signal intensity at ~550 °C. Further investigations on the dose rate dependence of the UV RF signal confirm theoretical findings that the signal intensity is linearly dependent on the dose rate and the slope of the initial UV RF signal is linearly-dependent on the squared dose rate. Numerical simulations are able to reproduce these characteristics after some modifications of charge carrier concentrations in the model parameters. It was remarkable that in all numerical investigations a simple three-energy-level model was able to simulate the main characteristics of the observed effects. Therefore, analytical solutions for the UV-RF signal dynamic were derived, implying that all UV RF signals can be best approximated by the sum of an increasing and a decreasing exponential function. This behavior is probably not restricted to the UV band and can also be transferred to other emission bands. The study of quenching mechanisms in quartz luminescence demonstrates the power of RF for fundamental investigations because RF offers the possibility to measure, e.g., thermal quenching more directly. Experimental data can then provide physical parameters for these quenching process for improvement of the models. Thermal and dose quenching effects were simulated and in good accordance with experimental results. Furthermore, long-known phenomena such as the UV-reversal effect were also analyzed more directly via UV RF, confirming the idea of reversibility of sensitivity changes through annealing and UV illumination. Another application is the determination of absorbed doses by means of UV RF, which was first predicted using numerical simulations. Experimental data indicate that the newly developed measurement protocol can recover doses up to ~300 Gy with an accuracy of ~10% with UV RF. Possible applications of this method range from source calibration to the dating of annealed materials, e.g., ceramics. Generating predictions from simulations (forward modelling) needs valid input parameters. To obtain these parameters, sensitivity analyses of such input parameter were carried out to identify those variables influencing the model output most. Subsequently, these parameters were adjusted by fitting of experimental luminescence data (inverse modelling). As a further step, first ideas and results from Monte-Carlo simulations of quartz RF are presented and compared to established numerical methods. In summary, this project shows that the combination of experiments and simulations enables a comprehensive understanding of luminescence production in quartz. Furthermore, it has been demonstrated that quartz RF has a wide range of applications and provides important insights into charge carrier distributions in quartz crystals.
Projektbezogene Publikationen (Auswahl)
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2016. Solving Ordinary Differential Equations to Understand Luminescence: ‚RLumModel‘, an Advanced Research Tool for Simulating Luminescence in Quartz using R. Quaternary Geochronology 35, 88-100
Friedrich, J. Kreutzer, S., Schmidt, C.
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2017. Quartz radiofluorescence: a modelling approach. Journal of Luminescence 186, 318-325
Friedrich, J., Pagonis, V., Chen, R., Kreutzer, S., Schmidt, C.
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2017. The basic principles of quartz radiofluorescence dynamics in the UV – analytical, numerical and experimental results. Journal of Luminescence 192, 940-948
Friedrich, J., Fasoli, M., Kreutzer, S., Schmidt, C.
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2018. Making the Invisible Visible: Observing the UV-reversal Effect in Quartz using Radiofluorescence. Journal of Physics D: Applied Physics 51, 335105
Friedrich, J. Kreutzer, S., Schmidt, C.
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2018. On the dose rate dependence of radiofluorescence signals of natural quartz. Radiation Measurements 111, 19-26
Friedrich, J., Fasoli, M., Kreutzer, S., Schmidt, C.
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2018. Radiofluorescence as a detection tool for quartz luminescence quenching processes. Radiation Measurements 120, 33-40
Friedrich, J., Kreutzer, S., Schmidt, C.
