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[SCG44-06] The kinetics and heat conduction model of thermoluminescence of orthoclase
Keywords:Thermoluminescence, feldspar, Heat conduction
One method of applying the thermoluminescence phenomenon of minerals such as quartz or feldspar to geothermal exploration is to estimate the underground temperature from the thermoluminescence intensity. In this study, we developed a model to relate the kinetics of thermoluminescence intensity to the heat conduction equation.
TL is a phenomenon in which the energy accumulated in a solid crystal due to the excitation and trapping of electrons by incident radiation is emitted as light by thermal stimulation. When the energy accumulation is caused by natural radiation, it is called natural TL (NTL), and when it is caused by artificial radiation, it is called artificial TL (ATL). A plot of TL intensity versus temperature is called a glow curve, and the Randall-Wilkins model exists as a simple model for drawing this glow curve (Randall and Wilkins., 1945). There is also a model that optimized Randall-Wilkins model using parameters obtained from experiments (Kitis et al., 1998). The integrated intensity of the glow curve is used to compare TL intensity.
The kinetics of TL is expressed in terms of the decay of TL by thermal stimulation and the accumulation of TL by radiation. The decay term is described by a reaction rate equation and fitted to the results of isothermal heating experiments. We measured TL from a feldspar sample in a granitic pegmatite collected in Ishikawa-machi, Fukushima Prefecture, and separated several TL peak from glow curves using the Kitis model described above (hereinafter referred to as Peak). The glow curves of the samples whose TL was decayed by isothermal heating were also separated in the same way. The isothermal heating temperatures were 90, 125, and 150℃, and the heating times were 24, 168, 672, 1512, and 3360 hour. The results of the decay of TL intensity were plotted for each peak, and the curves drawn by the reaction rate equation with the frequency factor and reaction order as variables were fitted. The reaction was 2nd or 3rd order. The decay term in the kinetics of TL intensity was described by the reaction rate equation of the 2nd or 3rd order reaction. Next, To investigate the accumulation term, we performed ATL measurements. After artificially irradiating the feldspar samples with γ-rays, the ATL intensity was measured according to the radiation intensity, and the TL intensity per radiation intensity for each Peak was determined. The irradiation doses were 80, 500, 1000, and 2000 Gy. The annual dose received by the samples used was set to 15 mGy/year, and the annual accumulation intensity of each Peak was determined. From the above, the accumulation term could be expressed as a function of time with the annual accumulation intensity as a constant.
The temperature structure of a hypothetical heat source was set up using a heat conduction model, and the time variation of TL intensity was examined in a one-dimensional unsteady heat conduction model. The temperature at which the decay/accumulation of TL is switched is in the range of 20~30℃ at the lowest Peak, and the TL of the feldspar located far from the heat source is accumulated.
References
Randall, J. T. & Wilkins, M. H. F. (1945, November). Phosphorescence and electron traps. I. The study of trap distributions. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences (Vol. 184, No. 999, pp. 365-389). The Royal Society.
Kitis, G., Gomez-Ros, J.M. and Tuyn J. W. N. (1998) Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics. J. Phys. D: Appl. Phys. 31 (1998) 2636–2641.
TL is a phenomenon in which the energy accumulated in a solid crystal due to the excitation and trapping of electrons by incident radiation is emitted as light by thermal stimulation. When the energy accumulation is caused by natural radiation, it is called natural TL (NTL), and when it is caused by artificial radiation, it is called artificial TL (ATL). A plot of TL intensity versus temperature is called a glow curve, and the Randall-Wilkins model exists as a simple model for drawing this glow curve (Randall and Wilkins., 1945). There is also a model that optimized Randall-Wilkins model using parameters obtained from experiments (Kitis et al., 1998). The integrated intensity of the glow curve is used to compare TL intensity.
The kinetics of TL is expressed in terms of the decay of TL by thermal stimulation and the accumulation of TL by radiation. The decay term is described by a reaction rate equation and fitted to the results of isothermal heating experiments. We measured TL from a feldspar sample in a granitic pegmatite collected in Ishikawa-machi, Fukushima Prefecture, and separated several TL peak from glow curves using the Kitis model described above (hereinafter referred to as Peak). The glow curves of the samples whose TL was decayed by isothermal heating were also separated in the same way. The isothermal heating temperatures were 90, 125, and 150℃, and the heating times were 24, 168, 672, 1512, and 3360 hour. The results of the decay of TL intensity were plotted for each peak, and the curves drawn by the reaction rate equation with the frequency factor and reaction order as variables were fitted. The reaction was 2nd or 3rd order. The decay term in the kinetics of TL intensity was described by the reaction rate equation of the 2nd or 3rd order reaction. Next, To investigate the accumulation term, we performed ATL measurements. After artificially irradiating the feldspar samples with γ-rays, the ATL intensity was measured according to the radiation intensity, and the TL intensity per radiation intensity for each Peak was determined. The irradiation doses were 80, 500, 1000, and 2000 Gy. The annual dose received by the samples used was set to 15 mGy/year, and the annual accumulation intensity of each Peak was determined. From the above, the accumulation term could be expressed as a function of time with the annual accumulation intensity as a constant.
The temperature structure of a hypothetical heat source was set up using a heat conduction model, and the time variation of TL intensity was examined in a one-dimensional unsteady heat conduction model. The temperature at which the decay/accumulation of TL is switched is in the range of 20~30℃ at the lowest Peak, and the TL of the feldspar located far from the heat source is accumulated.
References
Randall, J. T. & Wilkins, M. H. F. (1945, November). Phosphorescence and electron traps. I. The study of trap distributions. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences (Vol. 184, No. 999, pp. 365-389). The Royal Society.
Kitis, G., Gomez-Ros, J.M. and Tuyn J. W. N. (1998) Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics. J. Phys. D: Appl. Phys. 31 (1998) 2636–2641.