| 摘要: |
| 在光释光（optically stimulated luminescence，OSL）测年流程中，通常采用预热（180—260℃）来去除低温区不稳定的信号。尽管预热过程中浅陷阱（如110℃陷阱）电子被清空，但在之后的光照过程中由较深陷阱进入导带的电子会再次进入浅陷阱并发生积累，这种现象称为光转移（photo-transfer）。本文以110℃热释光（thermoluminescence，TL）信号的实验数据为基础，模拟了光转移现象，研究了不同实验条件（激发温度、光子通量）和不同动力学参数（110℃ TL陷阱的空间容量、入陷概率、光电离截面）对光转移过程的影响，结果显示：激发温度越高、光子通量越大、110℃ TL陷阱的光电离横截面越大，光转移信号越弱；110℃ TL陷阱的空间容量和入陷概率越大，光转移信号越强。模拟结果揭示了影响光转移程度的因素，加深了对光激发过程中OSL信号产生的物理机制的认识。 |
| 关键词: 光释光测年 光转移 数值模拟 能级模型 |
| DOI：10.7515/JEE192041 |
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| Numerical simulation of photo-transferred phenomenon in 110℃ thermoluminescence (TL) trap |
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ZHENG Yue, PENG Jun, WANG Xulong
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. School of Resource Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
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| Abstract: |
| Background, aim, and scope In optically stimulated luminescence (OSL) dating, preheating is widely applied to remove unstable signal without significantly affecting the TL trap at 325℃. Such depleted signals reappear after optical stimulation, which means that retrapping of OSL electrons is not a negligible process. This phenomenon is photo-transferred thermoluminescence (PTTL). Previous studies focused on experimental results and explanations, but there is a lack of understanding from simulation aspect. Here we presented simulation results associated with PTTL and try to understand PTTL in quartz. Materials and methods Coarse quartz is used experimentally to measure a PTTL glow curve. For simulation, our investigation is based on a simple kinetic model with three energy levels (two electron traps and one luminescence center). The simulations are carried out based on R program KMS that is designed to simulate luminescence phenomena in quartz. Results The results show the same qualitative behavior between experimental and simulated PTTL glow curves, but there are differences in magnitude of the signal at peak level. Variations of electron concentration in two traps can be obtained against stimulation time Electron concentration in 110℃ TL trap first rises and then decreases with stimulation time, and electron concentration in 325℃ TL trap always decreases and can be separated into two phases with different decay rates. In addition, several kinetic parameters are important in photo-transferred process. For parameters such as optical stimulation temperature (ST) and incident photon flux (F), which are associated with experiment condition, PTTL intensity decreases with the increasing ST and F. For parameters of electron trap concentration (N1), retrapping probability (A1) and photoionization cross-section (a1), which are associated with properties of 110℃ TL trap, adding N1 and A1 to a larger extent within a proper range is beneficial for the production of PTTL signal, but enhancing values of a1 imposes negative effects. Discussion Our simulation results suggest that a relatively larger retrapping probability of 110℃ TL trap is of vital importance in simulating PTTL phenomenon. Recombination probability of luminescence center is 2×10−7 cm3·s−1, retrapping probability of 110℃ and 325℃ TL trap are 5×10−8 cm3·s−1 and 5×10−12 cm3·s−1, respectively. This ensures that most of stimulated electrons recombine at luminescence center and still some are retrapped into 110℃ TL trap and the proportion of electrons which retrapped into 325℃ TL trap is extremely small. Different kinetics were assumed to study the properties of the glow curve by comparing recombination and retrapping coefficients. For process that dominates by recombination, it usually follows first-order kinetics and may change into non-first-order kinetics due to electron concentration change. For process that dominates by retrapping or has equal recombination and retrapping probability coefficients, it follows second-order kinetics. During optical stimulation, electron concentration in 325℃ TL trap follows first-order kinetics at early stage and then decays exponentially, as electron concentration decays to about 1‰ to its initial value, the retrapping rate is larger, the decay rate is slower and follows non-first-order kinetics since then. Electron concentration in 110℃ TL trap first increases and then decreases, which represents the process of electrons retrapping into and escaping from 110℃ TL trap. PTTL intensity could also be affected by experimental and 110℃ TL trap properties. Higher stimulation temperature and larger incident photon flux is beneficial for electrons in 110℃ TL trap to escape, thus experimental procedure could be improved to avoid PTTL phenomenon. By increasing electron trap concentration and retrapping probability, we get higher intensity of PTTL peak, but increasing photoionization cross-section, we get opposite results. Conclusions Energy band model with two electron traps and one recombination center represents key features of PTTL signal. During optical stimulation, electron concentration in 110℃ TL trap first rises and then decreases, and electron concentration in 325℃ TL trap always decreases with stimulation time. Different parameters (not only associated with experiment but also associated with trap properties) contribute differently for the production of PTTL intensity. By increasing electron trap concentration and retrapping probability, we get higher intensity of PTTL peak, but increasing stimulation temperature, incident photon flux and photoionization cross-section, we get opposite results. Recommendations and perspectives It is believed that simulating kinetic models of quartz luminescence system will play a more important role in the future. Establishing complex kinetic models helps to explain phenomenon that obtains during experiments, searches for exact physical mechanisms and sets a fundamental role for putting forward new dating methods with accuracy. |
| Key words: optical stimulated luminescence dating photo-transfer numerical simulation energy band model |
