| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 105 === This Ph.D. work started from the study of the annealing and readout process of the TLD measurement technique. The ineffectiveness of the oven annealing for the LiF:Mg,Cu,P thermoluminescent dosimeter with a thick annealing tray was demonstrated to be due to the fact that the temperature achieved on the annealing tray was significantly lower than the oven preset temperature. The 240 oC / 10 min annealing using a special-designed hot plate, for which the heating temperature on the thin annealing tray can be accurately controlled, has been presented to be capable to reduce the residual signal of the TLD-700H, TLD-7000, and TLD-2007 detectors to less than 0.02% and achieve a reproducibility for 10-cycle measurements of ~0.36% when an optimal readout cycle has been applied. Based on the achievement of the annealing technique study we established an optimum annealing and readout process.
The theme of this work is the QA measurement of gamma-ray dose and neutron activation using TLD-400 for BNCT beam. TLD-400 (CaF2:Mn) chips were applied for the gamma-ray dose measurement in a PMMA phantom exposed to a BNCT beam because of their very low neutron sensitivity. Since TLD-400 chips possess an adequate amount of Mn activator they have been employed in this work simultaneously for neuron activation measurement. The self-irradiation TL signals owing to the decay of the neutron induced 56Mn activity have been applied for a calibration of the TLD-400 chip in situ, where the activities were measured by an HPGe detector system and the energy deposition per disintegration of 56Mn was calculated by applying a Monte Carlo code.
An effort has been made to improve the measurement uncertainty furthermore and facilitate the measurement process. Each TLD-400 chip was treated as an individual detector with a given ID number and denoted orientation and an in situ calibration of the TLD chip by using small laboratory sources in direct contact with the TLD chips was proposed. The attenuation effect of the TL light has been determined by using a simplified attenuation model in couple with the energy deposition distribution acquired from Monte Carlo code simulations. Since the energy deposition distribution is highly non-uniform, it has been demonstrated that a correction of the TL light attenuation plays a critical role for the proposed in situ calibration to be feasible and effective.
In the reading out of the thermoluminescence lights of the TLD-400 chips we accidentally found that TLD-400 chips irradiated in the BNCT beam were capable of emitting prominent scintillation lights owing to the induced 56Mn activity just like conventional inorganic scintillators do. Therefore, we explored the scintillation property of TLD-400 with an intention to develop the extra scintillation application of TLD-400 chips. The relative SC light sensitivity among some different types of TLD has been roughly estimated. The SC light counting rates readout by the TLD reader have been smartly applied to measure the activity of the induced 56Mn in the TLD-400 chip exposed to mixed neutron and gamma-ray fields. The attenuation of the SC lights in the TLD-400 chip has been studied based on a simplified attenuation model. The SC lights emitted by TLD-400 chips have also been applied for the activity measurements of small isotopic sources in direct contact with the TLD chips.
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