Nonmonotonic dose dependence of OSL intensity due to competition during irradiation and readout

Analysis of alpha-alumina by variable dose and Tm -TSTOP methods and simulations of the whole process

The T_m -T_STOP and the variable dose methods were applied to alumina experimentally, and then all the steps of the methods were simulated numerically. The T_m -T_STOP experiments were performed between 70 and 300°C, and results showed that the glow curve of the used alumina had six distinct peaks between 40 and 400°C. The glow curves were analyzed using computerized glow curve fitting programs, the analytical equations were also fitted to the experimental data, and the trapping parameters of the peaks were measured. The samples were irradiated above the saturation level to ensure the complete filling of the electron traps and, therefore, the concentrations of traps were measured experimentally. Short-term anomalous fading behaviours of the peaks were investigated, and a new equation was proposed to compute the anomalous fading rate. In addition, the variable dose and T_m -T_STOP methods were simulated using the six trap-one recombination centre (6T1C) model and the experimental trapping parameters. Results showed that there was good harmony between numerical and experimental glow curves. After all the simulations were performed with acceptable accuracy, an estimate was made for the trapping coefficients (A_i = 1-6 ) that could not be measured directly from thermoluminescence experiments. Results showed that the trapping coefficients were affected by the applied dose.

CONDUCTION BAND-VALENCE BAND THEORY OF TL AND OSL: EMPHASIS ON DELOCALIZED TRANSITIONS AND EXPLANATION ON SOME UNUSUAL EFFECTS

We discuss some unusual thermoluminescence (TL) and optically stimulated luminescence effects. We focus on luminescence due to transitions of electrons or holes through the conduction or valence band, respectively. We deal with non-linear dose dependence and non-monotonic dose dependence and also dose-rate effects sometimes reported. Also, is discussed the sensitisation of a sample due to the combined effect of irradiation and annealing, occurring in quartz samples and other materials. Another effect presented is the occurrence of anomalously high activation energies and frequency factors and its possible theoretical interpretation. Also, are considered the effects of anomalous fading and anomalous stability. Yet another phenomenon is concentration quenching. Here, the intensity of emitted TL depends non-monotonically on the concentration of the impurity responsible for the emission. The explanations given to these phenomena are based on the numerical solutions of the relevant sets of differential equations as well as approximate analytical treatment.

The effect of different bleaching wavelengths on the sensitivity of Al(2)O(3):C optically stimulated luminescence detectors (OSLDs) exposed to 6 MV photon beams

PURPOSE

To determine the effect of different bleaching wavelengths on the response of Al(2)O(3):C optically stimulated luminescence detectors (OSLDs) exposed to accumulated doses of 6 MV photon beams.

METHODS

In this study the authors used nanoDot OSLDs readout with a MicroStar reader. The authors first characterized the dose-response, fading, and OSL signal loss of OSLDs exposed to doses from 0.5 to 10 Gy. To determine the effect of different bleaching wavelengths on the OSLDs' response, the authors optically treated the OSLDs with 26 W fluorescent lamps in two modes: (i) directly under the lamps for 10, 120, and 600 min and (ii) with a long-pass filter for 55, 600, and 2000 min. Changes in the OSLDs' sensitivity were determined for an irradiation-readout-bleaching-readout cycle after irradiations with 1 and 10 Gy dose fractions.

RESULTS

The OSLDs presented supralinearity for doses of 2 Gy and above. The signal loss rates for sequential readouts were (0.287 ± 0.007)% per readout in the reader's strong-stimulation mode, and (0.019 ± 0.002)% and (0.035 ± 0.007)% per readout for doses of 0.2 and 10 Gy, respectively, in the reader's weak-stimulation mode. Fading half-life values ranged from (0.98 ± 0.14) min to (1.77 ± 0.24) min and fading showed dose dependence for the first 10-min interval. For 10 and 55 min bleaching using modes (i) and (ii), the OSL signal increased 14% for an accumulated dose of 7 Gy (1 Gy fractions). For OSLDs exposed to 10 Gy fractions, the OSL signal increased 30% and 25% for bleaching modes (i) and (ii) and accumulated dose of 70 Gy, respectively. For 120 and 600 min bleaching using modes (i) and (ii), the OSL signal increased 2.7% and 1.5% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 14% for bleaching mode (i) (120 min bleaching) and decreased 1.3% for bleaching mode (ii) (600 min bleaching) for an accumulated dose of 70 Gy. For 600 and 2000 min bleaching using modes (i) and (ii), the signal increased 2.3% and 1.8% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 10% for mode (i) (600 min bleaching) and decreased 2.5% for mode (ii) (2000 min bleaching) for an accumulated dose of 70 Gy.

CONCLUSIONS

The dose-response of nanoDot OSLDs read using the MicroStar reader presented supralinearity for doses of 2 Gy and above. The signal loss as a function of sequential readouts depended on dose. Fading also depended on dose for the first 10-min interval. For dose fractions of 1 and 10 Gy, OSLDs may be reused within 3% and 5% accuracies up to the maximum accumulated dose of 7 and 70 Gy investigated in this study, respectively. These accuracies were obtained after the OSLDs were bleached with a light source with wavelengths above about 495 nm. The authors also concluded that changes in sensitivity of OSLDs depended on bleaching time, accumulated dose, and wavelength spectrum of the bleaching source.