Simulation of thermoluminescence signals at very low dose rates and low doses: Implications for dosimetric applications

Abstract

During luminescence dosimetry applications, samples are irradiated in the laboratory with irradiation dose rates of the order of 0.1 Gy/s. By contrast, samples in nature are irradiated with a dose rate many orders magnitude smaller, typically 1 mGy/year. In this paper, the effect of very low dose rates and also low doses on thermoluminescence (TL) signals is investigated using the basic one-trap one-recombination center model (OTOR). The simulations showed that at very low dose rates the assumptions of quasi-equilibrium conditions (QE) are violated. This violation of QE conditions results in a significant distortion of the shapes of TL glow curves, with the result that the peak shape methods of analysis fail to produce the correct activation energy E of the traps. At the same time, the estimated half-life of the electrons traps is found to increase significantly at very low doses. The dose response of the sample at very low dose rates is simulated for both the irradiation stage and the subsequent heating stage. The dose response of the integrated TL signal is found to coincide with the dose response during the irradiation stage. However, the TL dose response measured in terms of the peak height shows a significant under-response at very low doses, making the whole dose response curve superlinear before the onset of saturation. The peak height under-response is due to the clear violation of QE conditions at very low doses and dose rates. The reported distortion of the TL glow curves and the existence of superlinearity of the TL signals at low dose rates, have important implications for the analysis of TL signals. Researchers need to be aware of the possibility that both of these effects may be present, since they can influence the analysis of experimental data and can lead to the wrong conclusions in dosimetric applications.