An idea to reduce angular dependence of dosimeter having a disk-shaped detection region

Abstract When dose measurement is carried out using an optically stimulated luminescence (OSL) dosimeter which uses a disk-shaped dosimeter in the diagnostic X-ray examination, angular dependence of the dosimeter should be taken into consideration. Angular dependence is the characteristic in which the detector response varies depending on incident angle of X-ray to the dosimeter and this becomes one of the causes of systematic uncertainty. In order to measure exposure dose accurately, it is important to reduce the systematic uncertainty caused by angular dependence. In this study, we propose two types of novel disk-shaped dosimeters for trials to reduce angular dependence: one is annular disk-shaped dosimeter, and the other is a composite-disk-shaped dosimeter. The commercially available dosimeter has a ϕ5 mm disk-shaped detection region which is composed of Al2O3;C. The annular disk-shaped dosimeter, which is proposed in this study, is a ϕ5 mm disk having a ϕ3 mm hole in it. Moreover, we propose a novel composite-disk-shaped dosimeter, which has two sensitive regions; a ϕ3 mm disk detection region is additionally inserted into the annular disk-shaped dosimeter. We performed Monte-Carlo simulation and experimentation using 40–140 kV X-rays to evaluate angular dependences and detection efficiencies of these dosimeters. In the simulation, incident angles of X-rays to the dosimeter varied every 15°. Experiments were performed under the same conditions. We also focused on the difference of angular dependence between 0° and 90°, and the ratio of responses at 90° divided by 0° was evaluated for analyzing the reduction rate of angular dependence. The results of ratios of the disk-shaped dosimeter, annular disk-shaped dosimeter and composite disk-shaped dosimeter were 82%, 89% and 88%, respectively. These results mean that the novel shaped dosimeters have better angular dependence compared with that of the original disk-shaped dosimeter. However, the detection efficiencies for disk-shaped dosimeter, annular disk-shaped dosimeter and composite disk-shaped dosimeter were 100%, 64% and 100%, respectively. From these facts, we concluded that the composite disk-shaped dosimeter can accomplish better angular dependence without loss of sensitivity. We verified that the characteristics of the conventionally-used disk-shaped dosimeter can be improved by adding some effort.