Session Item

To evaluate the dosimetry characteristics of photoconductive materials for high energy beam in radiation therapy, we fabricated detector unit-cell using the material such as HgI₂ (Mercuric Iodide), HgI₂-TiO₂ (Mercuric Iodide–Titanium Oxide), PbO (Lead Oxide), PbI₂ (Lead Iodide) and HgO (Mercuric Oxide) by Particle-In-Binder method.

To evaluate the fabricated unit-cells, we measured the dosimetric characteristics that the radiation detector should have such as a high energy response, dose rate dependence, linearity, and reproducibility using 4 and 10 MV photon beam. The radiation field size and source-to-surface distance was 10x10 cm² and 100 cm, respectively. The unit-cells were placed in a dark room to make it only respond to the radiation because it has light sensitive property. To obtain output signals of the unit-cells, we use a current to voltage converter circuit because an initial output of the fabricated unit cell is the current. Then, the output signal with voltage was processed by NI DAQ (NI SCB-68). Also, the output signals of the LINAC (Linear Accelerator) beam pulse was obtained by NI DAQ (NI USB-6363) [Figure 1].

Figure 1 Schematic diagram of (a) measurement set-up and (b) signal acquisition circuit

As a result, HgI₂ and HgI₂-TiO₂ cell shows a good response with 1.2, 2.5 and 1.4, 2.6 voltages at the 4 and 10 MV, respectively. Especially, the sensitivity of the HgI₂ unit-cell was higher than that of LINAC output at 10 MV. However, the PbO and PbI₂ unit-cell shows a poor response less than 0.1 voltage. Therefore, further measurement was done using the HgI₂ and HgI₂-TiO₂ unit cell. The linearity measurement shows that R² value is 0.9999 (HgI₂-TiO₂) and 0.9998 (HgI₂) at 10 MV. To evaluate the dose rate dependency, the result compared with the LINAC output. The results shows that the HgI₂ unit cell was almost same with the LINAC output as 0.0154 of the RMSE. The reproducibility of the HgI₂ and HgI₂-TiO₂ unit cell was also compared with the LINAC beam pulse. All the RMSE value shows almost 0. And the standard deviation of the HgI₂ and HgI₂-TiO₂ unit cell was 0.45% and 0.6% at the 4 MV beam and 0.95% and 0.48% at the 10 MV beam, respectively [Figure 2].

Figure 2 Results of the unit-cell measurement

In this study, we demonstrated that the photoconductive material has dosimetric characteristics for high energy radiation dosimeter and it’s possible as an alternative of indirect material. Therefore, it is considered to be of great significance as a fundamental study in applying photoconductive materials to high energy radiation dosimeters.