Solid Type Sensor Optimization according to the Mass Mixing Ratio of Semiconductors for Radiotherapy Quality Assurance

Our study aims to improve detection stability and reproducibility, which has been highlighted as a shortcoming of existing radiation therapy detecting sensors. To this end, we developed a solid sensor by mixing photo conductors and oxide semiconductors in a specific weighting percent (wt%), and evaluate the sensor characteristics to test its applicability as a QA sensor for radiation therapy. For the photoconductor, which forms the core of the sensor, we select polycrystalline PbI2, which shows stable signal detection for high-energy radiation.[1] For the oxide semiconductors to be mixed with the photoconductor, we select SiO2, TiO2, and PbO. First of all, comparing the reproducibility of polycrystalline PbI2 unit cell sensors with no oxide semiconductor against unit &#8211; cell sensors with 5 to 20 wt% oxide semiconductor, the wt% with the best reproducibility are selected as the final sensors. These sensors are then assessed in terms of response coincidence, linearity, dose rate dependence, and percent depth dose (PDD), and compared with the polycrystalline PbI2 sensor. Based on these results, the unit-cell sensors selected for the final assessment are PbI2 + SiO2 15 wt%, PbI2 + TiO2 5 wt%, and PbI2 + PbO 15 wt%. All these sensors show excellent reproducibility, which are relative standard deviation of less than 1.5%. The consistency of detection response time relative to the linear accelerator beam emission time is assessed for the three selected sensors, all of which show PAE < ± 1%. All three selected sensors show excellent dose linearity, with coefficients of determination close to 1. Detection characteristics were examined for the various dose rate from 100 to 600 MU/min, and after normalizing the signal relative to 100 MU, coefficient variation of the oxide semiconductor PbO 15 wt% sensors were less than 1%, indicating dose rate independence. Therefore, with varying dose rate, the sensors show constant detection characteristics with out any change in the measured signal. Finally, as a result of evaluating the PDD, 15 wt% of PbO among the three selected mixtures showed the most similar graph distribution to commercialized thimble ionization chamber.