Modeling of an X-ray calibration system using the Monte Carlo simulation with the PHITS code

In this study, we performed modeling of an X-ray calibration system in the energy range from 1 keV to 80 keV with a resolution of 0.5 keV using Monte Carlo simulation PHITS code. The accuracy of the model was evaluated based on the ISO 4037–1:2019 standard by establishing the characteristics of the narrow-spectrum N-80 X-ray beam, which include four parameters: the first half-value layer (HVL₁), the second half-value layer (HVL₂), the homogeneity coefficient (h), and the mean energy (E_mean). The calculated results from the proposed PHITS model show good agreement with the reference values in ISO 4037–1:2019. Next, to enhance the reliability of the simulation model, we conducted experimental measurements of these parameters. The experimental results showed that the relative deviations between the simulation and experiment for HVL₁ and HVL₂ were 1.5 % and 2.7 %, respectively. The homogeneity coefficient h, as determined from PHITS simulation and experiment, was 0.92 ± 0.06 and 0.91 ± 0.07, respectively, which falls within the recommended range of ISO 4037–1:2019 (0.88–1). In addition, field uniformity, the contribution from scattered radiation, and the attenuation of radiation intensity with distance were also compared, showing good agreement between simulation and experiment. Finally, we used the simulation model to calculate ambient dose equivalent rates at various positions and compared them with experimental measurements. The results showed that the maximum deviation between PHITS model calculations and experimental values was 0.6 % at a position 100 cm from the X-ray tube, and did not exceed 7.7 % over the entire range from 60 cm to 450 cm. This study suggests that modeling an X-ray calibration system using the PHITS code may be an effective and low-cost approach for calculating necessary quantities in X-ray radiation dose calibration and for conducting further studies.