Radiation Physics and Engineering

Abstract In this study, using the FLUKA code (version 2011.2c.6), a 2300C/D linear accelerator head manufactured by Varian (18 MeV) were simulated within a radiation therapy room. Electron source and transporting particles using the FLUKA code from the source in each calculation replaced by an alternative photon which is an important aspect of the work. After verifying this alternative source, a composite shielding layer was considered as the interior covering of the treatment room, and the effect of six polymer-based shielding materials with different compositions on the photon and neutron doses at different locations was calculated. As a general conclusion from this research, using a polymer shielding layer of material including 32.5 wt% elastomer +60 wt% tungsten +7.5 wt% boron carbide on the interior of the roof, floor, and concrete walls of the treatment room is recommended, except for the second wall at the maze entrance. Additionally, we found that a 2 cm thickness of this shield is almost equivalent to 3 mm of pure lead. The optimized thickness depends on the specifications of the LINAC, its energy, the dimensions of the treatment room, and the thickness of the concrete walls which can be calculated based on these specifications.

Abstract For storage of a spent fuel during the life of the facility, testing or evaluation may be required to determine the integrity of the spent fuel. In addition, various tests of the spent fuel are carried out at nuclear facilities for many purposes. Therefore, it is necessary to theoretically and experimentally evaluate the fuel burnups as one of the most important issues for nuclear spent fuel storage. The present study discusses the ¹³⁴Cs/¹³⁷Cs activity ratio method in determination of the spent nuclear fuel burnup, which is very important in view of the spent fuel storage as well as its dose rate calculation. For this aim, ORIGEN and MCNPX codes were used to calculate the radioisotopes' concentrations during the nuclear fuel burnup and different cooling times of Tehran Research Reactor (TRR) fuel assembly. The obtained results showed the burnup estimation might be maximum 2 times different from its real value by using this method if the fuel burnup history has not been regarded and the cooling time is unknown. For the specific cooling times, the discrepancy may be 36% if the fuel burnup history has not been modelled. The discrepancies between the estimated burnup and the real value may be less if the TRR fuel assembly burnups are less than 55%.

Abstract To design a hot cell, it is essential to consider all safety requirements and radiation protection acceptance criteria. In this research, the design of a hot cell with specific geometric dimensions and materials was simulated using MCNP code. Then, the gamma dose rate was calculated for a 60Co source with 1.8×10¹³ Bq activity and a spent fuel plate with 90% burnup and a cooling time of 30 days to determine the appropriate shielding thickness. In these calculations, the source intensity and the gamma spectrum of the spent fuel plate were obtained using ORIGEN code. According to the references, the gamma dose rate criterion of 10 μSv.h^-1 was used to determine the thickness of the hot cell wall, which is made of barite concrete with 3.35 g.cm^-3 density and a combination of concrete and paraffin, in different orientations. The results indicate that the necessary optimal thicknesses of shielding for different locations are 80, 65 and 75 cm respectively regarding the irradiation safety criteria.

Abstract This study investigates the power upgrade of the Tehran Research Reactor (TRR) to enhance neutron flux for various applications. Two strategies are proposed: (1) utilizing spent fuel for economic feasibility and (2) adopting a compact core layout to maximize neutron flux. Neutronic simulations reveal that the compact core with 26 fuel assemblies can safely operate at 8.5 MW, achieving a thermal neutron flux greater than 1.5×10¹⁴ n.cm^-2·s^-1. Thermal-hydraulic analysis shows that, with a power peaking factor (PPF) of 2.7, safety criteria are met at 8.5 MW. Reducing the power to 8.3 MW ensures full compliance with all safety requirements. Under a conservative approach with a PPF of 3.0, a safe operational power of 7.5 MW is achievable. Moreover, lowering the coolant inlet temperature by 3 °C improves reactor performance, allowing safe operation at 8.5 MW, with potential to exceed 9 MW at high mass flow rates while maintaining safety margins.

Abstract This study presents the design and construction of a quality control phantom for evaluating the clinical imaging performance of electronic portal imaging devices (EPIDs) used in radiation therapy. The phantom, incorporating lead sheets of varying thicknesses and a plexiglass scaling component, enables the assessment of key image quality parameters, including Noise, Uniformity, Contrast-to-Noise Ratio (CNR), Signal-to-Noise Ratio (SNR), Contrast, and pixel-to-millimeter scaling. A dedicated software application, developed using LabVIEW, facilitates automated and standardized analysis of EPID images. The phantom was tested across two radiotherapy centers, revealing differences in EPID performance, with Center A demonstrating lower Noise (0.5 vs. 1.6), better Uniformity (1.9 vs. 9.3), and higher CNR (69.3 vs. 45.6) and SNR values compared to Center B. The phantom's design, inspired by established models like the Las Vegas and PTW phantoms, supports both qualitative and quantitative evaluations, meeting TG142 standards for spatial resolution (>2 mm). This work highlights the efficacy of tailored quality assurance tools in enhancing EPID performance, ensuring precise radiation therapy delivery and improved patient outcomes.

Abstract Despite the established negative effects of radon on lung and skin cancers, radon therapy has historically been considered a potential treatment for various skin and respiratory diseases. In this study, the impact of such therapy on the population density of hematopoietic stem cells, including both WT and Lnk types, which are crucial for regenerative medicine, was investigated. MCNPX simulations were used to perform dosimetry for radon in a MIRD phantom. The calculated dose rates for both air and water were integrated into a stem cell differential equation, which was solved using the robust fourth-order Runge-Kutta method to model changes over a four-day period. It was shown that the radon dose reduces the population density of stem cells. This minimal reduction suggests that low-dose radon exposure does not have a destructive effect on stem cells, but rather a tolerable one. By the end of day 4, the stem cell density was observed to have decreased by 12.33% in the air environment and 2.65% in the water environment, suggesting a more pronounced effect in the air scenario.

Abstract In tandem accelerators, charge exchange can be achieved using either foil-based or gas-based methods. This study presents the design of a vacuum system specifically for the gas-based charge exchange method in tandem accelerators, aiming to maintain optimal atomic gas density in the interaction region and minimize gas leakage toward the accelerating tubes. To achieve high charge exchange efficiency, precise design of the Beam Transfer Lines (BTL) is essential. In this study, the impact of length and diameter of the BTL on the pressure profile along the tube was investigated. A geometry was selected that concentrates gas in the interaction region while minimizing leakage into accelerating tubes. Subsequently, gas flow simulations were used to determine the optimal operating window for gas throughput and pumping speed in the charge exchange region, ensuring an atomic gas density of 7.24×10¹⁶ cm^-2 required for nearly 100% charge exchange efficiency. Keeping the gas injection rate fixed, the effective range of pump speeds on charge exchange section and accelerating tubes was then identified to create a pressure ratio of about 10⁻³ mbar between the charge exchange section and accelerating tubes. Results showed that beyond a certain threshold, increasing pumping speed does not significantly reduce the pressure due to conductance limitations.

Abstract Immobilization devices are critical for ensuring precise and reproducible patient positioning during radiotherapy. These devices, along with the treatment couch, can alter dose distribution by increasing skin dose, reducing target coverage, and introducing attenuation and scattering. Guidelines like TG-176 emphasize incorporating the dosimetric impact of devices into treatment planning system (TPS) calculations, but current practices often neglect or only partially consider these effects, usually modeling the treatment couch as low-density material. This study evaluated the impact of immobilization devices on dosimetric parameters for target volumes and organs at risk (OARs) in 20 head and neck IMRT plans. Five scenarios were compared, from no devices to full immobilization and a double-layer couch in TPS calculations. Two-tailed paired t-tests assessed significance, with p-values reported. The most significant effect was for parameter GTV D100, showing a maximum difference of 5.5% and an average of 2% (p<0.0001). OAR doses had smaller differences, ranging from 1% to 3% (p<0.0001). Pearson correlation analysis indicated a relatively strong correlation between the posterior beam contribution and dose parameters of the target region (r = 0.93 to 0.95). To enhance accuracy, include all immobilization devices and the carbon fiber treatment couch in external contour and TPS calculations. If CT simulation equipment differs from treatment setups, apply appropriate CT number overrides. Additionally, use the same immobilization devices as in the treatment room for CT imaging, ensuring the field of view (FOV) is large enough to capture all devices and integrate a complete couch model into the images.