Radiation Physics and Engineering

Abstract Combining nanoparticles (NPs) with proton therapy holds promise for improving treatment gain. Prior simulation studies often lack clinical beam realism and comprehensive radiobiological endpoints, leading to conflicting results. This study employs a Geant4-DNA Monte Carlo simulation to compare the physical and radiobiological enhancement of Au, Pt, and Ir NPs in a human cell under a clinically relevant Spread-Out Bragg Peak proton beam. A 62.8 MeV SOBP was simulated, then, phase-space files at the beam's entrance, plateau, and distal edge were obtained. They used to irradiate a fibroblast cell containing 30 mg/g NPs in the cytoplasm. Three NP sizes of 10, 50, and 100 nm were investigated at each phase-space. We calculated the dose enhancement factor (DEF) and total DNA damage enhancement. Furthermore, cell survival curves were predicted using the Two-Lesion Kinetic model. The results indicated that Ir NPs yielded the highest physical dose enhancement (up to 4.21% for 10 nm size), followed by Pt NPs (up to 4.10%). Smaller NPs tend to present a higher DEF than larger NPs. DNA damage yields increased with linear energy transfer (LET), with the distal SOBP distal edge showing the greatest enhancement. Cell survival curves indicated a detectable reduction in survival fraction for Ir > Pt > Au NPs at the distal edge, correlating with increased complex DNA damage. Under clinically realistic simulation conditions, high atomic number and high density NPs like Ir provide a modest but consistent physical and radiobiological enhancement in proton therapy, most pronounced at the high-LET Bragg peak.

Abstract Proton therapy is an effective cancer treatment due to its precise dose distribution and the presence of the Bragg peak. The incorporation of high-Z nanoparticles has emerged as a promising strategy to further enhance local dose deposition in tumor cells. This study aims to evaluate the dose enhancement effect of metal and metal oxide nanoparticles in cellular environments under proton irradiation. Monte Carlo simulations were performed using the GEANT4 toolkit with the GEANT4-DNA extension to model proton interactions at the microscopic scale. The influence of nanoparticle material (gold, iron oxide, and hafnium oxide), concentration (10-90 mg.ml^-1), and size (5-25 nm) on the dose enhancement ratio in the nucleus and cytoplasm of a single cell was investigated. Results show that the dose enhancement ratio (DER) increased linearly with nanoparticle concentration, while increasing nanoparticle size caused a nonlinear decrease in the DER. Among the studied nanoparticles, gold nanoparticles showed the highest dose enhancement due to their higher atomic number and density. Nanoparticle type, size, and concentration are critical factors for maximizing dose enhancement in proton therapy, with gold nanoparticles offering the greatest potential to increase therapeutic efficacy.

Abstract In this study, a thermal neutron beam suitable for neutron radiography (NR) was designed based on the thermal column of the Tehran Research Reactor (TRR). The existing air-filled channel inside the graphite thermal column was utilized to implement a dedicated beamline consisting of a gamma filter slab, a boron carbide thermal neutron absorber with a central aperture, and a conical collimator. A comprehensive parametric optimization was performed using the MCNPX Monte Carlo code. A total of 144 configurations were evaluated by varying the gamma filter material, aperture thickness, aperture radius and the distance between the aperture and image position. Bismuth demonstrated superior performance compared with lead due to its lower neutron absorption and effective gamma attenuation. The optimized configuration, employing 5 cm of Bi filter and a 5 cm B₄C aperture with a 2 cm radius, achieved a thermal neutron flux of 1.0×10⁶ n·cm^-2·s^-1 at L/D = 114 under full-core simulation conditions at reactor full power. The neutron-to-gamma ratio and fast neutron suppression were significantly improved, while the gamma dose rate was substantially reduced compared with the existing E-beam tube NR facility at TRR. A secondary surface-source methodology was implemented to accelerate the parametric study and was subsequently validated against full-core simulations. Although the simplified model overestimated absolute flux values, it accurately reproduced relative performance trends, confirming its suitability for design optimization. The results demonstrate that the TRR thermal column can provide an efficient and high-quality neutron beam for advanced NR applications with enhanced beam purity and radiation safety.

Abstract This work presents a systematic optimization of an electrostatic quadrupole (ESQ) doublet designed for the ES200, a 200 keV Cockcroft-Walton ion accelerator through high-fidelity COMSOL Multiphysics simulations. The study aims to enhance beam focusing performance by rigorously analyzing critical field characteristics, including field linearity, electric potential distribution, and fringe field effects. Aperture diameters ranging from 30 mm to 70 mm were evaluated while maintaining a fixed electrode radius of 22 mm, consistent with mechanical and electrical constraints. The optimized configuration, featuring a 50 mm aperture, demonstrated superior field linearity with a minimal relative deviation of 0.8% at a 10 mm radial distance, ensuring uniform focusing forces across the beam profile. Furthermore, the implementation of integrated shielding discs and a structural support frame resulted in a 37.5% reduction in fringe field leakage, thereby improving field confinement and overall beam stability. These findings provide a validated design framework for fabricating a high-performance ESQ doublet, contributing to enhanced beam quality and operational reliability in compact, low-energy ion accelerator systems.

Abstract This study presents the implementation and validation of the CIEMAT/NIST efficiency tracing method for the absolute standardization of Carbon-14 using an ultra-low-level Quantulus 1220 liquid scintillation counter, toluene-based cocktails, and the EFFY-9 code. Due to the absence of a dedicated profile for classical toluene cocktails in the software library, Ultima Gold was used as a substitute. To achieve this, a universal curve was constructed using a series of tritiated standards as a tracer, correlating the instrumental quench index with the model’s free parameter. Subsequently, the activity of two certified Carbon-14 standards was computed across a range of Birks parameter (kB) values from 0.004 to 0.014 cm.MeV^-1. The results exhibited excellent agreement with certified values, with relative deviations consistently remaining below 2.1%. Detailed analysis indicated that the minimum bias corresponds to kB=0.004 cm.MeV^-1. This finding confirms that, in this specific configuration, kB serves as an effective parameter, compensating for the residual mismatch between the actual properties of toluene and the surrogate computational profile. This research emphasizes the necessity of experimentally determining the Birks parameter for each specific laboratory setup to ensure maximum accuracy.

Abstract In this study, the effect of external neutron beam tubes of MNSR research reactor on increasing the neutron and gamma dose rates in different zones of the reactor building was investigated and the dose distribution was determined through calculation and measurements. This study was conducted to investigate the radiation protection conditions at different operation conditions of these beam tubes and to ensure the radiation safety of the reactor operators and researchers. The results showed that when both beam tubes are operated simultaneously, the average total dose rate in the reactor hall, pneumatic room and corridors increases to 4.58 μSv.h^-1, 1.7 μSv.h^-1 and 4.9 μSv.h^-1, respectively at the maximum power of reactor, i.e. 30 kW. The major part of the dose rate of neutrons and gamma rays distributed in the environment is caused by the neutron radiography channel, and more than 80% of the dose rate is related to gamma rays. On the other hand, the performance of the neutron radiography beam tube radiation protection system showed that even when the reactor is at its maximum power and these beam tubes are inactive, the total dose at the edges of the reactor pool is about 2% of the annual dose limit. This indicates that the radiation protection of the beam tube has a good performance in preventing the increase in the dose rate when the beam tube is deactivated.

Abstract Dimensionality reduction can play an important role in radioisotope identification from gamma-ray spectra by compressing spectral information, reducing model complexity, and improving learning efficiency. The importance of this approach becomes more evident under real-world conditions, where spectra are typically characterized by high dimensionality, low counts, peak overlap, and calibration instabilities, all of which make direct analysis more difficult. On this basis, in the present study, two dimensionality-reduction approaches -principal component analysis as a linear method and an autoencoder as a nonlinear method -were compared to evaluate their effectiveness for radioisotope identification. Using a simulated dataset of 1024-channel NaI(Tl) spectra of six common radioisotopes (Co-60, Cs-137, I-131, Ba-133, Am-241, and Tc-99m) for training, we evaluated both approaches with a common multilayer perceptron (MLP) classifier on unseen data, including laboratory-measured spectra and scenarios with gain drift. Under ideal conditions, both approaches achieved nearly perfect identification performance (F1 ≈ 0.98-0.99). In more challenging regimes, including experimental spectra and severe gain drift, the autoencoder’s latent features consistently outperform PCA. The autoencoder+MLP model generalized better to real spectra (e.g., achieving F1 ≈ 0.99 versus 0.91 for PCA) and maintained higher accuracy under a ±20% gain drift (F1 ≈ 0.81 versus 0.71). These results suggest that the nonlinear latent representation learned by the autoencoder is less sensitive to gain drift and experimental variability than variance-based linear projections, resulting in improved robustness for multi-label radioisotope identification. This insight can support the design of more reliable field-deployable gamma spectroscopy systems, where maintaining high identification performance amid noise and calibration variability is essential.

Abstract A thyroid shield composed of 70 wt% micro-sized Bi₂O₃ dispersed in a silicone rubber matrix was evaluated for radiation protection efficacy in periapical dental radiography. Absorbed dose to the thyroid gland was quantified using both Monte Carlo N-Particle (MCNP) simulations with a modified MIRD phantom and experimental measurements employing TLD-GR200 dosimeters positioned in the superior and inferior thyroid regions of a Rando anthropomorphic phantom. Irradiations were performed at 60 kV, 7 mA, and 0.32 s exposure time with a fixed 60° vertical angulation. Experimental results revealed thyroid absorbed doses of 72.7±25.7 µGy (unshielded), 40.9±15.6 µGy (commercial collar equivalent to 0.5 mm Pb), and 20.8±12.5 µGy (composite shield), corresponding to a 71.4% dose reduction with the composite shield, (compared to 43.7% for the commercial collar). Monte Carlo simulations at 60° angulation demonstrated dose reductions of 85.1% (commercial), and 83.3% (composite). Additionally, under SSDL inverse broad-beam conditions based on the IEC 61331-1 (RQR5, 70 kV), the composite shield achieved a 99.97% air kerma rate reduction, markedly superior to the 99.08% (commercial) and 98.13% (0.5 mm pure Pb). Although the composite shield exhibited superior shielding performance, its weight remains higher than commercial alternatives, indicating the need for further optimization.