Le Tuan Anh

Ionization chambers (ICs) are the primary reference instruments for measuring absorbed dose to water ([[EQUATION]]) in clinical photon beams from medical linear accelerators (Linacs). However, because IC electrodes are fabricated from non-water-equivalent materials, thus investigation of perturbation correction factors for the central and outer electrodes ([[EQUATION]]) of ICs is a crucial task before applying ICs in use. This work applied PHITS code to calculate [[EQUATION]] values for 04 Freiburg PTW ICs [i.e., PTW-31021 (0.070 cm3), PTW-31010 (0.125 cm3), PTW-31013 (0.300 cm3), and PTW-30010 (0.600 cm3)]. The beam spectrum was reconstructed based on the IAEA-distributed phase-space data corresponding to a 6 MV photon beam from a Varian TrueBeam Linac. Reference conditions followed established clinical protocols: 10 cm depth in a water phantom, a 10 cm × 10 cm field size at 100 cm source-to-phantom surface distance. The [[EQUATION]] values were determined as the ratio of absorbed dose to water scored in a fully water-equivalent (dummy) geometry to that scored in a semi-realistic geometry retaining the actual electrode materials - aluminium for the central electrode and graphite-coated PMMA for the outer electrode. As results, the [[EQUATION]] values exceeded unity for all 04 chambers, ranging from 1.0003 to 1.0138, with combined statistical standard uncertainties below 1.0%. The largest correction of 1.4% was observed for the smallest-volume chamber (PTW-31021), demonstrating an inverse dependence between the magnitude of the electrode perturbation correction and the sensitive volume of the chamber. The [[EQUATION]] values are small but non-negligible, particularly for sub-0.1 cm3 ICs. The computed [[EQUATION]] values provide benchmark data applicable to both clinical and metrological dosimetry, confirming the importance of accounting for electrode material perturbations in high-accuracy absorbed dose to water measurements using ICs in clinical photon beams.

The accurate characterization of simulated workplace neutron fields is a prerequisite for the meaningful calibration of radiation protection instruments under realistic spectral conditions. This work presents Monte Carlo simulation data (including: neutron fluence rate spectrum, neutron ambient-dose-equivalent-averaged energy, as well as total neutron fluence and ambient dose equivalent rate) of the 241Am-Be based simulated workplace neutron calibration facility at the Institute for Nuclear Science and Technology (INST, Hanoi, Vietnam) using the Particle and Heavy Ion Transport code System (PHITS, version 3.36). A 241Am-Be neutron source of X14 capsule type was modelled at the centre of a concrete-walled calibration room (7.0 m × 7.0 m × 7.0 m), surrounded by spherical moderating assemblies of high-density polyethylene (HDPE, ρ = 0.95 g cm−3) and graphite (pure carbon, 12C) with wall thicknesses ranging from 15 to 35 cm. Volumetric track-length estimators (“T-Track” tally) were implemented along the diagonal calibration axis at source-to-detector distances of 100 to 250 cm to resolve both the thermal and fast spectral components with the statistical uncertainty less than ≤ 1.0%. Neutron fluence spectra due to HDPE configurations were benchmarked against published experimental data obtained at the same facility using a Bonner sphere spectrometer, showing excellent agreement. Total neutron fluence rates and ambient dose equivalent rates, H*(10), were derived by scaling T-Track outputs by the calibrated source emission rate and by applying ICRP Publication 74 fluence-to-dose conversion coefficients, respectively. Neutron ambient-dose-equivalent-averaged energies due to HDPE and graphite configurations were also calculated and compared with published data. The comparative analysis demonstrates that HDPE efficiently thermalizes and absorbs fast neutrons, whereas graphite thermalizes the field while preserving a significantly higher total neutron population. Bigger-thickness configurations (60 cm and 100 cm-diameter graphite spheres; 60 cm-diameter HDPE sphere) further illustrate the unique capability of carbon-based moderators for generating intense thermal neutron fields. Both moderator types are confirmed to produce simulated workplace neutron fields compliant with ISO 12789.

The local pH environment is a critical factor in radiobiological response, as it governs the yields and chemical equilibria of key reactive oxygen species (ROS). In this work, we present the systematic validation of the new pH-dependent chemical solver within the Geant4-DNA ’UHDR’ application, utilizing a mesoscopic Reaction-Diffusion Master Equation (RDME) approach. The solver treats hydronium and hydroxide ions as a stationary homogeneous background, enabling the accurate simulation of the time-dependent evolution of species such as hydroxyl radicals (OH∙), hydrated electrons (eaq− ), hydrogen atoms (H∙), hydrogen peroxide (H2O2), molecular hydrogen (H2), and the hydroperoxyl/superoxide system (HO2∙ ∕O2∙−). The implementation was benchmarked against two reference datasets: (i) radiolytic yields (𝐺- values) for 300 MeV protons across acidic to neutral pH, and (ii) the long-term second-order disproportionation kinetics of superoxide radicals produced by 1 MeV electrons, across acidic to alkaline pH. Results show good agreement with experimental data and reference Monte Carlo (MC) codes within the pH 2.0–9.0 range. While this interval is currently defined by the current neglect of ionic strength effects, it extensively covers the physiological and pathological pH spectrum of biological tissues, including the acidic tumor microenvironment. This benchmarking establishes a robust computational tool for mechanistic FLASH research, enabling the exploration of radical- chemistry hypotheses in high-dose-rate radiobiology.

A Faraday cup (FC) suitable for nA intensive beams has been developed to be used in nuclear reaction studies using the pelletron at Hanoi University of Science (HUS) - Vietnam National University (VNU). Firstly, the FC operation was tested with proton beams of 0.8 and 2.63 MeV energies bombarding a 0.081 μm ¹⁹⁷ Au target. By varying the high voltage (-HV) applied to the FC’s repeller ring, current increase due to negative charge loss as secondary electron escape was investigated. Saturation was observed at about 50 V in both experiment and simulation using the CST Studio Suite software. Subsequently, the HV was set to -350 V to ensure the measuring current’s precision. Afterward, the FC was used in an experiment to measure the cross sections of p +¹² C elastic scattering with E_p =0.95-3.2 MeV at the laboratory angles of 50°, 106°, 120°, 130°, and 160° for its validation and further study for this reaction. Good agreement was obtained between our new data and those reported in the literature. The new data were analyzed with the R-matrix approach. The extracted level parameters are consistent with those of previous studies.