Ngoc-Thiem Le

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 ISO-4037 low air-kerma rate series X-ray reference fields were characterized using the SpekPy toolkit in terms of X-ray fluence spectrum, radiometric quantities (i.e., spectral averaged energy, half-value layer, and spectral resolution) as well as a physical quantity (air-kerma rate). Those X-ray fluence spectra, radiometric and physical quantities were presented in comparison with those specified in ISO-4037 and those from experimentally published data.

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.

Evaluation of potential annual effective doses (PAED) for a radiation worker and a public adult due to inhalation of naturally occurring radioactive materials (NORM) were conducted around NORM sites in the northern Vietnam. The value of PAED is the most important indicator for radiation safety assessment. The values of PAED due to inhalation for a radiation worker and a public adult were found as 6.02 mSv/y and 6.01 mSv/y, respectively. The NORM site can be categorized into “type A” and a “controlled area”.

The design of Bonner Sphere Spectrometer (BSS) was optimized using Bayesian approach for two different measurement purposes in the ISO 8529 neutron standard field of 241Am-Be source as follows: (i) for the best estimation of neutron fluence spectrum and (ii) for the best estimation of neutron ambient dose equivalent, H*(10). As results, two optimal sets of BSS system (with sequential specific diameters) were proposed: (i) for the first problem, an optimal BSS system with a maximum of six different diameters is needed to obtain neutron fluence spectrum with the standard uncertainties in each energy bin as low as reasonably achievable and (ii) for the second problem, if one would just like to achieve the output H*(10) with a standard uncertainty <10%, only two optimal BSS measurements are needed.

This paper presents the technical parameters of neutron calibration fields available in Viet Nam for calibrations of neutron measuring meters (i.e., neutron ambient dose equivalent/rate meters and/or neutron personal dose equivalent/rate meters − referred to neutron meters). The technical parameters of neutron calibration fields, in this work, consist of physical quantity (i.e., the integrated neutron fluence rate over a neutron spectrum, ϕ); operational dosimetric quantities (i.e., ambient dose equivalent rate, Ḣ^*(10)) and radiation quantities (i.e., fluence-spectrum-averaged energies, Ē; fluence-to-ambient dose equivalent conversion coefficient, h*). The technical parameters of neutron reference fields were reported for different radiation beam qualities following the ISO-8529 and the ISO-12789 series (i.e., for direct neutron component from a neutron reference fields of a bare ²⁴¹Am-Be, complied with the ISO-8529 series criteria; and for total neutron component from simulated workplace neutron reference fields of a ²⁴¹Am-Be source moderated by different polyethylene spheres and from a neutron reference field of a bare ²³⁹Pu-Be source, complied with the ISO-12789 series criteria). The technical parameters of the ISO-8529 neutron reference field of the bare ²⁴¹Am-Be source were theoretically calculated based on the total neutron source strength and presented for direct neutron component. In addition, the technical parameters of the ISO-12789 simulated workplace neutron reference fields of the ²⁴¹Am-Be source moderated by different polyethylene spheres as well as of the ²³⁹Pu-Be source were characterized using the Bonner sphere spectrometer system accompanying with the FRUIT neutron spectrum unfolding code for the total neutron component. The technical parameters as well as their combined standard uncertainties of neutron fields were also presented in conjunction with different distances from the sources’ centers.

Half-value layer (HVL) measurement is the method most often used to validate an X-ray reference field as defined in ISO 4037. This work presents Particle and Heavy Ion Transport code System (PHITS) simulated values of HVL and their standard deviations for an infinitesimal radiation field size. The HVL values, calculated from simulated values of air kerma at a detector position for various radiation field sizes, were extrapolated to an infinitesimal field size for different ISO 4037 narrow spectrum X-ray beam qualities at two distances from an X-ray source.