Response of a TLD badge to the new operational quantity Hp(θ): Monte Carlo approach

Abstract

In 2020, the International Commission on Radiation Units and Measurements’ Report Committee 26 (RC 26) proposed new operational quantities to be used in personal monitoring for external radiation. METROBRAS is a Brazilian laboratory that uses a TLD-based personal dosimeter calibrated in terms of the personal dose equivalent, Hp(10, θ). In this paper, an evaluation of the METROBRAS personal dosimeter to the new operational quantity personal dose, Hp(θ), was performed. This assessment was based on the determination of Hp(10, θ) and Hp(θ) using Monte Carlo simulation. The basic personal dosimeter design includes three CaSO4:Dy plus polytetrafluoroethylene (PTFE) detectors sandwiched between different filters inside of an acrylic badge. The determination of the relative response for six X-ray qualities and for gamma-rays from Cs-137 (662 keV) was carried out using the Monte Carlo transport code MCNPX. TLD detectors, filters and the plastic badge were simulated on the surface of a water ISO phantom with 30 cm × 30 cm x 15 cm with PMMA walls. The effect of the direction of radiation incidence has been determined for angles of 0° (anterior-posterior, AP), ±45° and ±60°. The new Hp(θ) quantity affected the relative energy and angular response of the TLD badge mainly for the low photon energies with maximum factors of ∼1.94 and ∼2.91 for 33 keV and 24 keV, respectively. The TLD-badge exhibited not only photon energy but also angular dependence, that was larger for Hp(θ). The major differences between the relative angular responses with respect to the response at 0° were found for the exposures at ± 60°. To comply with the requirements of the new quantity, an upgrade on the calculation algorithm and a new design of the filter set (that allows energy discrimination) should be made. The information provided by the Monte Carlo data helped to predict the METROBRAS badge response to the new operational quantity Hp(θ).