Development of a portable alpha–beta–gamma radioactive material continuous air-monitoring system

Abstract

During the decommissioning process of nuclear facility sites, such as the Fukushima Daiichi Nuclear Power Station (FDNPS), radioactive contamination due to alpha, beta, and gamma nuclides is present. To accurately detect these nuclides, a new phoswich alpha/beta/gamma detector is proposed and developed. A portable alpha/beta/gamma radiation continuous air-monitoring system is also developed using the proposed phoswich detector for detecting unknown radioactive air contamination. The developed phowich detector features a 6 mm × 6 mm × 6 mm-stilbene scintillator (first layer) and a 6 mm × 6 mm × 6 mm-Gd3(Ga, Al)5O12(Ce) (GAGG) scintillator (second layer). The bottom of the GAGG scintillator is optically coupled to a silicon photomultiplier (SiPM). The proposed phoswich detector is capable of successfully separating alpha, beta, and gamma nuclides by applying the pulse-shape discrimination (PSD) technique. The alpha energy resolution of the 5.5 MeV alpha particles using was 22.1 ± 0.21% at the full width at half maximum (FWHM), whereas the gamma energy resolution of the 0.662 MeV gamma rays was 10.3 ± 0.18%FWHM. The energy spectra obtained from the simulations agree well with those obtained from measurements. Continuous alpha-nuclide air measurements were conducted in the basement of a concrete building, where the 222Rn concentration was approximately 200 Bq/m3, using the developed portable continuous air-monitoring system. The measured peaks, which correspond to 212Bi (6.1 MeV), 214Po (7.7 MeV), and 212Po (8.8 MeV), were observed in the alpha spectrum by separating beta and gamma rays. Therefore, the developed monitoring system can be potentially efficient for the early detection of unknown released radioactive materials.