Characterization of a Lanthanum Bromide Detector for Eye Lens Dosimetry at the Candu Nuclear Power Plants Based on Direct Measurements of the Gamma-Ray Spectra

Abstract

Gamma-ray spectra were measured using a LaBr$_{3}$(Ce) spectrometer during the outage periods, aiming at quantifying the gamma source term of radiation workers’ exposure, at the CANDU nuclear power reactors, for the purposes of eye lens dosimetry. The spectra were measured inside the boiler rooms, of the Bruce Power and Ontario Power Generation (OPG) CANDU nuclear power plants, where workers are exposed to relatively high dose rates radiation fields during the maintenance work. Prior to measurements at the CANDU reactors, the pulse shaping parameters of the gamma spectrometer were optimised for high rates gamma fields, up to an input rates of 120 kcps, in order to accomplish a high output rate with a reasonable energy resolution. In parallel, the response of the LaBr$_{3}$(Ce) detector was characterized by experiments and Monte Carlo simulations. The gamma spectra measured at the CANDU reactors were reported in terms of the gamma-ray fluence rate spectrum. In all measured data, $^{60}$Co and $^{95}$Nb were main contributors of the gamma fields. The measured spectra have been used to calculate the dosimetric quantities of interest: personal dose equivalents H$_{p}$(10) and H$_{p}$(0.07) and eye lens absorbed dose.

High-rate Gamma Spectrometry Using a LaBr3(Ce) Scintillator with a Fast Pulse Shaping

“…The PMT high voltage was tuned from +850 to +900 V, and the pulses from the PMT were digitized and shaped by a trapezoidal algorithm that was implemented into a FPGA processor of the system. In the original configuration (Laranjeiro et al 2020), a standard plug-in type preamplifier (model AS2612, Saint-Gobain) provided by the manufacturer was used for extracting the signal from the PMT; however, it is eliminated in the present study due to the reason described in the introduction section above. For this modification, a custom junction box was built for connecting the PMT with the high voltage supply and the pulse processing system.…”

“…Owing to its excellent energy resolution, high gamma detection efficiency, fast timing performance, and operation at room temperature, the LaBr 3 (Ce) scintillator has been employed for both fundamental and applied nuclear physics, including contemporary nuclear physics experiments (Bonesini et al 2023; Löher et al 2012; Régis et al 2010), gamma source characterization at nuclear power plants (Laranjeiro et al 2020), spent nuclear fuel characterization and non-destructive assay of nuclear material (Drescher et al 2017; Favalli et al 2018), nuclear fusion study (Nocente et al 2013; Cazzaniga et al 2013), and gamma-ray imaging (Okazaki et al 2021; Pani et al 2015). Its fast scintillation decay time of 10–20 ns (van Loef et al 2002) makes the LaBr 3 (Ce) scintillator a good candidate detector for high-rate gamma spectrometry, and many studies geared for high-rate measurements were reported (Löher et al 2012; Drescher et al 2017; Nocente et al 2013; Stevanato et al 2012).…”

“…In our previous study (Laranjeiro et al 2020), we employed a LaBr 3 scintillator and a standard digital pulse processing system with a trapezoidal shaping for high-rate spectrometry. Although a fast pulse shaping should not cause a notable degradation in energy resolution in principle, given the time scale of the pulse shaping is much longer than the scintillation decay time scale, the standard preamplifier provided by the detector manufacturer led to poor energy resolution when a fast pulse shaping was applied.…”

Current status of eye-lens dosimetry in Canada

“…In 2015, OPG, Bruce Power and McMaster University initiated a five-year program to assess the need for eye lens dosimetry inside CANDU NPPs (Atanackovic 2018, Hanu 2018, Laranjeiro et al 2020, Bohra et al 2021. The method selected was to make detailed in-situ measurements of gamma-ray and beta source terms using a variety of detectors.…”