Development of a multi-layer silicon beta-ray spectrometer for beta spectrometry and dosimetry at CANDU power plants

A compact multi-layer silicon beta-ray spectrometer has been developed for beta spectrometry and dosimetry for the beta-gamma mixed fields encountered at CANDU nuclear plants. Its design is based on the principle that the coincidence operations between silicon detectors make most gamma detection events be rejected while beta detection events are saved in the beta energy region of 0.7 to 3 MeV. A prototype spectrometer consists of a collimator, an entrance window, a stack of silicon detectors, an interface board, and a quad-input pulse processing system. Geant4 Monte Carlo simulations were carried out to optimize the configuration of the detector stack and compute the spectrometer responses to beta and gamma radiations, which led to an optimum detector stack consisting of a 500 μ m front detector followed by three 1500 μ m detectors. To characterize the gamma rejection and beta spectrometric performance, comprehensive measurements were carried out for various mixed beta-gamma fields with different beta count rates and beta-gamma count ratios that were created by varying the positions of a 90 Sr / 90 Y beta source and a 137 Cs gamma source. The coincidence spectra showed excellent gamma rejection performance in most energy regions above 250 keV while notable gamma perturbation events were identified in the low energy region for the coincidence spectrum between the first two detectors and the anti-coincidence spectrum of the front detector. A further study will be conducted soon to improve the spectrometer performance in the low energy region.