The design of a plastic scintillating fiber-based array for online tritium monitoring device in water

BackgroundTritium is a radioactive isotope of hydrogen that is primarily released from nuclear power plants in the form of tritiated water. Once discharged, it can enter natural water bodies and the human living environment. Due to its biological toxicity, continuous monitoring of tritium in water is required. Regulatory limits on tritium activity in drinking water vary by region, with the lowest threshold reaching 100 Bq·L⁻¹, which places stringent demands on the detection limit of monitoring systems.PurposeThis study aims to meet the requirement for real-time monitoring of low-level tritium water by developing a detection system with a low minimum detectable activity concentration (MDAC).MethodsBased on a laboratory-developed real-time tritium-in-water monitoring system using plastic scintillating optical fibers, Geant4 Monte Carlo simulations combined with ROOT-based data analysis were employed to investigate the key factors affecting MDAC of the monitoring system. Under fixed chamber geometry and electronic parameters, the effects of fiber length, diameter, number, spatial arrangement, cladding thickness, fiber curvature, and impurity radionuclide background were systematically investigated.ResultsThe simulation results indicate that: (1) a fiber length of approximately 20 cm provides a practical balance between mechanical feasibility, cost and MDAC reduction; (2) for a fixed chamber geometry, MDAC decreases with the increase of fiber number or fiber diameter; (3) the influence of fiber arrangement on MDAC is significant only at low filling densities; (4) fiber cladding affects MDAC reduction only when the filling density is low and the cladding thickness is below 0.1 μm; (5) it is necessary to ensure that the fibers are straight and without obvious bends as much as possible during the industrial manufacturing and processing of scintillator fibers; and (6) impurity radionuclides can significantly degrade detection efficiency and increase MDAC, hence, should be minimized.ConclusionFor practical detector design, achieving an MDAC of 100 Bq·L⁻¹ requires a single 46 mm-diameter chamber filled with approximately 1 800 unclad plastic scintillating fibers of 1 mm diameter and 20 cm length. At high filling densities, a uniform fiber arrangement is recommended to reduce optical crosstalk. In addition, background suppression and the integration of radionuclide pre-treatment or purification units should be considered in practical applications.