In-situ Real-time Monitoring of Waterborne Low Energy Betas

Dixon, Nile and Cheneler, David and Monk, Stephen (2025) In-situ Real-time Monitoring of Waterborne Low Energy Betas. PhD thesis, School of Engineering.

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Abstract

Nuclear Licenced sites like Sellafield, Dounreay, and Dungeness B handle hazardous radioactive material which is stored before it can be reprocessed for permanent storage. To ensure the safety of these sites, groundwater sampling is performed from an array of onsite boreholes. Boreholes containing low energy betas are of additional interest due to the difficulty in their detection, requiring advanced measurement techniques which are not yet suited to an in-situ setting. This research has implemented granulated scintillators within a manufactured novel flow cell detector aimed at detecting low energy betas. The detector is to be placed within the borehole itself to take direct measurements of the groundwater, removing the delay of sampling and transporting the samples offsite. Methods have been tested for granulating and separating solid scintillators, with finer particulates achieving higher detection efficiencies for tritium. Silicon Photomultipliers (SiPMs) in contact with the flow cell have been used to detect radiation interacting with the scintillator. A Field Programmable Gate Array (FPGA) has been implemented to shape pulses from a SiPM into spectra that are accumulated and stored within a database hosted within the detector. This system has been validated experimentally using four sealed gamma sources by identifying and fitting photopeaks within the captured spectra to known gamma energies. LED blink testing has been conducted to study how the pulse shaper delay coefficients affect recorded spectrums. The electrical system and flow cell have then been combined into a benchtop system and tested with multiple concentrations of tritiated water, results show a correlation between detected count rate and the activity concentration within the flow cell. Experiments with strontium-90 tracers show an increase in count rate as the tracer entered the flow cell, and two borehole groundwater samples taken from the Sellafield site have been passed through the detector obtaining two spectra. Finally, the benchtop system has been compacted to fit within the limited confines of standard 50 mm boreholes.