In situ radiometric discrimination and depth profiling of strontium-90, yttrium-90 and caesium-137 in aqueous media

The research, design, development, and testing of a bespoke depth monitoring probe, referred to as ‘ABACUS’, is described. ABACUS is a prototype groundwater X-/γ-ray and potentially direct β-particle logging instrument conceived for in situ, real-time spectroscopy of radioactivity in boreholes and sumps on contaminated land. It provides data complimentary to traditional sampling, especially in high-dose areas where radiological risk limits access. ABACUS comprises a commercially available Ø10×9.5 mm cerium bromide (CeBr3) detector connected to a small full-featured Topaz-SiPM MCA unit, shielded in a Ø7 × 28 cm aluminium case. It is controlled and readout via Ethernet-based logging cable compatible with a motorised winch for measurements up to 50 m. The probe fits narrow, metallic-lined, end-capped boreholes and it’s suited for laboratory settings. Four experimental studies are described: Firstly, a semi-empirical algorithm is developed and tested for dual-mode detection and discrimination of caesium-137 (137Cs) from β--emitting radionuclides, like strontium-90/yttrium-90 (90Sr/90Y), based on X- and γ-ray spectral comparison. These are long-lived, high-yield fission key products of the nuclear fuel cycle. Direct measurement of 90Sr and 90Y is challenging due to their short-range β- emissions and lack of characteristic γ rays, unlike 137Cs. The assessment of 137Cs and 90Sr in surrounding soil close to a borehole testbed and their location is achieved by detecting both γ-rays and bremsstrahlung X-rays, respectively, the latter arising from the interaction of β- particles emitted by 90Sr and 90Y in e.g. the steel of borehole linings and the aluminium of the probe case. The algorithm detects and discriminates a primary source of radiation associated with distinct full-energy γ-ray peak(s) from background. Secondly, the depth of individual source species is determined by processing count profiles using a Moffat-based point-spread function and verified via a series of tests with 137Cs sealed point sources. The combined algorithm is shown to identify changes in one-dimensional vertical source distributions from a baseline anomaly count-depth response curve, over successive log runs on a laboratory testbed (also conceived and commissioned in this research). Thirdly, the relative contributions of 137Cs and 90Sr/90Y in solution are discerned via qualitative spectral-shape analysis based on a comparison of distinct X- and γ-ray spectra. This yields count-independent parameters from full-energy peak and bremsstrahlung responses, and the extent of X- and γ-ray mixing and their relative proximity to the probe. This approach isolates characteristic responses consistent with continuous β- spectra and their semi-characteristic end-point energies, given the high-energy β- emission of 90Y. Fourthly, the potential to discern the ingrowth of 90Y from its parent 90Sr in solution-based samples using an unshielded ABACUS probe in aqueous, open-source samples stripped of yttrium is demonstrated. Direct β- detection is suggested from a response observed consistent with the high β- emission energies of 90Y. A temporal count profile confirms 90Y ingrowth in the stripped sample, consistent with the timescale for secular equilibrium being achieved (~ 8 half-lives of 90Y). This research led to a deployable commercial product that can be readily used on-site to support timely environment radiological assessments. It has demonstrable potential to advance both practical and analytical characterisation of legacy fission-product contamination in groundwater and near-surface waste disposal environments.