Discerning ⁹⁰Y in-growth from ⁹⁰Sr for in-situ groundwater monitoring with the ABACUS logging probe

¹ Lancaster University, United Kingdom
² Hybrid Instruments Ltd., United Kingdom
³ Nuclear Restoration Services, Dounreay, Scotland

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Published online: 6 November 2025

Abstract

Measurements showing the potential to discern the in-growth of yttrium-90 (⁹⁰Y) from its parent strontium-90 (⁹⁰Sr) in aqueous media, based on direct β^- particle detection, are described. These have been made using an unshielded cerium bromide (CeBr₃) detector derived from a dedicated depth monitoring probe, hereafter referred to as ‘ABACUS’ in media stripped of yttrium. Whilst similar radiologically, ⁹⁰Y and ⁹⁰Sr differ in terms of their aqueous solubilities and associated tendencies to migrate as elemental species. Consequently, they may not always be in secular equilibrium due to their relative proportions in solution changing with time especially where groundwater flow or disturbances act to resuspend or wash out yttrium precipitates. A temporal count profile has been obtained of aqueous samples stripped of yttrium indicating the in-growth of ⁹⁰Y in this initially, Sr-only-containing groundwater sample. A plateau is observed 21 days consistent with secular equilibrium having been achieved (~ 8 half-lives of ⁹⁰Y) based on a combined scintillation response indicating not only characteristic features in the X-/γ-ray response but also the potential for the direct detection of ⁹⁰Y β^- particles in CeBr₃, evidenced by spectra consistent with the anticipated ⁹⁰Y β^- particle response. ABACUS is a groundwater X-/γ-ray prototype logging instrument designed for in-situ assessment of radioactivity in underground, water-logged environments, particularly in boreholes and sumps comprising a commercially available, 10 × 9.5 mm CeBr₃ scintillation crystal attached to a small, full-featured MCA TopazSiPM module, contained in a Ø 7 cm & 28 cm long cylindrical aluminium case. A first study presented at ANIMMA 2023 focused on the mathematical algorithm used to analyse the X-/γ-ray photon spectra from ABACUS. Here, the focus is on the implementation of a qualitative approach to discern the relative contribution of ¹³⁷Cs and ⁹⁰Sr/⁹⁰Y in solution with an unshielded ABACUS probe, based on comparison of Xand γ-ray photon spectra arising from the bremsstrahlung yield from ⁹⁰Sr/⁹⁰Y, the ¹³⁷Cs γ-ray full-energy peak response and the direct β^- response, using spectral shape analysis. This not only enables spectral shapes linked to different source distributions and isotopes but also, given the very high-energy, β^- particle stimulus from ⁹⁰Y, the isolation of characteristic responses consistent with continuous β^- particle spectra and their semicharacteristic end-point energies, especially where detectors might be fabricated to enhance this response, i.e., to incorporate a thin window. Direct assessment of ⁹⁰Sr and ⁹⁰Y is known to be particularly challenging due to the short range of their β^- emissions in matter, relative to X-rays and γ rays, and their lack of characteristic γ rays in contrast to ¹³⁷Cs but not impossible especially given the high energies of said β^- particles, as this research will illustrate.