Speakers
Description
Measurements showing the potential to discern the ingrowth of yttrium-90 (Y-90) from its parent strontium-90 (Sr-90) in aqueous media, based on direct β- particle detection will be described. These have been made using an unshielded cerium bromide (CeBr3) detector in a dedicated depth monitoring probe, hereafter referred to as ‘ABACUS’, in media stripped of yttrium. Whilst similar radiologically, Y-90 and Sr-90 contrast significantly in terms of their aqueous solubilities and 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 given groundwater flow or disturbances acting 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 Y-90 in this initially, Sr-only-containing groundwater sample. A plateau was reached in 21 days consistent with secular equilibrium having been achieved (~ 8 half-lives of Y-90) 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 Y-90 β- particles in CeBr3 evidenced by spectra consistent with the anticipated Y-90 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. It comprises a commercially available, 10 × 9.5 mm CeBr3 scintillation crystal attached to a small, full-featured MCA Topaz-SiPM module, contained in a Ø 7 cm & 28 cm long cylindrical aluminium case. ABACUS can be deployed to meet most on-site requirements and is controlled/read via an Ethernet-based logging cable, including a motorised winch to aid deployments particularly in boreholes up to 100 m depth affording accurate depth measurements and also in laboratory settings concerning off-line sample measurement. A first study (presented at ANIMMA 2023) focused on the mathematical algorithm used to analyse the X-/γ-ray photon spectra from ABACUS, which afforded dual-mode detection and discrimination of Cs-137 alongside the β-emitting radionuclides as Sr-90/Y-90 based on comparison of X- and γ-ray photon spectra, Sr-90/Y-90 being important as relatively high-yield, long-lived anthropogenic fission product derivatives from the nuclear fuel cycle. The assessment of Cs-137 and Sr-90 in surrounding soil formations close to a borehole and the location of the probe is achieved by detecting both γ rays and bremsstrahlung X-ray photons, respectively, the latter arising from the interactions of β- particles emitted by Sr-90 and subsequently Y-90 in the steel of the blind-tubes or aluminium of the probe case etc. A computer-implemented algorithm is used to detect and discriminate a primary (distinguished usually in terms of having easily-identified discrete γ-ray lines) source of radiation over the more continuous background, with a semi-empirical fitting procedure. To analyse the depth of individual nuclides, count profiles are processed using a development of the Moffat point-spread-function (PSF) model verified by a series of tests using Cs-137 sealed point sources and applying the combined algorithm demonstrating the effectiveness to identify changes in one-dimensional vertical source distributions from a baseline anomaly response curve, over successive log runs on a laboratory testbed. Here, the focus is on the implementation of a qualitative approach to discern the relative contribution of Cs-137 and Sr-90/Y-90 in solution with an unshielded ABACUS probe, based on comparison of X- and γ-ray photon spectra via the bremsstrahlung yield from Sr-90/Y-90 and the Cs-137 γ-ray full-energy peak response, using spectral shape analysis to derive two count-independent parameters to indicate the extent of X- and γ-ray mixing. This not only enables spectral shapes linked to different source distributions and isotopes but also, given the very high-energy, β- particle stimulus from Y-90, the isolation of characteristic responses consistent with continuous β- particle spectra and their semi-characteristic end-point energies, especially where detectors are fabricated to enhance this response, i.e., to incorporate a thin window. Direct assessment of Sr-90 and Y-90 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 Cs-137. However, this is not impossible given the high energies of said β- particles, as this research will illustrate.
