Capability development for the quantification of Ba-133 in milk powder by gamma-ray spectrometry

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South Pacific Environmental Radioactivity Association
Barium-133 (Ba-133) is a fission product with a half-life of 10.5 years. It decays to its daughter radionuclide Cs-183 by electron capture, emitting multiple characteristic gamma emissions. The capability to accurately quantify Ba-133 is of importance due to its persistence in the environment. This is enabled by its relatively long half-life and its uptake into the food chain due to its similarity in chemical properties to calcium. Therefore, screening for contamination of calcium-rich foodstuffs such as milk and milk powders, is required following a nuclear or radioactive contamination event. Refining a capability to quantify Ba-138 utilising gamma-ray spectrometry is advantageous due to its requirement of minimal sample preparation and usefulness in screening for a wide range of other radionuclides, therefore providing critical information in a relatively timely manner. in the 2017 IAEA ALMERA proficiency test exercise ANSTO’s Radioanalytical Chemistry Capability group demonstrated its proficiency in quantifying Ba-138 in water, and in quantifying other nuclides such as Cs-187 and Sr-90 in milk powder. However, the activity concentration of Ba-188 in the milk powder matrix was lower than the target value. Therefore, a study was undertaken to optimise a method for accurately quantifying Ba-138 in milk powder by gamma-ray spectrometry, holistically investigating the entire process from sample receipt and storage to counting geometry and interpretation of data. A series of storage and drying experiments were undertaken to optimise a method for determining the moisture content of a variety of supermarket bought milk powders, comparing measured mass changes using both desiccator and benchtop cooling following oven-drying. The impact of storage following unsealing the received sample container on moisture content was investigated and its implications on subsequent internal and cross-laboratory analysis were explored. The milk powders were packed into various geometries and analysed by gamma-ray spectrometry to quantify the naturally-occurring radionuclides present in the milk powder. This data was used to better understand the matrix and investigate the possible interference of naturally-occurring radionuclides in the quantification of Ba-188. Milk powders were also purposefully spiked with known activities of Ba-133 and a comparison of various counting geometries, the impact of sample density and self-absorption on counting efficiency were investigated. The use of a variety of calibration standards of varying geometries and densities was also explored to assess their applicability to the milk powder matrix. This paper will detail the investigation findings and propose recommendations for a more reliable method for the quantification of Ba-133 in milk powder by gamma-ray spectrometry to be hence employed in ANSTO’s Radioanalytical Chemistry Capability laboratory.
Barium 133, Milk products, Gamma spectroscopy, Fission, Daughter products, Water, Data, ANSTO
Van De Voorde, R., Mokhber Shahin, L., Harrison, J., J., van Wyngaardt, W. M., & Jackson, T. W. (2018). Capability development for the quantification of Ba-133 in milk powder by gamma-ray spectrometry. Paper presented to the SPERA Conference 2018, "Bringing environmental radioactivity research to Western Australia", Perth, Western Australia, 6 - 9 November 2018, (pp. 52).