Co-precipitation approach to measure amount of 238U in copper to sub-ppt level using ICP-MS

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 941; p. 162335
• Main Authors: Ding, Y.Y., Liu, M.C., Zhao, J., Yan, W.Q., Wei, L.H., Zhang, Z.Y., Wen, L.J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 11.10.2019
• Subjects: Co-precipitation; ICP-MS; Ultra-low radioactivity; Uranium; Ultra-low radioactivity; Uranium; ICP-MS; Co-precipitation
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2019.162335

Inductively coupled plasma mass (ICP-MS) spectroscopy is widely used for screening materials of low background detectors in dark matter and double beta decay searches due to its high sensitivity to trace 238U and 232Th. This work describes a novel co-precipitation approach to measure the amount of 238U in high-purity copper to sub-ppt level. Such an approach allows the pre-concentration of U and removal of the matrix, by selecting a proper precipitator to co-precipitate with 238U and using excess ammonia water to separate the uranium hydroxide from copper by forming water-soluble tetra-amminecopper (II). The isotope dilution method and standard addition method were both used to mitigate the matrix effect and cross-check each other. The latter was also used to measure the recovery efficiency of 238U by using 233U as the tracer. The method detection limit (MDL) reached ∼0.1 pg 238U/g Cu for both methods while the recovery efficiency of uranium robustly remains 65%–85%. Various sources of interference in the ICP-MS analysis were thoroughly evaluated, and the contamination from reagents were found to be the dominant factor that affected the MDL. Further purification will allow significant improvements in the MDL. This co-precipitate approach can be easily extended to measure 232Th by using 229Th as the tracer.