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The performance of ²¹⁰Pb AMS for the radioassay of materials considered for the construction of ultra-low background experiments in Astroparticle Physics
One of the most important tasks in the design and construction of ultra-low background experiments is the radioassay of the materials used. This requires the selection of the materials and enables the calculation of expected detector background. The ASTREA project (Accelerator mass spectrometry Survey of Trace Radionuclides for Experiments in Astroparticle physics) addresses AMS radioassay challenges for a few rare event experiments. Some examples are nEXO, which is searching for neutrinoless double beta decay; and NEWS-G and DarkSide, which are attempting to directly detect dark matter. This project, led by the André E. Lalonde AMS Laboratory (AEL-AMS) at the University of Ottawa, is performed in collaboration with Carleton University, Queens University and University of Alberta.
The main focus of the project is screening Pb-210 in various detector construction materials, with emphasis on low background copper and high-performance polymers. We have studied the possibility of using 2 different materials for the AMS measurements: lead fluoride (PbF2) and lead oxide (PbO) targets, producing respectively (PbF3)- and (PbO2)- ions on the LE side. In both cases, the 210Pb/206Pb blank ratio is in the 1e-14–1e-13 range. Measurements on 1-2 g Kapton films have established upper limits in the range 850-2500 mBq/kg at 90% C.L.
Future ASTREA activities will focus on the Pb-210 assay in acrylic, which is considered for future low background dark matter detectors. Previous best results, obtained in 2014 by γ-counting 2 kg of acrylic, have established an upper limit for the Pb-210 concentration of 0.3 mBq/kg. Our proposed method, using AMS, should provide a limit of detection in the 0.01-0.1 mBq/kg range.
Other important study looks at the Pb-210 contamination in the electroformation process of the copper for the NEWS-G and nEXO detectors. For the Pb-210 concentration in the copper, we estimate a limit of detection in the 0.3-1.0 mBq/kg range.
Keywords: ²¹⁰Pb contamination; Accelerator mass spectrometry; Rare event searches; Astroparticle physics; Radiopurity
The 15th International Conference on Accelerator Mass Spectrometry, 19.11.2021, Online, Online