Ultra-low background mass spectrometry for rare-event searches

摘要

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) allows for rapid, high-sensitivity determination of trace impurities, notably the primordial radioisotopes ~(238)U and ~(232)Th, in candidate materials for low-background rare-event search experiments. We describe the setup and characterisation of a dedicated low-background screening facility at University College London where we operate an Agilent 7900 ICP-MS. The impact of reagent and carrier gas purity is evaluated and we show that twice-distilled ROMIL-SpA~(™)-grade nitric acid and zero-grade Ar gas delivers similar sensitivity to ROMIL-UpA~(™)-grade acid and research-grade gas. A straightforward procedure for sample digestion and analysis of materials with U/Th concentrations down to 10 ppt g/g is presented. This includes the use of ~(233)U and ~(230)Th spikes to correct for signal loss from a range of sources and verification of ~(238)U and ~(232)Th recovery through digestion and analysis of a certified reference material with a complex sample matrix. Finally, we demonstrate assays and present results from two sample preparation and assay methods: a high-sensitivity measurement of ultra-pure Ti using open digestion techniques, and a closed vessel microwave digestion of a nickel-chromium-alloy using a multi-acid mixture.