Abstract
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) allows for rapid, high-sensitivity determination of trace impurities, notably the primordial radioisotopes 238U and 232Th, 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-SpATM-grade nitric acid and zero-grade Ar gas delivers similar sensitivity to ROMIL-UpATM-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 233U and 230Th spikes to correct for signal loss from a range of sources and verification of 238U and 232Th 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.
Highlights
Rare event search experiments such as those seeking interactions from galactic dark matter scattering or evidence of neutrino-less double beta decay (0νββ) require construction from intrinsically radio-pure materials to limit radiogenic backgrounds
With a focus on high-throughput and fast-turnaround of time-critical assays, straightforward cleaning, sample preparation and analysis procedures have been developed for materials with U/Th concentrations in the 10 ppt to 1 ppb range: Samples are microwave-digested in pre-cleaned TFM vessels using ultra-high purity acids
Unlike the TFM digestion vessels, they do not come into contact with concentrated acids at elevated temperatures and we have found that using new labware that has been rinsed three times with DI water is sufficient for most purposes as typical U/Th BECs seen in calibrations and samples prepared are below < 30 ppq
Summary
Rare event search experiments such as those seeking interactions from galactic dark matter scattering or evidence of neutrino-less double beta decay (0νββ) require construction from intrinsically radio-pure materials to limit radiogenic backgrounds. Comprehensive radio-assay campaigns increasingly deploy multiple techniques to build a high-precision background model, and to meet sample throughput requirements during the experiment’s construction phase. Mass spectrometry techniques such as ICP-MS can be used to directly measure trace quantities of 238U and 232Th, delivering assays from small samples that are digested and screened with turnaround times on the order of 1–2 days.
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