A novel assay method for the trace determination of Th and U in copper and lead using inductively coupled plasma mass spectrometry

This study describes a novel sample preparation and assay method developed, primarily in support of the Majorana Demonstrator neutrinoless double-beta decay experiment, for the determination of extremely low levels of Th and U in copper and lead shielding components. Meticulously clean sample preparation methods combined with anion exchange separations for analyte pre-concentration and matrix removal were developed. Quantification was performed by isotope dilution inductively coupled plasma mass spectrometry. Detection limits of 0.0084 pg 232Th/g (0.034 µBq 232Th/kg) and 0.0106 pg 238U/g (0.131 µBq 238U/kg) were determined for copper, while detection limits of 0.23 pg 232Th/g (0.94 µBq 232Th/kg) and 0.46 pg 238U/g (5.7 µBq 238U/kg) were achieved for lead. These methods allow the Majorana Collaboration to accurately assay detector components and ensure that the experiment׳s stringent radiopurity requirements are met.