Direct method for the quantitative analysis of surface contamination on ultra-low background materials from exposure to dust

In this work we present a method for the direct determination of contaminant fallout rates on material surfaces from exposure to dust. Naturally occurring radionuclides ⁴⁰K, ²³²Th, ²³⁸U and stable Pb were investigated. Until now, background contributions from dust particulate have largely been estimated from fallout models and assumed dust composition. Our method utilizes a variety of low background collection media for exposure in locations of interest, followed by surface leaching and leachate analysis using inductively coupled plasma mass spectrometry (ICP-MS). The method was validated and applied in selected locations at Pacific Northwest National Laboratory (PNNL) and the SNOLAB underground facility. A comparison between data obtained from direct ICP-MS measurements and those estimated from current model-based predictors is also performed. Discrepancies of one order of magnitude or higher are observed between estimated and directly measured accumulation rates.