Speaker
Description
Materials showing exceptionally low content of radioactive impurities and excellent mechanical properties (e.g., yield strength) are needed as structural and shielding materials in current and next-generation low-background detectors. Impurities of naturally occurring radionuclides Th-232, U-238, and their progeny, are typically required to be at or below the microBq/kg range, equating to concentrations in the parts per trillion (ppt) or parts per quadrillion (ppq) range. These levels can be several orders of magnitude lower than levels generally found in earth materials (i.e., soils and rocks) or in most commercially available materials. Such stringent radiopurity constraints pose a challenge in the sourcing and validation of materials, for which the most sensitive analytical methods are required. When sufficiently radiopure materials are not available off-the-shelf, new manufacturing methods are investigated and developed (e.g., ultra-pure copper electroforming).
Thanks to exceptional yield strength (750 MPa), high atomic number, and high density, tungsten has garnered interest as a potential structural and shielding material. Previous searches, however, have determined commercially available tungsten in the milliBq/kg range, from impurities of U-238 and its progeny, preventing the use of this material in low background studies.
In this work, we have investigated chemical vapor deposition (CVD) processes as a mean to produce tungsten material with sufficiently low content of Th-232 and U-238 impurities for use in ultra-sensitive detectors. To overcome challenges associated with measuring low-levels of Th-232 and U-238 in a tungsten matrix, a novel ICP-MS based analytical method was developed for tungsten assay. Detection limits of a few tens of ppq were obtained for both radionuclides. Produced samples of CVD tungsten were measured at levels ranging from single-digit ppt to less than a few tens of ppq of Th-232 and U-238, with a purification efficacy greater than three orders of magnitude compared to off-the-shelf tungsten. This work advances analytical chemistry capabilities for ultra-pure material validation, and paves the way for the use of ultra-pure tungsten in low-background detectors.
