High-purity tungsten produced via chemical vapor deposition for use in low-background detectors

Abstract

Strict radiopurity constraints limit the type and amount of materials available for use in low-radioactivity detectors. Additional properties (e.g., yield strength and density) are required for materials to be used as structural or shielding components, further limiting availability. In this work, chemical vapor deposition (CVD) was investigated as a means to produce ultra-pure tungsten, a material with desirable density and mechanical properties. The radiopurity of the samples produced via CVD was verified using inductively coupled plasma mass spectrometry, focusing on impurities of ²³²Th and ²³⁸U. Due to the formation of tungsten-based polyatomic species in the plasma affecting ²³²Th and ²³⁸U determinations, an analytical method involving matrix separation and the use of a triple-quadrupole mass spectrometer was developed. Method detection limits in the ppq (parts per quadrillion, or fg g⁻¹ of solid tungsten) were obtained. Method recoveries ranged from 10%–50% and were determined using non-natural ²²⁹Th and ²³³U tracers. The CVD technique proved to be effective at producing ultra-pure samples of tungsten, which were measured in the ppt (parts per trillion, or pg g⁻¹ of solid tungsten) to sub-ppt range for ²³²Th and ²³⁸U.