Identification of radiopure tungsten for low background applications

Abstract

In this article we explore the availability of radiopure tungsten and itspotential as high density shield material for low background applications. Forcompact shield designs, conventionally, lead is used. Metallic tungsten andtungsten pseudo-alloys reach higher densities up to 19.3 gcm$^{-3}$ and do notexhibit a significant $^{210}$Pb activity, which is a typical intrinsiccontamination in lead. Within several $\gamma$-ray screening campaigns we wereable to identify tungsten samples with activities similar or better than 1mBqkg$^{-1}$ in $^{232}$Th, $^{40}$K, $^{60}$Co and the second part of the$^{238}$U decay chain. In cooperation with a manufacturer we further reduced apersisting contamination in the first part of the $^{238}$U decay chain by afactor of $\sim$2.5 down to (305$\pm$30) mBqkg$^{-1}$. With Monte Carlosimulations, the construction of prototype tungsten-based setups and dedicatedmeasurements, the shield capability of tungsten in comparison to lead wasextensively studied. Specifically, the impact of cosmogenic radiation on thebackground at shallow depth was investigated. We showed that a 6-40% reduction(depending on the exact shield configuration) in the muon-induced neutronfluence is achievable by replacing lead with an equivalent amount of tungstenregarding the suppression of external $\gamma$-radiation. Overall, manybenefits using tungsten especially for low energy applications below a few 100keV are found. The pseudo-tungsten alloy presented in this work paves the wayfor several applications especially regarding background suppression inparticle and astroparticle physics search programs.<br