Assay of lead-210 in metallic copper via accelerator mass spectrometry

Abstract

This study focuses on the precise quantification of ²¹⁰Pb contamination in metallic copper, an essential construction material of ultra-sensitive detectors in astroparticle physics, by Accelerator Mass Spectrometry. For the most stable and durable anion beam current from the Cs sputter source, PbF₂ in a mixture with fine Ag powder was found to be preferable over PbO + Ag. After tandem acceleration and electron stripping with helium, the maximum ²⁰⁶Pb³⁺ current was 0.11 μA, and the ²¹⁰Pb detection efficiency was 1.3 × 10⁻⁴. Investigation of several methods to extract lead from copper, with HNO₃, NH₄OH, and H₂SO₄, led to chemical recovery rates up to 92%. Following the development of chemical and target preparation procedures, an upper limit of ²¹⁰Pb = 62 mBq kg⁻¹ was determined for the NEWS-G copper (Aurubis) sample, consistent with previous results obtained by α-spectrometry. Tests with copper plates exposed to an atmosphere of ²²²Rn (1625 Bq m⁻³) for 182 hours showed significant ²¹⁰Pb surface contamination (0.56 ± 0.16 Bq m⁻²), underscoring the importance of minimizing radon in spaces where metallic copper is chemically and/or physically processed.