Ultra-pure Nickel for Structural Components of Low-Radioactivity Instruments

TL;DR

This study evaluates chemical vapor deposition nickel as a promising ultra-low-radioactivity structural material for rare-event physics experiments, demonstrating its high strength and extremely low contaminant levels, with insights into welding effects and surface contamination.

Contribution

It provides the first comprehensive assessment of CVD nickel's mechanical properties and radioactive contamination levels, highlighting its potential for low-background physics applications.

Findings

CVD Ni has a tensile strength of ~600 MPa, surpassing standard nickel.

Welding reduces tensile strength to standard levels, with porosity likely causing this.

Measured radioactive contaminants are at record low levels in nickel.

Abstract

The next generation of rare-event search experiments in nuclear and particle physics demand structural materials combining exceptional mechanical strength with ultra-low levels of radioactive contamination. This study evaluates chemical vapor deposition (CVD) nickel as a candidate structural material for such applications. Manufacturer-supplied CVD Ni grown on aluminum substrates underwent tensile testing before and after welding alongside standard Ni samples. CVD Ni exhibited a planar tensile strength of ~600 MPa, significantly surpassing standard nickel. However, welding and heat treatment were found to reduce the tensile strength to levels comparable to standard Ni, with observed porosity in the welds likely contributing to this reduction. Material assay via inductively coupled plasma mass spectrometry (ICP-MS) employing isotope-dilution produced measured bulk concentration of 232-Th, 238-U, and nat-K at the levels of ~70 ppq, <100 ppq, and ~900 ppt, respectively, which is the lowest reported in nickel. Surface-etch profiling uncovered higher concentrations of these contaminants extending ~10 micrometer beneath the surface, likely associated with the aluminum growth substrate. The results reported are compared to the one other well documented usage of CVD Ni in a low radioactive background physics research experiment and a discussion is provided on how the currently reported results may arise from changes in CVD fabrication or testing process. These results establish CVD Ni as a promising low-radioactivity structural material, while outlining the need for further development in welding and surface cleaning techniques to fully realize its potential in large-scale, low radioactive background rare-event search experiments.