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

Technical Summary: Ultra-pure Nickel for Structural Components of Low-Radioactivity Instruments

Problem Statement
The next generation of rare-event search experiments in nuclear and particle physics, such as the tonne-scale neutrinoless double beta decay experiment nEXO, requires structural materials that combine exceptional mechanical strength with ultra-low levels of radioactive contamination. Standard construction materials often introduce background noise that obscures rare decay events. While ultra-pure chemical vapor deposition (CVD) nickel (Ni) has been used historically (e.g., in the Sudbury Neutrino Observatory's Neutral Current Detectors), there is a need to re-evaluate its mechanical properties after modern fabrication and processing (specifically welding and heat treatment) and to characterize its radioactive impurity profile with current analytical sensitivity.

Methodology
The study utilized CVD Ni panels fabricated by CVMR (Toronto, Canada) via a Mond CVD process on aluminum substrates. The material was evaluated through two primary avenues:

1. Mechanical Strength Evaluation:

   • Samples: CVD Ni panels were cut into sections for welding, heat treatment, and control testing. Commercial off-the-shelf (COTS) Ni-200 plates served as a baseline comparison.
   • Welding: Tungsten inert gas (TIG) welding was performed on CVD Ni and COTS Ni samples. Welds were prepared with 40-degree chamfers and performed from the top side only (no backing weld) to simulate nEXO vessel constraints. No post-weld grinding or machining was permitted to maintain cleanliness protocols.
   • Heat Treatment: Sub-size dog-bone samples were subjected to a heat cycle (ramped to 1000°C, held for 4 minutes, cooled naturally) to simulate thermal effects of welding without melting.
   • Testing: Tensile strength, yield strength, and displacement at break were measured using an MTS 312.21 frame. X-ray radiography was employed to inspect weld integrity for voids.

2. Radioactive Contamination Assay:

   • Technique: Inductively coupled plasma mass spectrometry (ICP-MS) with isotope dilution was used to quantify bulk and surface concentrations of $^{232}\text{Th}$, $^{238}\text{U}$, and natural potassium ($^{\text{nat}}\text{K}$).
   • Sampling:
     • Bulk: Acid etching removed surface layers before stoichiometric digestion of the remaining material.
     • Surface/Depth Profiling: Sequential acid etching (both mass-based and stoichiometric) was performed to map contaminant concentrations from the surface down to $\sim10\,\mu\text{m}$ and deeper.
   • Controls: Experiments were conducted in Class 10000 cleanrooms with rigorous blank validation to ensure detection limits in the femtogram/gram range.

Key Results

• Mechanical Properties:

  • As-Supplied CVD Ni: Exhibited a planar ultimate tensile strength of $\sim600$ MPa, significantly higher than the COTS Ni baseline ($\sim400$ MPa). The CVD Ni showed brittle failure characteristics with minimal necking compared to the ductile COTS Ni.
  • Effect of Welding: Welding reduced the tensile strength of CVD Ni to levels comparable to COTS Ni ($\sim200\text{--}300$ MPa for some samples, generally lower than the base material). X-ray imaging revealed significant voids (up to 1 mm) in the weld roots of CVD Ni, which were not observed in COTS Ni.
  • Effect of Heat Treatment: Heat treatment alone reduced the tensile strength of CVD Ni to $\sim400$ MPa, aligning it with the strength of un-welded COTS Ni. This suggests that the thermal cycle associated with welding (annealing) contributes to the strength reduction, alongside the observed porosity.

• Radioactive Contamination:

  • Bulk Concentrations: The bulk CVD Ni demonstrated the lowest reported contamination levels for nickel:
    • $^{232}\text{Th}$: $\sim70$ parts per quadrillion (ppq) ($\sim0.09\text{--}0.37\,\mu\text{Bq/kg}$).
    • $^{238}\text{U}$: $<100$ ppq ($<0.79\text{--}1.22\,\mu\text{Bq/kg}$).
    • $^{\text{nat}}\text{K}$: $\sim900$ parts per trillion (ppt) ($\sim25\text{--}31\,\mu\text{Bq/kg}$ of $^{40}\text{K}$).
  • Surface Contamination: Depth profiling revealed that concentrations of Th, U, and K are significantly higher within the first $\sim10\,\mu\text{m}$ of the surface compared to the bulk. This near-surface contamination is attributed to the aluminum growth substrate.
  • Comparison to SNO: The bulk Th and U levels reported here are more than an order of magnitude lower than those reported for the SNO NCD CVD Ni. The authors suggest this may be due to changes in the CVD fabrication process over 30 years, differences in sample geometry (surface-to-volume ratio), or the SNO's inability to fully separate bulk and surface contamination with their historical methods.

Significance and Claims
The paper establishes CVD Ni as a promising low-radioactivity structural material for large-scale rare-event search experiments. The bulk material possesses ultra-low radioactive contamination levels that meet the stringent requirements for next-generation detectors.

However, the authors modestly conclude that the material's potential is currently limited by processing techniques. Specifically:

1. Welding: The current TIG welding process introduces porosity and reduces tensile strength to levels comparable to standard commercial nickel, negating the superior strength of the as-supplied CVD material.
2. Surface Cleaning: The presence of substrate-derived contaminants extending $\sim10\,\mu\text{m}$ deep indicates that standard surface cleaning may be insufficient for applications requiring the lowest possible background.

The paper recommends further development in welding techniques (to eliminate voids and preserve strength) and surface cleaning methods (to remove the contaminated substrate-affected layer) before CVD Ni can be fully realized for large-scale instrument construction. It also suggests that future fabrication should consider higher-purity substrates or alternative substrate materials to mitigate surface contamination.