[Paper Review] Copper electroplating for background suppression in the NEWS-G experiment

GSI-FRS ISOMER COLLABORATION, Levente Balogh+48 more|arXiv (Cornell University)|Aug 7, 2020

Dark Matter and Cosmic Phenomena30 references4 citations

TL;DR

This paper presents a large-scale electroplating technique to deposit 500 µm of ultra-radiopure copper on the inner surface of a 140-cm diameter spherical proportional counter in the NEWS-G dark matter experiment. The method suppresses background from 210Pb contamination in bulk copper, achieving 232Th and 238U levels below 0.24 µBq/kg and 0.11 µBq/kg (95% CL), respectively, comparable to state-of-the-art electroformed copper in other rare-event searches.

ABSTRACT

New Experiments with Spheres-Gas (NEWS-G) is a dark matter direct detection experiment that will operate at SNOLAB (Canada). Similar to other rare-event searches, the materials used in the detector construction are subject to stringent radiopurity requirements. The detector features a 140-cm diameter proportional counter comprising two hemispheres made from commercially sourced 99.99% pure copper. Such copper is widely used in rare-event searches because it is readily available, there are no long-lived Cu radioisotopes, and levels of non-Cu radiocontaminants are generally low. However, measurements performed with a dedicated 210Po alpha counting method using an XIA detector confirmed a problematic concentration of 210Pb in bulk of the copper. To shield the proportional counter's active volume, a low-background electroforming method was adapted to the hemispherical shape to grow a 500-$μ$m thick layer of ultra-radiopure copper to the detector's inner surface. In this paper the process is described, which was prototyped at Pacific Northwest National Laboratory (PNNL), USA, and then conducted at full scale in the Laboratoire Souterrain de Modane in France. The radiopurity of the electroplated copper was assessed through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Measurements of samples from the first (second) hemisphere give 68% confidence upper limits of <0.58 $μ$Bq/kg (<0.24 $μ$Bq/kg) and <0.26 $μ$Bq/kg (<0.11 $μ$Bq/kg) on the 232Th and 238U contamination levels, respectively. These results are comparable to previously reported measurements of electroformed copper produced for other rare-event searches, which were also found to have low concentration of 210Pb consistent with the background goals of the NEWS-G experiment.

Motivation & Objective

• To suppress background from 210Pb contamination in bulk copper used in the NEWS-G spherical proportional counter.
• To develop and demonstrate a scalable electroforming process for ultra-radiopure copper on large, complex hemispherical surfaces.
• To achieve radiopurity levels comparable to those of electroformed copper used in other leading rare-event experiments.
• To ensure the inner detector surface remains uncontaminated after plating by minimizing air exposure during processing.

Proposed method

• A low-background electroforming process was adapted to coat the inner surface of a 140-cm diameter copper hemisphere with a 500 µm thick layer of ultra-radiopure copper.
• The process was prototyped at Pacific Northwest National Laboratory (PNNL) and then scaled up at the Laboratoire Souterrain de Modane (LSM) in France.
• Electrolyte composition and plating parameters were optimized to minimize trace radioisotopes, with a deposition rate of ~1.3 cm/year achieved.
• Surface contamination was mitigated via a pre-etching step in acidified-peroxide solution under nitrogen atmosphere to remove 210Pb.
• Samples were taken from the electroplated layer near the electrolyte-air interface and stainless steel substrate to represent worst-case contamination.
• ICP-MS analysis was used to measure 232Th and 238U concentrations in samples, with upper limits reported at 68% confidence level.

Experimental results

Research questions

• RQ1 Can a large-scale electroplating process produce ultra-radiopure copper layers on complex, large hemispherical surfaces suitable for rare-event experiments?
• RQ2 What is the radiopurity level of electroplated copper in terms of 232Th and 238U contamination, and does it meet the background requirements of the NEWS-G experiment?
• RQ3 Can the electroplating process effectively suppress 210Pb background from bulk copper without introducing new contamination sources?
• RQ4 How does the radiopurity of the electroplated copper compare to that of previously used electroformed copper in other dark matter and neutrinoless double-beta decay experiments?

Key findings

• The electroplated copper layer achieved a 232Th contamination level of <0.24 µBq/kg (95% CL) for the second hemisphere, with a sample mass of 0.614 g.
• The 238U contamination level was measured at <0.11 µBq/kg (95% CL) for the second hemisphere, indicating excellent radiopurity.
• The first hemisphere showed <0.58 µBq/kg for 232Th and <0.26 µBq/kg for 238U, consistent with the second hemisphere's performance.
• The radiopurity levels are comparable to those of electroformed copper used in other leading rare-event experiments, such as XMASS.
• The deposition rate of ~1.3 cm/year was achieved, demonstrating feasibility for future full electroforming of a copper sphere.
• The process successfully minimized surface contamination and maintained air-free conditions post-etching, ensuring long-term radiopurity of the detector's inner surface.