Designing New Ultra-Radiopure, High-Strength Electroformed CuCr Alloys, for Rare Event Searches
Authors: Spathara Dimitra , Knights Patrick , Nikolopoulos Konstantinos 
Published: December 2024

Immense progress has been achieved in understanding the Cosmos over more than 45 orders of magnitude in length. Alas, fundamental questions remain about the nature of; a) Dark Matter (DM); and b) neutrinos. DM’s existence, corresponding to 84.5% of the Universe’s matter, is established via astrophysical observations and precise measurements. Several search approaches have been employed, but “direct detection” is the most prominent: aiming to observe DM from the Milky Way halo via its coherent elastic scattering off a nucleus.

Copper is the material of choice for rare event searches: it can be procured commercially at low cost and high purity, and it has no long lifetime isotopes. Alas, it may be contaminated during manufacturing and can be activated by fast neutrons from cosmic rays. In an effort to achieve even higher radiopurity, attention is focused on electroformed copper (EFCu), which has favorable radiochemical, thermal, and electrical properties. To fulfil the unique radiopurity requirements, experiments pioneer large-scale additive-free Cu electroformation, e.g. the on-going project ECUME. This novel technique [1] leads to extreme radiopurities with contamination below 10-
14 grams of 232Th and 238U per Cu gram. However, Copper is highly ductile and of low strength, limiting its use for moving mechanical, high-pressure, and load-bearing parts. This would improve the capability for experiments such as DarkSPHERE [2], a large-scale fully electroformed underground spherical proportional counter operating under high pressureto probe uncharted territory in the search for DM, and is vital for nEXO', a neutrinoless-double β-decay experiment aiming to clarify the nature of neutrinos.

The most promising alloying element for improved strength of EFCu is Chromium (Cr). It has been experimentally demonstrated that small additions of Cr, combined with heat treatment and aging, improve strength by 70% [3]. However, Cr additions lead to impurities, and a compromise between strength and radiopurity is required by exploring a complex parameter space of compositions and strengthening mechanisms [4].

Cr solubility in Cu is very limited and small additions of Ti can allow for improved mechanical strengthening due to reinforced precipitation. A critical evaluation of the ternary phase diagram of the Cr-Cu-Ti system is underway to inform the particular parameter space of compositions and strengthening mechanisms within the CALPHAD framework. This system has gained recently significant interest in the development of CuCrTi alloys for contact wires of high-speed, large-scale integrated circuit lead frames and electronic connectors [5].

Our work addresses materials challenges by developing high radiopure CuCr and CuCrTi alloys with significantly higher strength compared to Cu. The project will deliver a novel approach based on the Integrated Computational Materials Engineering (ICME) framework enabling rapid design of new, application-specific alloys for fields where thermodynamic and kinetic description of the system are crucial. Electroformation in additive-free bath is the focus of the project, which will push the boundaries in many fields, from fundamental science to industrial applications.

References

[1] NEWS-G Collaboration "Copper electroplating for background suppression in the NEWS-G experiment", Nucl. Instrum. Meth. A, 988 (2021): 164844

[2] NEWS-G Collaboration "Exploring light dark matter with the DarkSPHERE spherical proportional counter electroformed underground at the Boulby Underground Laboratory", Phys. Rev. D 108, (2023): 112006

[3] Overman N. et al. "Mechanical property anisotropy in ultra-thick copper electrodeposits", Appl. Phys. A 120 (2015): 1181

[4] Vitale A. et al. "A preliminary investigation into the electrodeposition and synthesis of radiopure Copper–Chromium alloys for rare-event physics detector systems", Nucl. Instrum. Meth. A, 1003 (2021): 165291

[5] Zhaokuo H. et al. "Mechanism investigation on high-performance Cu-Cr-Ti alloy via integrated

computational materials engineering", Mater. Today Commun. 27 (2021): 102378