Speaker
Description
Materials with extremely low content of radioactive impurities are required for the construction of successful rare-event detectors, where constraints on naturally occurring radionuclides such as 232Th and 238U typically fall in the microBq/kg range or lower, corresponding to parts-per-trillion (ppt) or parts-per-quadrillion (ppq) concentrations. These levels are several orders of magnitude lower than levels found in soil, rocks, and most commercially available materials. Additive-free, ultra-pure electroformed copper, originally developed at PNNL, has been a key material for several neutrinoless double beta decay and dark matter experiments, with 232Th and 238U impurities measured below a fraction of a microBq/kg, in the ppq range. However, its relatively low strength (~33 MPa) means that copper components must be made unusually thick to meet structural requirements. At growth rates on the order of 1 mm/month, producing large-scale, thick electroformed structures becomes both costly and extremely time-consuming. Developing structural materials that retain ultra-low radioactivity while offering significantly higher strength would have a great impact on next-generation low-background experiments.
Previous work has demonstrated that alloying individually electroplated layers of copper and chromium can yield a Cu-Cr alloy with substantially increased yield strength (by ≈50 % relative to pure copper) and acceptable levels of radiopurity, with only ~1% Cr added to the copper matrix. This has driven interest in Cu–Cr alloys with 0.6–1.2 wt% Cr in bulk copper as candidates for low background structural components, potentially enabling reduced copper mass without sacrificing mechanical performance or radiopurity. Here, we investigate the manufacturing of a Cu-Cr alloy by co-electrodeposition, which overcomes drawbacks associated with stacking and subsequently alloying individually plated layers. Furthermore, we replace highly toxic and carcinogenic hexavalent chromium with trivalent chromium.
We present preliminary results from co-deposition of Cu and Cr from low-additive aqueous chloride baths with future radiopurity requirements in mind. Given the complexity of Cr(III) in aqueous solution and its electronegative reduction potential beyond that of water, we also investigate non aqueous baths based on deep eutectic solvent (DES) solutions as an alternative co deposition medium. Early experiments in DES indicate that Cu and Cr can be co deposited, and we are optimizing bath composition and process parameters to approach the desired Cr content while maintaining coherent, structurally robust deposits. We outline our initial electrochemical and microstructural characterization of these deposits, and discuss planned next steps toward alloying and hardening heat treatments, as well as radiopurity assessment for application in low background detector structures.