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Materials Design for the Synthesis of High Strength Radiopure Copper Alloys for Rare Event Detection

Dimitra Spathara, Patrick Knights, Konstantinos Nikolopoulos

TL;DR

The paper addresses the need for mechanically robust yet radiopure detector materials for rare-event searches by designing Cu-based alloys (CuCr and CuCrTi) through a CALPHAD-guided workflow. It combines sequential electrodeposition with heat-treatment simulations (via CALPHAD and DICTRA) to predict homogenization and precipitation strengthening while maintaining radiopurity. Key findings show feasible pathways to homogenize CuCr to $0.5$ wt% Cr within practical times at $1050^{ deg}C$, and show that trace Ti can enable CuCrTi alloys with hardness comparable to mild steel while preserving conductivity; higher processing temperatures can further accelerate homogenization but risk intermetallic formation. The results are illustrated through case studies of DarkSPHERE and XLZD, highlighting how radiopure, high-strength CuCr-based vessels could dramatically improve detector performance and reduce data-taking times in next-generation experiments.

Abstract

Additive-free electroformed copper has emerged as the material of choice in exceptionally radiopure detectors for rare-event searches, based on its radiopurity, physical properties, and affordability. However, copper is ductile and of limited mechanical strength posing challenges for its use in future experiments. Electroformed copper-based alloys have been identified as a promising solution. However, their synthesis needs refining by exploring a complex parameter space of compositions and strengthening mechanisms. Here we show how a materials design approach may address current challenges and optimize alloy synthesis and processing. Alloy properties are predicted following thermal processing, using computational thermodynamics. The findings suggest a methodology to design high-performance, radiopure copper-based alloys suitable for next-generation rare-event experiments, while minimizing lengthy and expensive trial-and-error approaches. The impact on future experiments is exemplified through case-studies of the DarkSPHERE and XLZD experiments.

Materials Design for the Synthesis of High Strength Radiopure Copper Alloys for Rare Event Detection

TL;DR

The paper addresses the need for mechanically robust yet radiopure detector materials for rare-event searches by designing Cu-based alloys (CuCr and CuCrTi) through a CALPHAD-guided workflow. It combines sequential electrodeposition with heat-treatment simulations (via CALPHAD and DICTRA) to predict homogenization and precipitation strengthening while maintaining radiopurity. Key findings show feasible pathways to homogenize CuCr to wt% Cr within practical times at , and show that trace Ti can enable CuCrTi alloys with hardness comparable to mild steel while preserving conductivity; higher processing temperatures can further accelerate homogenization but risk intermetallic formation. The results are illustrated through case studies of DarkSPHERE and XLZD, highlighting how radiopure, high-strength CuCr-based vessels could dramatically improve detector performance and reduce data-taking times in next-generation experiments.

Abstract

Additive-free electroformed copper has emerged as the material of choice in exceptionally radiopure detectors for rare-event searches, based on its radiopurity, physical properties, and affordability. However, copper is ductile and of limited mechanical strength posing challenges for its use in future experiments. Electroformed copper-based alloys have been identified as a promising solution. However, their synthesis needs refining by exploring a complex parameter space of compositions and strengthening mechanisms. Here we show how a materials design approach may address current challenges and optimize alloy synthesis and processing. Alloy properties are predicted following thermal processing, using computational thermodynamics. The findings suggest a methodology to design high-performance, radiopure copper-based alloys suitable for next-generation rare-event experiments, while minimizing lengthy and expensive trial-and-error approaches. The impact on future experiments is exemplified through case-studies of the DarkSPHERE and XLZD experiments.

Paper Structure

This paper contains 13 sections, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Manufacturing electroformed CuCr and CuCrTi alloys
  4. Single and double-sided layer configuration for Cu-0.5Cr alloy
  5. Towards an electroformed CuCrTi alloy
  6. Case studies: DarkSPHERE and XLZD
  7. The DarkSPHERE detector
  8. The XLZD detector
  9. Methods
  10. Data availability
  11. References
  12. Author contributions
  13. Declaration of competing interest

Figures (5)

  • Figure 1:
  • Figure 2: Phase diagrams. Calculated phase diagrams for the \ref{['fig:2a']} Cu-Cr \ref{['fig:2b']} Cr-Ti and \ref{['fig:2c']} Cu-Ti systems. The TCHEA6 and MOBHEA3 High Entropy alloys Database is used TCdatabase1TCdatabase6. Several phases are formed, including faced centered cubic (fcc), body centered cubic (bcc), and hexagonal close-packed (hcp). The logo of the Thermo-Calc software is shown on the lower left corner of each panel.
  • Figure 3: Ti concentration profiles. Simulated Ti concentration profiles as a result of diffusion after solution heat treatment of 0.7 $\mu m$ Ti in contact with 9.8 $\mu m$ Cr. \ref{['fig:3a']}$1050\,^{\circ}$C, \ref{['fig:3b']}$1300\,^{\circ}$C and \ref{['fig:3b']}$1395\,^{\circ}$C. The duration of solution heat treatment is indicated for each concentration profile. The logo of the DICTRA module is shown on the lower left corner of each panel.
  • Figure 4: Cr and Ti concentration profiles. Simulated concentration profiles as a result of diffusion after solution heat treatment at $1050\,^{\circ}$C of 1.5 $\mu m$ Cr-6.23Ti (in wt%) in contact with 228.5 $\mu m$ Cu of \ref{['fig:4a']} Cr and \ref{['fig:4b']} Ti, resulting in homogenized alloy Cu-0.5Cr-0.032Ti composition, 230 $\mu m$ thick. The duration of solution heat treatment is indicated for each concentration profile. The logo of the DICTRA module is shown on the lower left corner of each panel.
  • Figure 5: DarkSPHERE sensitivity. Expected sensitivity of DarkSPHERE for \ref{['fig:limitsSI']} spin-independent dark matter-nucleon interaction cross-section, including the enhancement due to the Migdal effect (Mig.) from Ref. NEWS-G:2023qwh, and spin-dependent \ref{['fig:limitsSDp']} dark matter-proton and \ref{['fig:limitsSDn']} dark matter-neutron interaction cross-section. All assume a nominal exposure of 300 days and a gas at a pressure of $5\;\bar{$NEWS-G:2023qwh. The spin-dependent proton sensitivity includes the contribution from both the He:i-C$_4$H$_{10}$ and the Ne:CH$_4$ data taking campaigns. The sensitivity in \ref{['fig:limitsSDn']} comes from using Ne:CH$_4$ isotopically enriched in $^{21}$Ne and i-C$_4$H$_{10}$ that is isotopically enriched in $^{13}$C to 99% while in Ref. NEWS-G:2023qwh the $^{13}$C natural abundance was considered. Also shown is the sensitivity where $5\;g$ of helium are replaced with $^3$He. Existing constraints are summarised both for spin-independent Agnese:2018gzeAbdelhameed:2019hmkAgnes:2018vesNEWS-G:2017pxgAprile:2019jmx and spin-dependent SuperCDMS:2017nnsCollar:2018ydfCRESST:2022dtlBehnke:2016lskPICO:2019vscAprile:2019jmxCRESST:2022dtl searches.