PNNL

The Science   

Background radioactivity in everything from dust to the surrounding environment can interfere with ultra-sensitive physics experiments, including natural radioactivity in the very electronics that are designed to detect signals. To reduce the amount of background radiation from cables used in these experiments, researchers systematically examined the amount of radioactive contaminants introduced during the cable production process using inductively coupled plasma mass spectrometry. The scientists then identified alternative methods of cleaning and preparing the cables to reduce their contamination levels. Their method resulted in cables with 10× to 100× less ­²³⁸U and ²³²Th levels compared to commercial cables—reaching radiopurity levels sufficient for use in ultra-sensitive physics experiments.

The Impact

Certain physics experiments—like the search for neutrinoless double beta decay or dark matter—are looking for extremely rare events that could help understand the origin and nature of matter in the universe. However, radioactive contaminants inside these detectors, even at concentrations as tiny as one part-per-billion, can mimic the elusive signals that physicists are seeking. Cables are needed to extract the signals from these detectors but commercial options exceed the radioactivity background requirement for use in the next generation of detectors. The use of these new custom low-radioactivity cables can increase the sensitivity of these experiments by not only reducing the amount of interfering radioactivity but also allowing researchers to deploy additional sensors within the detector.

Summary

Researchers at Pacific Northwest National Laboratory, in collaboration with a small business partner Q-Flex Inc., investigated radioactive contamination levels in the cable fabrication processes using small detachable “coupons” that act as surrogates for the cables. At each step of the process, the scientists would remove an individual coupon from a panel for analysis using inductively coupled plasma mass spectrometry—a highly-sensitive, yet destructive technique for sub-parts per trillion (ppt) detection of ²³⁸U, ²³²Th, and ^natK. Their results revealed that, even when starting with carefully-selected radiopure materials, radioactive contaminants were introduced via chemical solutions during the photolithography and plating steps. Working with the company, the scientists explored new fabrication techniques as well as substitute materials, and developed a cleaning recipe to reduce the ²³⁸U and ²³²Th contamination to a few 10s of ppt.

Once the key contaminating steps and suitable alternative methods were identified, the scientists produced two sets of fully-functional cables using the newly formulated low-background fabrication process. These cables represent the types used in the nEXO neutrinoless double beta decay experiment and the Dark Matter in Charged coupled devices at Modane (DAMIC-M) experiment. The resulting cables were measured to have extremely low levels of radiation, at roughly 10 – 30 ppt of ²³⁸U and ²³²Th. These “quiet” cables will enable more sensitive searches for extremely rare events, such as neutrinoless double beta decay and dark matter interactions.

Contact

Richard Saldanha
Pacific Northwest National Laboratory
richard.saldanha@pnnl.gov

Funding

This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics, under the Early Career Research Program and the SBIR program.