Description
Readout cables for signal sensors are a fundamental component of rare event searches such as neutrinoless double beta decay. While possessing unique electrical and mechanical properties, polyimide-based flexible cables can be a significant contributor to the total detector background, due to their relatively high content of natural radionuclides. Contaminations of Th-232 and U-238 in commercially-available flexible cables have been measured in the mBq/kg range, making them incompatible with the stringent levels required for next-generation rare event detectors.
In a previous work, we have demonstrated the possibility of obtaining low-background (µBq/kg) copper-polyimide laminates which serve as the starting material for flexible cable manufacturing. However, we have found that even when starting with low-background laminates, the cable manufacturing process results in finished flexible cables with high (mBq/kg) levels of radioactivity.
In this work, each step of the flexible cable manufacturing process was systematically investigated as a potential vector of radioactive impurities. Through the investigation of process modifications, the development of cleaning procedures, and surveys of alternative materials, we have demonstrated that the radioactivity content from Th-232 and U-238 can be reduced to a few tens of µBq/kg. We will discuss our key findings, report the current best levels of radiopurity achieved, and discuss future plans for making ultra-low background flexible cables commercially available.
