Scientists from 25 universities around the world are collaborating on a particle physics project designed to expand our knowledge of the universe. The EXO (Enriched Xenon Observatory) Collaboration is building a series of detectors designed to observe a theorized —but not yet seen — type of radioactive decay known as a zero-neutrino double beta decay (0νββ).1 The EXO-200 detector uses liquid xenon (Xe) enriched with an 80.6% concentration of the radioactive isotope 136Xe as the detection medium.
At the heart of the detector is a time projection chamber (TPC), a copper vessel containing liquid xenon and electronics designed to detect radioactive decay events. The TPC is housed in a vacuum-insulated cryostat, which is shielded in lead. Front-end electronics located outside the lead shield are connected to the TPC through specially designed flat 25μm polymide cables. Six rectangular copper tubes serve as conduits for the cables between the TPC and the inner wall of the cryostat. At the end of each tube is a copper flange in which feedthroughs are formed for passing the cables between the TPC and the cryostat and between the cryostat and the front end electronics.
The first step in fabricating each feedthrough was to use a small amount of low outgassing Master Bond EP29LPSP adhesive to attach u-shaped acrylic pieces around a width of the cable, forming a cup. The acrylic cup was then filled with liquid EP29LPSP epoxy, and the cable and cup were bonded to a thin copper lip on each flange. Because ultra-low background radioactivity construction materials were a necessity in the design of the detector, the low outgassing property of Master Bond EP29LPSP was key to its selection.