Master Bond EP29LPSP was selected for use in the construction of a highly sensitive liquid argon (Ar) detector designed and built as part of a Princeton University doctoral candidate’s dissertation.1 Liquid argon is used in multi-ton dark matter detectors designed to sense the presence of elementary particles of non-luminous matter left over from the Big Bang. Significant concentrations of a radioactive isotope of argon, 39Ar, in atmospheric argon reduces the sensitivity of dark matter detectors, but argon mined from underground natural gas reserves has been found to contain much lower levels of 39Ar. The levels are so low that they could not be measured by established techniques, so researchers at Princeton University designed a device to measure the concentration of 39Ar in underground argon.
A key component of the Princeton argon detector is a cryogenic photomultiplier tube (PMT), which had to be modified to reduce background radioactivity by replacing a glass feedthrough with a high purity ceramic plate. When the modified PMT failed high voltage tests in pure argon gas, researchers hypothesized that the source of the electric breakdown was the narrow space (~1mm) between the anode lead passing through the ceramic plate and the outer rim of the plate. Because the rim is metalized to form a seal with the metal PMT housing, and the housing shares the cathode potential, there is only a 1mm separation between the anode potential and the cathode potential, presenting a risk of high voltage electric breakdown.
To solve the problem, a thin layer of cryogenic Master Bond EP29LPSP epoxy was used to coat the metalized rim of the ceramic plate, the anode lead, and all other leads passing through the ceramic plate. The low outgassing property of EP29LPSP was critical for this application, because the sensitivity of argon-based dark matter detectors is highly dependent on the purity of the liquid argon used. A customized oven was designed for curing the epoxy, which requires a minimum cure temperature (55°C) that is above the maximum operating and storage temperature (50°C) of the PMT. The oven was designed to enclose just the rear end (feedthrough and leads) of the PMT in a PVC case, which had an inlet for hot nitrogen gas, so that the epoxy could be cured at 60-70°C inside the oven while the temperature-sensitive part of the PMT remained at room temperature outside the oven. The epoxy coating passed cryogenic testing and enabled the PMT to operate at 1600V.