Customizing a CRDS Trace Gas Analyzer for Dark Matter Experiments

Some proposed dark matter detectors require large amounts of noble gases with ultra-low levels of impurities such as H$_{2}$O, O$_{2}$, and N$_{2}$. There are no commercially available gas analyzers sensitive enough to detect these impurities at the low levels required by these detectors ($<$1 ppb). In order to reach the required impurity detection limits we will customize a Cavity Ring-Down Spectroscopy system. CRDS involves trapping monochromatic light between two highly reflective super-mirrors placed on both sides of the sample chamber, leading to an extremely long ($>$1 km) effective path length through the sample. By comparing the decay rates of light intensity at a resonance frequency of the impurity with that of light that is off-resonance, the concentration of absorbing material (the impurity) is obtained. We will gain improved sensitivity over existing technology by lengthening the gas cavity, optimizing the gas-flow routes, and enhancing the laser and detection electronics. The BHSU CRDS system will first be used for the depleted argon-based dark matter detectors DARKSIDE and MAX. The system will then also be available for xenon-based dark matter detectors such as LUX. This system may also be of interest commercially.