Noble liquid-based dark matter detectors are sensitive to parts-per-billion (ppb) concentrations of impurities of O 2 , N 2 , and H 2 O, which reduce the scintillation light yield and disrupt the ionization signal. In order to achieve an optimal light yield and collection of the ionization signal, DarkSide is designing a cryogenic distillation unit to achieve large quantities of depleted argon with a purity at sub-ppb levels. Critical to the success of the depleted argon dark matter detector will be the development of a system capable of analyzing these impurities at the sub-ppb level.
A trace gas analyzer based on cw-Cavity Ring-Down Spectroscopy (CRDS) technology is being built at Black Hills State University that exceeds the current limits of commercially available systems. CRDS involves measuring the decay rate of the intensity of monochromatic light transmitted at the mirrors enclosing a cavity filled with a sample of gas. By comparing the decay rates for light at a resonance frequency of the contaminant of interest with that of light slightly off-resonance, a direct measurement of the amount of absorbing material (the contaminant) is obtained. Designed to measure ultra-low levels of elemental impurities, the BHSU system will be critical to such experiments as DarkSide and other dark matter searches based on liquid argon and xenon. * This work is supported in part by NSF Grants 0903335 and 0923557, and the SD 2010 Center for Ultra-low Background Experiments at DUSEL.