Direct detection of dark matter with noble liquid detectors

The search for non-baryonic, non-luminous dark matter that comprises approximately 23% of our universe is an exciting endeavor. However, detecting this matter has proved difficult as it does not interact through the electromagnetic force but only by scattering elastically off of target nuclei on the weak scale; therefore evidence of dark matter must be demonstrated through the observation of nuclear recoils induced by dark matter candidates. Because nuclear recoils can be caused by any type of elastic scattering reactions induced by radiogenic and cosmogenic processes, a dark matter detector must have an extremely low background. Moreover, the low energy signal of a dark matter event requires building detectors with large volumes of target material with low background. Noble liquids provide a promising target for the detection of dark matter. Of the noble elements, argon and xenon have been shown to be ideal targets in dark matter searches as they have excellent scintillation yield and are relatively inexpensive and scalable. However, natural argon contains a radioactive isotope, 39Ar, that must be reduced in order to observe a rare dark matter event. Several technologies exist that can be utilized to reduce the concentration of this element including thermal diffusion, underground water sources and laser isotope separation. Thermal diffusion employs a temperature gradient in order to separate gaseous argon isotopes along the length of a cylindrical column. A test bench thermal diffusion column has been constructed which resulted in the significant depletion of 36Ar in a natural argon sample. Underground water sources have also been evaluated in the pursuit of natural argon depleted of the 39Ar isotope. Since the water in these sources has not been in contact with atmospheric air for several thousand years that 39Ar should have decayed away. A water source at Wall, SD has been obtained and evaluated for depleted argon using a water degassing apparatus and a proportional counter. The water sample obtained from this source showed a reduction of 39Ar by a factor of three.