Control of Radon and Pollutants in Gas-based Directional Dark Matter Detectors Using Molecular Sieves

The most compelling explanation for the so-called Dark Matter of the Universe is the postulation of particles beyond the standard model, with Weakly Interacting Massive Particle (WIMP) dark matter being well-motivated. While there are many different methods to search for WIMPs, the most sensitive dark matter experiments in the world employ liquid noble gas targets to detect WIMP-induced recoils. As the next generation of liquid noble detectors become more sensitive, they are confronted by an inevitable background of solar neutrinos, which inhibit the conclusive identification of dark matter in such searches. Directional dark matter detectors have the capability to distinguish against the otherwise irreducible solar neutrino background by adding information about the direction of the WIMP-induced recoil events. Most directional detectors reconstruct recoil tracks using low-pressure gas Time Projection Chambers (TPC). In gas TPC operation, it is important to remove radon and common pollutants from the target gas. Radon contamination provides a source of unwanted background able to mimic WIMP-induced recoils, while common pollutants can significantly suppress the gain of the detector. SF6 is an ideal target gas for directional dark matter searches, so the ability to remove radon and common pollutants from SF6 during TPC operation is crucial. A method that also recycles SF6 is required as it is a potent greenhouse gas. This thesis describes work toward a gas recycling system that removes radon and common pollutants from target gases during TPC operation. The removal of radon from SF6 gas was demonstrated for the first time using a 5 angstrom type molecular sieve. A low radioactive 5 angstrom type molecular sieve that intrinsically emanated 98.9% less radon per radon captured compared to commercial sieves was found. To effectively implement the molecular sieves with TPC detectors, a gas system utilising a modified Vacuum Swing Adsorption (VSA) technique with a gas recovery buffer was designed. The VSA technique minimises the required amount of molecular sieve for long-term filtration, and the gas recovery buffer maximises the amount of recycled gas. The design was built into a prototype and tested with a small-scale gas TPC detector. Performance testing with the gas system prototype resulted in the low radioactive 5 angstrom type molecular sieve reducing the intrinsic radon contamination of the TPC detector setup within the background limits of the radon measurement apparatus (14.0±5.7 mBq). A TPC detector run with the gas system employing 3 angstrom and 4 angstrom type molecular sieves significantly reduced the impact of common pollutants suppressing signal amplification, with the detector signal remaining until detector operation was terminated after 340 hours. Without the gas system, the TPC detector could only maintain this level of signal amplification for 50 hours. The results presented in this thesis successfully demonstrate the feasibility of a molecular sieve-based gas recycling system that simultaneously removes radon and common pollutants from SF6-based directional dark matter detectors.