Physics & Astronomy ETDs

Publication Date

Summer 7-15-2019


Although dark matter comprises approximately 85\% of the matter content of the universe, direct detection of dark matter remains elusive. As the available parameter space for dark matter candidates is pushed to lower and lower limits, the demand for larger, more sensitive detectors continues to grow. Although upscaling the detector improves the sensitivity, it greatly increases the cost and complexity of the experiment. Even after a dark matter signal is detected, there remains the possibility that an unknown background mimics the dark matter signal. Consequently, verifying the dark matter origin of a detection signal is an issue for any dark matter experiment. The solution is to search for the so-called ``smoking gun" signatures for dark matter. There is the annual modulation of the event rate, and the modulation of the recoil direction over a sidereal day. The directional modulation is the more robust signal. It would not only unambiguously confirm the existence of dark matter, but pave the way for characterizing the properties of dark matter.

This thesis describes research toward advancing low pressure gas Time Projection Chamber (TPC) technology for directional dark matter detection. It begins by measuring the thermal negative ion behavior of the novel TPC gas, SF6, and thereby confirming SF6 as an ideal gas for directional dark matter experiments. The disadvantage of SF6 is the low fiducialization efficiency due to the relatively small secondary drift species, SF5-. This motivated studies of CF4-SF6 gas mixtures that led to the discovery of a new negative ion species hypothesized to be CF3-. We show that the relative production of the new species can be tuned by adjusting the SF6 concentration and the drift field. We also propose a model for CF3- production in CF4-SF6 gas mixtures that makes qualitative predictions, which are consistent with our measurements. Our studies show that a 20-3 CF4-SF6 mixture results in low thermal diffusion and a factor two enhancement of the fiducialization efficiency relative to that measured for pure SF6. Using this mixture our measurements demonstrate gamma/electron discrimination down to 15 keVee and head-tail directionality down to 30 keVee. These are the first such measurements in TPCs with SF6-based gases, and the first utilizing a 1D readout in any gas.

Degree Name


Level of Degree


Department Name

Physics & Astronomy

First Committee Member (Chair)

Dinesh Loomba

Second Committee Member

Michael Gold

Third Committee Member

Paul Schwoebel

Fourth Committee Member

Keith Rielage




Dark Matter, Directional Dark Matter, TPC, SF6, CF4, DRIFT

Document Type