The novel properties of SF6 for directional dark matter experiments

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SF6 is an inert and electronegative gas that has a long history of use in high voltage insulation and numerous other industrial applications. Although SF6 is used as a trace component to introduce stability in tracking chambers, its highly electronegative properties have limited its use in tracking detectors. In this work we present a series of measurements with SF6 as the primary gas in a low pressure Time Projection Chamber (TPC), with a thick GEM used as the avalanche and readout device. The first results of an 55Fe energy spectrum in SF6 are presented. Measurements of the mobility and longitudinal diffusion confirm the negative ion drift of SF6. However, the observed waveforms have a peculiar but interesting structure that indicates multiple drift species and a dependence on the reduced field (E/p), as well as on the level of water vapor contamination. The discovery of a distinct secondary peak in the waveform, together with its identification and use for fiducializing events in the TPC, are also presented. Our measurements demonstrate that SF6 is an ideal gas for directional dark matter detection. In particular, the high fluorine content is desirable for spin-dependent sensitivity, negative ion drift ensures low diffusion over large drift distances, and the multiple species of charge carriers allow for full detector fiducialization.