The novel properties of SF$_6$ for directional dark matter experiments

TL;DR

This paper explores SF$_{6}$ as a primary gas in a low pressure TPC for directional dark matter detection, highlighting its unique properties like negative ion drift, multiple charge species, and potential for improved detector fiducialization.

Contribution

It presents the first measurements of SF$_{6}$ in a TPC, demonstrating its suitability for dark matter detection due to its negative ion drift and multiple charge carriers.

Findings

Confirmation of negative ion drift in SF$_{6}$

Observation of multiple drift species affecting waveforms

Identification of a secondary peak useful for event fiducialization

Abstract

SF$_{6}$ is an inert and electronegative gas that has a long history of use in high voltage insulation and numerous other industrial applications. Although SF$_{6}$ 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 SF$_{6}$ 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 $^{55}$Fe energy spectrum in SF$_{6}$ are presented. Measurements of the mobility and longitudinal diffusion confirm the negative ion drift of SF$_{6}$. 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 SF$_{6}$ 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.