The novel properties of SF₆ for directional dark matter experiments

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.

Negative Ion Time Projection Chamber operation with SF6at nearly atmospheric pressure

“…The SF − 6 mobility we determined in pure SF 6 is consistent with the results from [9] in the region 10-30 Td of E/N and extend the measurements below 10 Td. The drift velocities measured in the range of cm/ms demonstrate that all the other mixtures exhibit negative ion behaviour as well, with the highest SF − 6 mobility obtained in the He:CF 4 :SF 6 360:240:10 Torr (as expected from the small SF 6 fraction), nearly 5 times higher than in pure SF 6 .…”

“…The metastable SF − * 6 de-excites into SF 6 , SF − 6 or SF − 5 through autodetachment, collisional stabilization or auto-dissociation respectively, with the relative abundances depend on the lifetime of SF − * 6 , the electron energy, gas pressure, temperature, and drift field. Auto-detachment is expected to be inconsequential at the operating conditions of our detector, as shown in [9]. Other negative ion species such as F − or SF − 4 can be produced, but at much lower probabilities, due to the much lower cross section and higher electron energies required.…”

“…The first experimental evidence for the possibility of using SF 6 as negative ion capture agent has been demonstrated by the pioneering studies by University of New Mexico [9], laying the groundwork for the future use of SF 6 in NITPC experiments. In this paper, they operate a single thick 400 µm GEM in the range 20-100 Torr of pure SF 6 and observed a gas gain between 2000-3000 on low energy 5.9 keV 55 Fe x-ray clusters.…”

Neutrinoless double beta decay with82SeF6and direct ion imaging

“…Reduced mobilities of ions in SF 6 are in the range of 0.4-0.6 cm 2 V −1 s −1 [21], and we shall assume similar mobilities in SeF 6 . This implies a drift time across a ton-scale experiment of order 0.5-1.0 s. It also allows us to estimate the effects of ion-ion recombination near the track corea vital consideration, given that substantial recombination, if present, could in principle spoil the energy resolution of such an experiment [29].…”

Prototype analog front-end for negative-ion gas and dual-phase liquid-Ar TPCs

“…As a result, the difference of arrival times between species correlates with total drift distance. For example, nuclear recoils in SF 6 primarily produce SF − 6 , but also trace amounts of SF − 5 (so-called a "minority carrier"), whose mobility is ≈ 5% larger [11]. Because the time-scale for events in a NI µ-TPC is orders of magnitude longer than for an electron-drift TPC, new electronics, such as the LTARS, are required to read out these detectors.…”