Negative ions as charge carriers in gaseous detectors: Results in Xe, SF6 and Xe-SF6 mixtures

In this paper, we characterize Xe, SF 6 and Xe-SF 6 mixtures as detection media in gas detectors. In particular, given the electronegative nature of SF 6 , the goal is to explore the advantages that may derive from this feature. The study, which assessed electron attachment to SF 6 , estimated the conditions of the electron detachment process and investigated if charge multiplication is an amplification option using a single GEM. In the experimental system a pulsed xenon lamp, whose photons impinge on a transmissive CsI photocathode, originates near-thermal electrons, which attach to the SF 6 molecules and form negative ions. These anions drift under a uniform electric field established between the photocathode and a GEM, kept at 51.2 mm distance. The GEM may act as a charge collecting device or detachment/multiplication device. A set of two grids 6.15 mm and 14.15 mm below the bottom electrode of the GEM are independent charge collecting points. The gas filling is pure Xe at 800 Torr, or SF 6 and Xe-SF 6 mixtures (SF 6 fractions down to 0.10%) at 400 Torr total pressure. Results show that no electron signal is observed at the expected electron arrival time within the sensitivity of the system, indicating a near total electron attachment efficiency for all mixtures studied. Electron multiplication occurs after electron detachment from SF 6 − ions, at thresholds that depend on the SF 6 fraction in the mixture. In pure SF 6 , multiplication begins at V G E M / p = 826 ± 49 V/bar (E a v g = 680 ± 41 Td) with gains up to 2.8 ± 0.2 at V G E M / p = 1219 V/bar (E a v g = 1000 Td); reducing the SF 6 fraction lowers the threshold for multiplication and raises gains up to 38.8 ± 2.6 at V G E M / p = 647 V/bar (E a v g = 532 Td) for 0.10% SF 6 fractions. Calculated density-reduced ionization coefficients ( α / N ) were found to be field-dependent as expected. • Near-total electron attachment in Xe–SF 6 down to 0.10% SF 6 . • Electron detachment achieved using a single GEM. • Multiplication onset field decreases with decreasing SF 6 content. • Charge gain increases as SF 6 fraction decreases. • Trace SF 6 enables attachment with moderate amplification.