Ion mobility studies in SF6 and Xe–SF6 mixtures

Within the scope of a broader study on the possibility of using negative ions as charge carriers on gas detectors, the transport properties of negative ions in Xe–SF 6 gas mixtures were studied under low reduced electric field conditions, focusing on the determination of reduced ion mobility. Measurements were performed at a total pressure of 400 Torr and reduced electric fields between 0.70 and 1.80 Td (typical values of the electric field on the drift region in these detectors), for gas compositions ranging from pure SF 6 to 0.10% dopant-level concentrations in Xe. Negative ions, produced by electron attachment to SF 6 molecules near a photocathode where they are created, drift through a uniform electric field, allowing the determination of their arrival times. The reduced mobility of SF 6 − ions was found to be independent of the reduced electric field over the investigated range, confirming operation in the low-field regime. For pure SF 6 , a reduced mobility of K 0 = 0 . 536 ± 0 . 010 cm 2 /(V ⋅ s) was determined, which is consistent with the literature. A systematic increase in reduced mobility was observed as the SF 6 fraction decreased, consistent with ion transport being dominated by binary ion–neutral (and cluster–neutral) collisions and the general trend well described by Blanc’s law. For the lowest fraction of the electronegative gas (0.10%), a value of K 0 = 0 . 732 ± 0 . 016 cm 2 /(V ⋅ s) was observed. From the measured mobilities, momentum-transfer collision integrals Ω were determined, showing a significant decrease from Ω = 234 ± 5 Å 2 in pure SF 6 to Ω = 176 ± 4 Å 2 in Xe-rich mixtures, reflecting the suppression of resonant charge-transfer interactions. At low reduced electric fields, the more pronounced asymmetries in the arrival-time distributions observed suggest the presence of heavier negative-ion species, tentatively attributed to SF 6 − (SF 6 ) cluster ions. The measured mobilities for SF 6 − (SF 6 ) are in good agreement with previously reported values: K 0 = 0 . 487 ± 0 . 010 cm 2 /(V ⋅ s) in pure SF 6 , and K 0 = 0 . 652 ± 0 . 011 cm 2 /(V ⋅ s) in 0.10% SF 6 ) and the calculated collision integrals match those derived from other authors, indicating that ion transport remains close to thermal equilibrium under the conditions studied. • Negative-ion mobility measured in Xe–SF 6 mixtures at p = 400 Torr, for E / N = 0 . 7 –1.8 Td. • Field-independent mobility confirms operation in the low-field regime. • Two ion species identified from asymmetric arrival-time distributions: SF 6 − and SF 6 − (SF 6 ). • Reduced mobility follows Blanc’s law across mixture compositions. • Collision integrals indicate suppressed charge transfer in Xe-rich mixtures.