Measurements of Drift Properties in Negative Ion Gas Mixtures for TPCs

Detectors capable of reconstructing the direction of electron and nuclear recoils have many promising applications such as experimental searches for dark matter, neutrino-less double beta decay, and the Migdal effect. Many of the current and proposed experiments involve gaseous time projection chamber detectors (TPCs) operating at pressures where particle tracks can be resolved. As diffusion can be a limiting factor, especially at low energies, the use of negative ion drift (NID) in the thermal regime by introducing electronegative gases, has been demonstrated as a promising solution. The slow drift speeds also provide the potential for a high resolution reconstruction along the drift direction as well as the ability to count individual primary electrons to improve the energy resolution. One of the main challenges, however, is that the detection target and the NID component are not the same, so designing a gas mixture that maximizes the former is desirable. Therefore, we have studied various mixtures containing targets such as CF4 and the noble gases Ar and He, doped with small amounts of the electronegative gas CS2. I will present measurements of gas gain, drift velocity, and diffusion, as a function of CS2 concentration in these mixtures. I will also report on progress towards primary electron counting in gas mixtures using CS2 for NID.