On the properties of a negative-ion TPC prototype with GridPix readout

The performance of a GridPix detector to read out a negative ion TPC was studied using a module with four GridPix chips that are based on the Timepix3 pixelated readout ASIC. The quad module dimensions are 39.6 mm$\times$28.38 mm, and the maximum drift distance is 40 mm. The TPC is operated using a 93.6/5.0/1.4 gas mixture (by volume) of Ar/iC₄H₁₀/CS₂ with a small amount of oxygen and water vapour at a pressure of 1030 mbar and a temperature of 297 K. Tracks were produced by a pulsed $N_2$ laser. The GridPix chips are sensitive to single drift ions, and allow for the determination of the drift distance using the velocities of the different ion species. The 1.56 ns time resolution of the Timepix3 chips allows for a precise determination of the drift properties in the longitudinal direction. The measured mobility of majority ion charge carriers is (1.391 $\pm$ 0 003) cm²/V/s. Using the high granularity pixel readout, the transverse and longitudinal diffusion coefficients were measured to correspond to an effective thermal diffusion temperature of 314 K and 384 K respectively. For 429 detected ions, the precision on the absolute drift distance is expected to be 1.33 mm for a mean drift distance of 20 mm.