Diffusion and drift studies of Ar-DME/CO2/CH4 gas mixtures for a radial TPC in the E > B field

We have performed systematic studies of electron diffusion and drift in three gas mixtures: Ar(50%)-DME(50%), Ar(50%)CO2(50%), and Ar(90%)CH4(10%) inside the E > B field, with the aim to build a high-resolution radial Time-Projection-Chamber (TPC). Our TPC design is unconventional in that: (1) it drifts electron clusters in a radial electric field perpendicular to a magnetic field; (2) it employs microstrip gas chamber (MSGC) with pads for readout. To achieve the required spatial resolution and two-track resolution, we find that one must use a gas mixture based on a “cool” gas such as CO2 or DME (dimethylether (CH3)2O). Such a gas mixture has low diffusion coefficients and small Lorentz angle in the E > B field. Its transport parameters are less sensitive to the magnetic field. Our measurements are compared with two transport calculations. For the Ar(90%)CH4(10%) gas mixture, the calculations based upon the moments method agree well with our measurements, while the calculations using the Magboltz program largely over-predict our measurements for transverse diffusion and Lorentz angle. For the Ar(50%)-DME(50%) and Ar(50%)CO2(50%) gas mixtures, which have slow drift and low diffusion, our measurements are consistent with calculations based upon either the moments method or the Magboltz program.