  A time projection chamber (TPC) is a strong candidate for the central tracker
of the international linear collider (ILC) experiment and we have been
conducting a series of cosmic ray experiments under a magnetic field up to 4 T,
using a small prototype TPC with a replaceable readout device: multi-wire
proportional chamber (MWPC) or gas electron multiplier (GEM). We first
confirmed that the MWPC readout could not be a fall-back option of the ILC-TPC
under a strong axial magnetic field of 4 T since its spatial resolution
suffered severely from the so called E x B effect in the vicinity of the wire
planes. The GEM readout, on the other hand, was found to be virtually free from
the E x B effect as had been expected and gave the resolution determined by the
transverse diffusion of the drift electrons (diffusion limited). Furthermore,
GEMs allow a wider choice of gas mixtures than MWPCs. Among the gases we tried
so far a mixture of Ar-CF4-isobutane, in which MWPCs could be prone to
discharges, seems promising as the operating gas of the ILC-TPC because of its
small diffusion constant especially under a strong magnetic field. We report
the measured drift properties of this mixture including the diffusion constant
as a function of the electric field and compare them with the predictions of
Magboltz. Also presented is the spatial resolution of a GEM-based ILC-TPC
estimated from the measurement with the prototype.
