Cosmic Ray Tests of the Prototype TPC for the ILC Experiment

TL;DR

This study evaluates the performance of a prototype TPC for the ILC, demonstrating GEM readout's advantages over MWPC under high magnetic fields and exploring optimal gas mixtures for improved resolution and stability.

Contribution

The paper provides the first cosmic ray test results of a prototype TPC with GEM readout under a 4 T magnetic field, highlighting its superior resolution and gas mixture options.

Findings

GEM readout is unaffected by E x B effects at 4 T, unlike MWPC.

GEM-based TPC achieves diffusion-limited spatial resolution.

Ar-CF4-isobutane mixture shows promise for stable operation under strong magnetic fields.

Abstract

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.