A 30 cm TPC prototype that the ILC Canada group built and used in magnetic field tests at TRIUMF and DESY.

The team of scientists from Canada, Germany, France and Japan who tested the two small TPC prototypes at KEK last fall.

Although it sounds like a device used on Star Trek, a Time Projection Chamber (TPC) is a gas-filled cylindrical chamber that acts like a three-dimensional electronic camera, making a photo-copy of a particle track as it flies through the detector. For about a decade now, a group of scientists in Canada has been developing and testing Micro-Pattern Gas Detectors (MPGD), contributing to the worldwide R&D for a high resolution TPC tracker for the future International Linear Collider.

Similar to a bubble chamber but with electronic readouts at either end, the TPC was invented by Dave Nygren of Lawrence Berkeley Laboratory in the 1970s. The TPC is a mostly empty, gas-filled cylinder with proportional wire readouts at each end. Any charged particle passing through the cylinder liberates electrons from the gas molecules along its path, and due to the presence of a strong electric field, the trail of electrons drift to the wires at each end. By measuring the position of these electrons on the endplate and the time they take to arrive, physicists can determine the momentum of the charged particle and reconstruct the particle trajectory in three dimensions. "The TPCs could make measurements quite well in those early days, but they never did make measurements as well as they could in principle," said Carleton University's Madhu Dixit.

Dixit has spent the past 15 years developing MPGDs for high energy physics and other applications. In the early eighties, a group from the National Research Council of Canada and Carleton University built the first TPC ever to be used in an experiment. About five years ago, the Carleton group teamed with the University of Victoria and the University of Montreal to actively start developing a TPC for the ILC. As a result of this Canadian collaboration combined with contributions from other international R&D groups, a TPC with MPGD readout is part of the design for the central tracker for two of the proposed detector design concepts for the ILC.

Having advantages over the conventional wire chambers used in traditional TPCs, the MPGD readout better preserves the spatial information of the drifting electrons, resulting in a much more precise tracker. "Because of its simplicity and its ability to measure a large number of three dimensional points with good resolution, there are many compelling reasons to use a TPC for charged particle tracking at the ILC," said University of Victoria's Dean Karlen. "A key requirement for the TPC is for the curvature of the particle tracks coming out of the event to be measured with extraordinary precision. This curvature tells us the momentum, and so far the results from cosmic-ray testing and small prototypes show us that it works." To reach the physics goals, the TPC must make about 200 position measurements along the particle track, each with a precision of about 0.1 mm.

Tests of small prototype TPCs built by the University of Victoria and Carleton University confirm the viability of using a TPC for the ILC. Proving that the TPC technology works on a small-scale prototype, however, is only the first step. Physicists must also prove that it can work on a much larger scale. For the ILC, the TPC tracker will be approximately 5 metres long and have more than one million readout channels at either end. Because more readout channels equate to larger costs (not to mention a spider web of wires), the Carleton group has been working to develop a new TPC readout technology of charge dispersion to reduce this number.

Already demonstrated in the small prototype tests, charge dispersion uses a special high resistivity MPGD anode to disperse the track charge in a controlled way. As the track charge disperses, physicists can measure its position precisely using relatively wide pads – hence reducing the number of channels. "It is a novel idea," said Carleton University's Alain Bellerive. "The goal of this charge dispersion technique is to reduce the number of channels and maintain the same high resolution that a conventional MPGD readout can achieve. The charge dispersion TPC readout will be less complex and cheaper."

Last fall, research groups from Germany, Japan, France and Canada collaborated to test two small charge dispersion prototypes – a 15 cm TPC built at Carleton University and a 30 cm TPC built at the Max-Planck Institute in Munich -- in a 4 GeV beam at KEK. "We got the best resolution that has ever been achieved with a TPC," Dixit said. The performance exceeded the 0.1 mm goal. In 2003 and 2004, the Canadian ILC group tested their 30 cm TPC prototype in magnets at TRIUMF and DESY and the results, which achieve the 0.1 mm goal, have recently been published.

The Canadian ILC group plans to continue conducting TPC R&D and will send another small prototype to DESY for testing this summer. They are also active participants in the international collaboration that is now drafting a proposal to build a larger TPC prototype. Discussions on this proposal will occur at the next Linear Collider Workshop in Vancouver this July. "This is a new idea, and it has to be proven every step of the way that you can build a large system," Dixit said. "So far, things look optimistic."

-- Elizabeth Clements