Time Projection Chambers
A time projection chambers is a particle detector that detects and identifies particles by recreating their track as they travel through a volume filled with gas. The chamber is placed in a large magnetic field causing the trajectory of any charged particles to curve. A large electric field is established by applying a high voltage across the chamber. Charged particles traversing the chamber ionize the gas producing ion electron pairs, with the ions drifting towards the electrode, and the electrons drifting towards the end plates. Traditionally, a TPC has many thin wires that act to amplify the drifting electron so that the electronic signal can be detected. When the electron cloud hits the wires, electronics can be used to determine its position using an anode readout system that samples the induced cathode charge. By also knowing the time that it took for the electrons to drift through the chamber and reach the wires, the track of the initial ionizing particle can be determined in three dimensions. Studying the curvature of this reconstructed track reveals the charge and momentum of the particle.
While the electric field is fairly uniform throughout the chamber, in close proximity to the endplate wires, the field lines begin to bend significantly. The magnetic field, which was initially parallel to the electric field, now results in what is known as the ExB effect. This effect, which is a significant systematic problem inherent to the wire/end cap design, results in measurably worse spatial resolution. Research at Carleton University aims to almost entirely eliminate the ExB effect by replacing the wires with Micro-Pattern Gas Detectors and modifying the end cap to use a resistive anode readout structure.
Full studies on position sensing from charge dispersion: