DF Thesis

Theory

Drift Chambers

Typical Construction and Principle of Operation

Drift chambers are charged particle detectors which are designed to collect data which allows for the reconstruction of the particle's path through the detector. This is important as it ultimately allows for the measurement of a variety of quantitites of scientific interest by means of inverse kinematics.

Typically, drift chambers are constructed out of a chamber filled with an ionizable gas and charged wire planes. The wire planes are arranged to be ideally perpendicular to the particle's direction of motion and come in two alternating types: cathode planes and sense planes. A cathode plane is designed such it creates a nearly-uniform static electric field which will drive electrons towards the sense planes. A sense plane is constructed with alternating anode and cathode wires. The anode wires are the sense wires, i.e. where the electrons ultimately end up and create an electronic signal. That electronic signal propagates away from the location of the hit in both directions and is read by a pair of coupled timers at either end of the wire. The "field-shaping" cathode wires within a sense plane are to prevent electrons from being caught roughly in the middle of two anode wires and thus taking a significant amount of time to reach either of them.

The general principle of operation is as follows. A charged particle moves through the detector and ionizes molecules along its path. The electrons, which have been separated from their ions, are now accelerated towards the anode wires (sense wires). If the anode wire is very thin, the electric field near to it becomes very strong, causing the electrons to pick up a great deal of speed and cause a Townsend avalanche. A Townsend avalanche is a cascading series of ionizations which help to amplify the signal. This signal is a voltage pulse traveling towards either end of the wire from the point where the electrons hit. Using the coupled timers at the ends, it is then possible to use the difference in time to calculate the position along the wire of the hit.

This position along the axis of the wire is good, but it is ultimately only one dimensional information about a particle traveling through 3D space. It is, however, possible to couple the timing information from multiple nearby sense wires and electron drift time information to calculate the distance of closest approach (doca) to each of them. This then gives a position along the axis of each wire as well as a radius perpendicular to that axis at that point. If all of this information is known perfectly, then the path of the particle will be tangent to the circle defined by the aforementioned radius for each wire, ultimately giving a full reconstruction of the path of the particle through the detector.

Addition of a Magnetic Field

The inclusion of a magnetic field into a drift chamber allows for the reconstruction of not just the path of the particle, but also the magnitude of its momentum. A uniform magnetic field perpendicular to the particle's direction of motion, for example, would cause the path to bend into some section of a circle, thus changing the expected hit position along the wires of the sense planes. Using these hits, it is then possible to reconstruct the radius of curvature of the path. With the assumption the particle carries the usual elementary charge, it is then possible to back out the particle's momentum as shown below.

<Insert Central Force = Magnetic Force Equation Here>