Drift Chamber
Next layer out from the vertex detector is the drift chamber, a horizontal, thin-walled cylinder, six feet long and six feet in diameter, with a small tube through the center to accommodate the beam pipe and vertex detector.
Photograph of the endplate of the SLD drift chamber.
Some 35,000 fine wires are strung the length of the cylinder between precisely placed holes in the aluminum ends. When the chamber is filled with a gas mixture and high voltage is applied to groups of wires it becomes a giant set of Geiger counters.
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Charged particles passing through the chamber temporarily knock a few electrons loose from gas atoms. These electrons are attracted to certain wires, known as sense wires, knocking loose more electrons on the way to give a large electronic signal when the cascade of electrons hits a sense wire. |
 Close-up view of the end plate of a drift chamber before insertion of wires |
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The wires are arranged in layers that pass through the cylinder at three different angles. The set of wires that give a signal can be used to allow computer reconstruction of the paths (or tracks) of all the charged particles through the chamber. |
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The "drift" in the name of this chamber refers to the time it takes electrons to drift to the nearest sense wire from the place where the high-energy particle ionized an atom. Any three sense wires are only nearby in one place so a set of "hits" on these three fix a particle track in this region. By measuring the drift time, the location of the original track can be determined much more precisely than the actual spacing between the wires. |
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 The SLD's magnetic field causes this cosmic ray to curve in the drift chamber. |
The drift chamber measures a particle's position to a few thousandths of an inch at eighty steps along its path from the inner to outer wall. Because these charged particles are in the strong magnetic field of the SLD, the path is a curve. Knowing this curvature, one can calculate the momentum of the particle. |
