Click HERE for a full view of the tracking chamber. |
A closeup of the original tracking chamber used between
1986 and 1996, currently on display in the CDF assembly
hall. Several panels of the metal skin that encased the
chamber were removed so visitors could see the wires
inside, and lights were added below to make
those wires more visible. Even the physicists who built
it were surprised to see this side of their creation;
they had not added the wires to the detector
until the outer skin was complete.
(Click for larger image.)
The Central Outer Tracker (COT) fills the remainder of the 3
square meters (nearly ten feet) of magnetic field—which is to say, most of it.
Like the silicon detector, the COT tracks charged particles as
they pass through it, leaving a trail of ions and electrons in their
wake (see ionization
But instead of layers of silicon, the COT
uses a chamber full of argon and ethane gas, with tens of thousands
of wires strung across it to “catch” those electrons and
transport the information to computers for processing.
If you look carefully at the picture to the right, you may notice that there
are two kinds of wires. The thinner ones are called sense wires
Their job is to pick up electrons that are released from the atoms
in the gas when a particle flies past. The thicker field wires
positive ions that are left behind when the electrons get ripped off. Engineers apply a more negative voltage
to the field wires than to the sense wires, creating an electric field
that causes electrons to move in one direction (toward the more positive
sense wires) and positive ions to move in the other (toward the more negative field wires).
An electron, once released, will travel toward the nearest sense wire.
A section of one of the endcaps on the original tracking chamber.
Notice the rows of gold-colored knobs along every other concentric layer.
These knobs line up with the sense wires
inside the detector; this is where the
readouts for the sense wires would be
located. (Click for larger image.)
A dynamic duo: Silicon and Gas
While the silicon detector and COT are both used for tracking charged particles,
the silicon is more precise than the COT. You might wonder, then,
why physicists didn’t use silicon to fill the rest of
the 3-meter diameter of magnetic field. The answer? It costs a lot less to fill a
large space with argon and ethane gas than it does to use silicon,
so physicists use the silicon
where they need it most—as close as possible to the collision point,
where particles are so close together that two particles can easily look like one.
Further away, in the region of the COT, the
particles formed in the collision have spread out, so scientists don't need
the same kind of accuracy to distinguish one particle from another.
Physicists want to pack as many wires as they can into the COT.
The denser your forest of wires, the less time it takes for an electron
to reach the nearest one. And with eight million collisions happening every second,
the faster the electrons reach the wires, the better your chances of distinguishing the
particles produced in one event from those produced in the next. The space between the wires in the original tracking chamber
(featured in the images on this page) is 5 millimeters (0.197 inches); in the tracking chamber currently used at CDF,
the wires are only 3.56 millimeters (0.140 inches) apart.