The first stop for these newborn particles is the silicon detector.
Seven concentric cylinders of silicon, arranged like barrels within
barrels, fit snugly around the
beam pipe. The innermost layer literally touches the beam pipe;
the outermost layer begins 28 centimeters (about eleven inches) outward.
Particles can pass through as many as all seven layers of silicon,
leaving a trail of ions and electrons in each layer. This trail is recorded as a
"hit" on that particular layer of silicon; by connecting the dots,
scientists can determine the path the particle took.
Because the silicon detector is located within a
particles (such as electrons
and charged hadrons
) are forced to curve in their paths.
The slower or less massive they are, the greater
the magnet's effect on them, and the more they curve.
Scientists therefore use the amount by which a particle's track curves to determine its mass
and how fast it is traveling.
This information helps them determine what kinds of particles were produced
immediately after the proton and antiproton collided.
Why doesn’t the silicon stop the particles?
Well, it may stop some of them. But remember, even dense matter like a
metal is mostly empty space to a subatomic particle.
Each layer of silicon is extremely thin— 1/85th of an inch thick.
Further outward on the particle’s journey through the detector, in areas
specifically designed to stop the particles, thick layers of steel or
lead are interwoven with the detection equipment. The thickness of the
metals increases the chances that a particle zipping through will be stopped
by one of the particles in the metal.
|Sometimes a particle will travel as far as a millimeter before it decays,
but often the distance is much smaller.
The silicon detector has to be sensitive enough to tell whether a particle's track
leads directly back to the point of collision, or whether it is offset by some fraction of a millimeter.
In this way we can tell whether a particle has come from the original collision,
or from the decay of another particle that was produced in that collision.
|The outermost layer of the silicon detector is two meters (about six-and-a-half feet)
long. At the bottom is a one-meter ruler for scale.|
Ionization: the smoking gun
|Strips etched into the surface of the silicon wafer collect charge from ionized
electrons and channel it to the readout electronics.
Watch an ANIMATION of the process of ionizaton.
When a charged particle passes through a layer of silicon, it
disturbs the electrons in the atoms of silicon,
attracting or repelling them (depending on whether it is a
negatively or positively charged particle) and knocking
the electrons out of their orbits. This is called ionization
and the silicon atoms that lose their electrons are called ions.
Driven by the force of the electric field produced by a difference in voltage between the top and bottom layers of the silicon wafer,
the newly liberated electrons are pulled toward the more positive edge of the silicon. Their energy is then carried
along strips etched into the wafer's surface to readout electronics that record their charge
as a signal that something passed by.
Neutral particles (such as photons and neutrons) do not ionize the atoms
in the silicon. The result is that neutral particles shoot straight
through the silicon without leaving a trace. They don't get seen by the
CDF equipment until later in the journey
outward from the beam pipe.