Cerenkov Detector
Outside the drift chamber is the Cerenkov detector, or more accurately, the Cerenkov Ring Imaging Detector (CRID). It is used to measure particle velocities through Cerenkov radiation. This form of radiation is somewhat like a sonic boom except it is light rather than sound. It occurs when a particle travels through a medium (here, freon gas) at a speed that is faster than the speed of light in the medium (but slower than the speed of light in a vacuum, of course), just as a sonic boom occurs when an object travels in a medium (air) faster than the speed of sound in the medium.
This two-step device converts light emitted by a particle in
the first box
into a ring of charge in the second box. Different speeds of
particles
can be identified by the different sizes of ring.
On entering the CRID, a particle passes through a small container of liquid Freon with just the right density to give it the desired optical properties. The particle radiates ultraviolet light in a cone-shaped pattern, with slower particles making narrower cones (note). The original particle carries on undisturbed and the light passes into another vessel containing ethane gas with a trace amount of an exotic chemical called TMAE that absorbs the light, leaving a ring of electric charge in its place.
The particle speed is related to the diameter of the ring, which now must be measured. After drifting the length of the box, the ring crosses a picket fence of high-voltage wires that produce signals. A computer analysis of when and where these signals originated allows one to reconstruct the ring - both its size and where in the chamber it was formed.
In the analysis of an event, the computer follows the tracks from the drift chamber through the CRID and looks up the size of the rings associated with each track. This allows it to calculate the particle's velocity.
Once both velocity and momentum (from the drift chamber) are known, the mass of the particle can be calculated and this identifies what kind of particle it was.
Note: This relationship
is defined by
where n is the refractive index of the material and ß
is the velocity of the particle in units of c (speed of light).
For
,
cos(cone angle) = 1 or angle = 0. So, as ß increases
(particle moves faster), the cosine of the angle decreases.
This means that the angle increases with the speed of the particle.
