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The BTeV RICH
In the BTeV experiment, now defunct, our group was responsible for building the Ring Imaging Cherenkov detector (RICH). The purpose of the RICH is to distinguish p, K, and protons from one another. Cherenkov photons are produced at a characteristic angle, called the "Cherenkov angle", given by:
cos(qCh) = 1/bn,
where
b = v/c = p / Ö(p2+m2)
is the relative velocity (v) of the charge particle compared to the speed of
light (c), and n is the index of refraction of the medium.
Hence, if one measures the momentum of the charged
particle, as is done in the charged particle tracking system, and uses the RICH
to measure the Cherenkov angle, qCh,
one
can infer the mass. In reality, due to finite resolution, one obtains a
"likelihood" for each particle type, ie., p,
K, p, e, m, and then one can select the most likely
hypothesis, for example.
The deign of the BTeV RICH is shown below. A dual radiator system (1 cm liquid C5F12
and 3 meters of C4F8O gas) was proposed which
provides excellent particle separation over the full momentum range of interest
(3-70 GeV/c). Particles from the interaction point (from right to left as
shown) pass through these radiators and produce Cherenkov photons. The photon
from the C5F12 emerge at large angles due to the
large value of n~1.24, and are detected using an array of 5000 conventional
3" photomultiplier tubes which line 6 sides of the vessel. Photons produces
in the C4F8O gas are reflected from a spherical mirror and
focused onto an array pf multi-anode photomultiplier tubes.
