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Overview of the BTeV RICH and Testbeam Results
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 / (p2+m2)1/2
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 a 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, i.e., p,
K, p, e, m, and then one can select the most likely
hypothesis, for example.
The design 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 liquid radiator emerge at large angles due to
its large value of n~1.24, and are detected using an array of ~5000 conventional
3" photomultiplier tubes which line six 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. We expect about 15
and 65 detectable photons from the liquid and gas radiators, respectively.
A closeup view of some possible 3" tubes being tested are shown here:
Here are the some initial bench test photos of 8 MAPMTs on a single high voltage baseboard.
 |
On the left you see 8 MAPMTs connected to a
high voltage divider board (capable of handling up to 16 MAPMTs). To set
the scale, the front window of the MAPMT is about 1" by 1" in
size. The MAPMTs are read out using a hybrid containing 2 custom
low-noise ASICs (Application Specific Integrate Circuits) and an FPGA
which controls the data flow.
Each MAPMT contains a 16 (4x4) independent photomultiplier readouts, so the effective pixel size is 6 mm x 6 mm.
|
The conceptual design was put to the test in a beamtest in the Summer of 2004.
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Schematic of the testbeam box. It contains two "arms". The beam particles enter down the central arm, are reflected off a spherical mirror, and are focused onto an array of MAPMTs (top-center). The mirror tilt was adjustable from the outside of the box via the three ports on the back. |
Here is what it actually looked like ! |
|
Here, is the array before and after it was populated:
| Here is the MAPMT array, showing the HV
baseboard array before being populate with MAPMTs.
|
Here is the array after populating it with
MAPMTs. The MAPMTs were arranged according to the expected position for
detection of the Cherenkov ring.
|
And here are the beautiful results obtained
 |
This is the cumulative distribution of many
Cherenkov rings on top of one another. Each square represents a sinle
MAPMT cell (6 mm x 6 mm). The size of the box is proportional to the number of photons detected in that cell. The Chernkov ring is clearly detected and the resolution and number of photons per track was shown to be in good agreement with the Monte Carlo simulation. The results of this work have been published. |