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People
and institutions that
designed and built Milagro
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Milagro: A Water Cherenkov Extensive
Air Shower Array
Just as relativistic charged particles generate Cherenkov light in
air, they also emit Cherenkov light when they pass through water. However,
there are several key differences that make a detector such as Milagro
possible.
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The index of refraction of water is much bigger than
that of air. This leads to two important effects.
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The number of Cherenkov photons per unit track
length of charged particles is much greater in water than in
air (by a factor of about 1400).
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The Cherenkov angle in water is 41o,
while in air it is only 1o.
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The gamma rays in the air shower interact much more quickly in
the water (simply because it is dense), and create high-energy electrons
and/or positrons. These charged particles then emit Cherenkov light
that can be detected.
Since the Cherenkov cone has such a large opening angle a sparse array
of light detectors (PMTs) can detect nearly every particle that enters
the pond. If the spacing of the PMTs is less then twice the depth of
the water above the PMTs there are no "holes" in the detector.
Below is a schematic view of the Milagro detector.
Milagro Operations
Milagro construction was completed in January 1998. Engineering data
was taken between February and August 1998. We took nearly 5 billion
events during this run. During this time the data acquisition system
was completed, the online reconstruction of events was implemented,
and the detector calibration was begun. During this time Milagro took
data at rates of up to 2000 events/second. At this event rate the data
throughput rate is about 5 Mbytes/second. While this can be stored on
tape we do not possess the necessary resources (money) to do so. Thus,
we must be able to reconstruct the events in real time so that we can
store the reconstructed parameters to tape. The reconstructed parameters
include the event time, the direction of the event, and information
to help us determine the energy and nature of (gamma ray or nucleus)
of the primary cosmic ray.
In early August the detector was turned off to began repairing the
underwater connectors. This entailed scuba diving operations to raise
all 273 of the bottom layer PMTs, applying a patch to the connectors
and more scuba diving operations to reinstall the PMTs. Pictures from
this repair operation may be found on the Milagro
in Pictures page.
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