Air Cerenkov Detectors
While a typical gamma-ray
detector must be flown with a balloon or on a satellite
above the Earth's atmosphere
to avoid absorption of the gamma-ray photon, the Air Cerenkov telescope uses this
problem to its advantage, by making the atmosphere part of the
detector. When gamma rays encounter Earth's atmosphere, they create an
"air shower." This process involves the original photon undergoing a pair
production interaction high up in the atmosphere, creating an electron
and positron.
These particles then interact, through
bremsstrahlung and Compton
scattering, and give up some of their energy to create energetic
photons. These in turn create more electrons, resulting in a
cascade of electrons and photons that travel down through the
atmosphere until the particles run out of energy.
These are extremely energetic particles, which means that they are
traveling very close to the speed of
light. In fact, these particles are traveling faster than the speed
of light "in the medium of the atmosphere." Remember that nothing can
travel faster than the speed of light "in a vacuum," but that the speed of
light is reduced when traveling through most materials (like glass,
water and air, for example). The resulting polarization
of local atoms as the charged particles travel through the atmosphere
results in the emission of a faint, bluish light known as "Cerenkov
radiation", named for
Pavel
Cerenkov, the Russian physicist who made comprehensive studies of
this phenomenon.
Depending on the energy of the initial cosmic gamma ray, there may
be thousands of electrons/positrons in the resulting cascade that are
capable of emitting Cerenkov radiation.
As a result, a large "pool" of Cerenkov light accompanies the particles
in the air shower. This pool of light is pancake-like in appearance,
with a thickness that is approximately 1/200 the size of its diameter.
Air Cerenkov detectors, as the name implies, rely on the detection of
this pool of light to detect the arrival of a cosmic gamma ray.
Basic operating principles
Air Cerenkov detectors begin with one or many large optical
reflectors,
and are usually placed at mountain sites where standard optical
observatories might be located. The mirrors used can be of lesser
quality than those used in optical telescopes, since they are
reflecting the light of this large local pool rather than directly imaging an
astronomical source. The Cerenkov light reflected from this mirror is
then detected in the focal plane by one or many photomultipliers that
convert the optical signal into an electronic signal to record the
gamma-ray event. The light in this pool is very faint and can only
be detected cleanly on dark, moonless nights. Even so, it helps that
the total pool passes through the detector in only a few nanoseconds.
This allows further separation of the faint signal from the ambient
light from the rest of the night sky.
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| The pioneering Whipple Observatory
Air Cerenkov detector in Arizona
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Once the light has been detected in a phototube, fast electronics
are used to record the signal. Many modern detectors use an array of
100 or more small phototubes in the focal plane rather than a single
phototube. In this way, a crude image of the Cerenkov light pool is
recorded. This is very important because these detectors, in addition
to detecting cosmic gamma-ray photons, detect a large cosmic ray
background. Cosmic ray protons and nuclei interact in the atmosphere
in much the same way, creating their own Cerenkov light pools. These
showers induced by cosmic rays come uniformly from all parts of the sky
and mask the desired photonic signal. Less than 1% of the events
detected are due to photons. The rest are cosmic rays.
The latest generation of Air Cerenkov detectors have worked around
this problem through the technique of imaging. Simulations of air
showers show that the light collected from gamma-ray primaries differs
from that produced by cosmic ray primaries in a few fundamental ways.
The Cerenkov light collected from a gamma-ray shower has a smaller
angular distribution and tends to have an ellipsoidal shape that aligns itself with the
direction of the incoming photon. Cosmic-ray induced air showers, on
the other hand, have Cerenkov light images that are much broader and
less well aligned with the arrival direction. By measuring the shape of
each shower image, and selecting only those events that are
gamma-ray-like in appearance, nearly all the cosmic ray contamination
can be removed, resulting in a much improved ability to detect an
excess number of counts from the source direction.
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The particles in an air shower (above) are
much more widely distributed for proton showers (shown on right) versus
gamma-ray showers (shown on left). This is
reflected in the corresponding distribution of photons in the detector
(shown below).
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Detector characteristics
A major difficulty of this technique is trying to determine the energy
of the incoming photon. Researchers don't have the luxury of calibrating
their instruments at an accelerator or other laboratory. As a result,
simulations are required to estimate both the collection area and the
energy response of these telescopes. One of the nicest properties of
these telescopes is that their collection area is not the size of the
mirror, but the size of the pool of Cerenkov light. As long as the
detector is somewhere in the pool, it can detect the event. In some
sense, the detector is smaller than the event. As a result, these
telescopes have collecting
areas that are much higher than for typical satellite-borne
telescopes. This is important because the number of photons emitted by
a typical source decreases as energy increases. Hence, these
TeV energy photons are rather rare and a large collection area is
important.
Incident photon energy is not well determined. Simulations
typically show that the energy of the incoming gamma ray can be
estimated to about 30-40%
accuracy. Unfortunately, the absolute energy threshold must also be
determined through simulations. However, time resolution is good
because the arrival time of the shower can be determined at the
sub-millisecond level. Most detectors have relied on "on/off"
observations to detect a source. In this mode, the detector looks at
the region of the sky containing the source of interest for some period
of time, then alternates with a background region. An excess of events
in the on/off observations indicates a source detection. However, the
advent of imaging detectors has changed this, and it is becoming
possible to detect a source in the field-of-view without background
subtraction. In this mode, sources can be located to within a few
arc-minutes, good even by satellite-borne
gamma-ray instrument methods.
Detectors
As with more conventional detectors, larger is better. Researchers are
looking into arrays of reflectors that cover areas on the order of
hundreds of meters on a side to greatly increase the collecting area,
improve the measurement of image parameters and decrease the energy
threshold of these instruments.
VERITAS (Very
Energetic Radiation Imaging
Telescope Array System), located at Fred Lawrence Whipple Observatory
in southern Arizona, uses an array of seven 10m optical reflectors
for
gamma-ray astronomy in the energy range of 50 GeV - 50 TeV. VERITAS
began operation in April 2007.
MAGIC
is composed of two imaging atmospheric Cerenkov telescopes, located on the
Canary island of La Palma off the African coast. It is run by the IAC, and is composed of
an assemblage of nearly 1,000 individual mirrors with an active surface
of 234 square meters. It is sensitive to air showers of lower energy,
and achieved a low threshold of 25 GeV.
Milagro, which was
built by
a collaboration headed by Los Alamos National Laboratory, uses a 5,000
square meter pool of water (about the size
of an American football field) covered with a
light-tight barrier as a Cerenkov
detector to study TeV gamma rays and cosmic
rays. Just like the charged particles emitted Cerenkov light in air,
they do the same thing in water. The principle behind Milagro is that
the index of refraction of these particles in the water is much greater
than that in air, because it is much denser.
Though no longer in operation,
STACEE (the Solar
Tower Atmospheric Cherenkov Effect Experiment) was an example of how
light-collecting apparatus might serve a dual purpose in research. The
experiment took place from October 2001 to June 2007 and used the large
field of 220
solar heliostat mirrors as its primary collection mirror at the
National Solar Thermal Test Facility (NSTTF) of Sandia National
Laboratories. During the
day, the NSTTF was used for solar energy research. STACEE used the
array at night to collect and study Cerenkov light that resulted
from gamma ray air showers.
Last Updated: November 2010
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