Air Cerenkov Detectors
Air Cerenkov telescopes represent an interesting and challenging type of
gamma-ray
detector
technology. While a typical 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 nullifies this problem by
making the atmosphere part of the detector! Gamma-rays interacting in the
atmosphere create what is called an air shower. This describes the process of
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, giving up some of their energy to creating energetic photons,
which in turn pair produce creating more electrons which then
bremsstrahlung..., well, you get the idea. The result is a cascade of
electrons and photons which 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 media (like glass,
water, air, etc.). The resultant
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 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 which 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, about 200
meters in diameter
but only a meter or so in thickness. 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,
usually at mountain sites where you might also find standard optical
observatories. The mirrors used can be of reduced quality as compared to
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 which 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 night
sky.
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A picture of the pioneering Whipple Observatory Air Cerenkov detector
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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 cosmic ray induced showers come uniformly from all parts of the sky and
seriously mask the desired photonic signal. Less than 1% of the events
detected are due to photons rather than 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 which aligns itself with the direction of the
incoming photon. Cosmic-ray induced air showers, on the other hand, have
Cerenkov light images which are much broader and less well aligned with the
arrival direction. By measuring the shape of each shower image, and selecting
only those events which 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 versus gamma-ray showers. This is reflected in the
distribution of photons in the detector (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 in order 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 around 108 cm2,
much higher than for typical satellite-born telescopes. This is important
since the number of photons emitted by a typical source decreases as energy
increases, so
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. Time resolution is good, however, since 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 versus off observations indicates a source
detection. The advent of imaging detectors has changed this, however, 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.
Future developments
As with more conventional detectors, larger is better. Many researchers
are looking into arrays of reflectors which 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), being built by a collaboration headed by the
Whipple Observatory, uses an array of seven 10m optical reflectors for
gamma-ray astronomy in the energy range of 50 GeV - 50 TeV.
The Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE)
will use as its primary collection mirror the large field of 220
solar heliostat mirrors at the National Solar Thermal
Test Facility (NSTTF) of Sandia National Laboratories. During the
day, the NSTTF is used to for solar energy research. STACEE will use
the array at night to collect and study Cerenkov light that result
from gamma ray air showers.
Milagro, being built by a collaboration headed by Los Alamos National
Laboratory, will use a 5000 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 trillion electron-volt (TeV) gamma rays and cosmic rays.
In addition, it could prove possible to further enhance
the ability to distinguish gamma-ray from cosmic-ray showers based on the
spectral content of the shower light or on a timing analysis of the shower
development. |
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