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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.

Physics of Air Cerenkov detection

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.

The Whipple Air Cerenkov Telescope
The pioneering Whipple Observatory Air Cerenkov detector in Arizona

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.

A vertical cross-section of gamma-ray and proton air showers
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).
Distribution of photons from air showers

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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This page last updated: Thursday, 31-Mar-2011 15:45:07 EDT