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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.

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 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.

The Whipple Air Cerenkov Telescope
A picture of the pioneering Whipple Observatory Air Cerenkov detector

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.

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 versus gamma-ray showers. This is reflected in the distribution of photons in the detector (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 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.

Artist's impression of the Telescope Array Project
An artist's impression of the Telescope Array Project

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