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8. Gamma-ray

Gamma-rays are the highest energy, shortest wavelength electromagnetic radiations. Usually, they are thought of as any photons having energies greater than about 100 keV. (It's "gamma ray" when used as a noun, and "gamma-ray" when used as an adjective.)

The techniques needed to detect the highest energy photons have only become available since the late 1960's - a blink of the eye in terms of mankind's involvement in astronomical research. Gamma-rays simply pass through most materials and thus cannot be reflected by a mirror like optical or even X-ray photons. The tools of high-energy physics, however, are borrowed to detect and characterize gamma-ray photons and allow scientists to observe the cosmos up to energies of 1 TeV (1,000,000,000,000 eV, where an optical photon has an energy of a few eV) or beyond!

Unfortunately, gamma-ray detectors have to contend with a large contamination from cosmic rays. Cosmic rays - elementary particles which are come from all parts of the sky - often affect gamma-ray detectors in a similar manner to the source photons. This background must be suppressed in order to obtain a pure photonic signal. This is even more important when you consider that sources of cosmic gamma-rays are extremely weak and require long observations, sometimes several weeks, to get a significant detection or accurate measurement of a source.

Diagram of the EGRET spark chamberGamma-ray detectors can be placed in two broad classes. The first are what would typically be called spectrometers or photometers in optical astronomy. These are instruments which are "light buckets" and focus on a region of the sky containing the object of interest collecting as many photons as possible. These types of detectors typically use scintillators or solid-state detectors to transform the gamma-ray into optical or electronic signals which are then recorded. The second class are detectors which perform the difficult task of gamma-ray imaging. Detectors of this type either rely on the nature of the gamma-ray interaction process such as pair production or Compton scattering to calculate the arrival direction of the incoming photon, or use a device such as a coded-mask to allow an image to be reconstructed.

Gamma-ray detectors have come a long way, but the quest for better angular resolution (and therefore source identification) and spectral resolution (for more information on source behavior) is a continuing activity. Gamma-ray detectors are meant to measure the same things detectors at other wavelengths measure, but the challenge of working in this difficult energy range makes more demands on instrument developers than most other fields. Future detectors are beginning to use more advanced solid-state technology to overcome some of these problems and provide large, sensitive detectors which will further establish gamma-ray astronomy as an integral part of astrophysical research.

Specific types of Gamma-ray Detectors:

(Source: "Gamma-Ray Telescopes & Detectors" - "NASA's Imagine the Universe."

Topic Image: An image showing the gamma ray emissions detectable from Earth's Moon with Goddard's EGRET on board the Compton Gamma Ray Observatory. (NASA Photo PAO_E97_01.JPG)


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