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Gamma-Ray Telescopes & Detectors

Gamma-ray astronomy is a late bloomer. The techniques required to detect the highest energy photons have only been available since the late 1960s, which is just a blink of the eye in terms of mankind's involvement in astronomical research. Gamma-rays pass through most materials, so they cannot be reflected by a typical mirror as for optical photons, or using a special configuration of mirrors, as for X-ray photons. However, the tools of high-energy physics 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 must contend with a lot of interference from cosmic rays, or elementary particles that come from all parts of the sky, which often affect gamma-ray detectors in a way that is similar to the source photons. This background must be suppressed to get a pure gamma-ray signal. This suppression is critical because 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 chamber

Gamma-ray detectors can be placed in two broad classes. The first class includes what would typically be called spectrometers or photometers in optical astronomy. These instruments are "light buckets" that focus on a region of the sky containing the target and collect 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 includes detectors that 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 many other fields. Current designs for future detectors incorporate more advanced solid-state technology to overcome some of these problems and provide large, sensitive detectors that will further establish gamma-ray astronomy as an integral part of astrophysical research.

Topics about specific types of Gamma-ray Detectors:

Last Modified: October 2010

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