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.
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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