Gamma-Ray Telescopes & Detectors
Gamma-ray
astronomy is a
late bloomer. The techniques needed to detect the highest energy photons have
only become available in the last 30 years - 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.
Gamma-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 in 30 years, 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.
Topics about specific types of Gamma-ray Detectors:
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