The Pair Telescope
The pair telescope is a technology that was borrowed directly from
the world of high-energy physics. They have a long history of use in
high-energy
astrophysics,
with experimental spark chamber detectors having been flown on balloons
in the late 1960s. At energies above about 30 MeV, pair
production is the dominant photon interaction in most materials.
A pair telescope uses this process to detect the arrival of the
cosmic photon through the electron/positron pair created in the detector.
With the advent of the large, sophisticated spark chambers such as COS-B
and EGRET, high-energy gamma-ray
astronomy has gone from being a discipline for instrumental specialists
to being an integrated part of multiwavelength astronomy.
At energies above 30 MeV, celestial objects such as pulsars, active galaxies,
and diffuse emission are studied with pair telescopes. Advances in
energy and spatial resolution were made with the use of silicon strip
detectors, such as those used in Fermi.
Basic operating principles
The standard instrument design is to have a layered telescope, with
converter layers interleaved with tracking material. The converter is
typically a heavy metal such as lead) which provides the
target for creating the initial pair while the tracking material
detects the pair. One type of tracking material is a spark chamber,
which is a gas-filled region criss-crossed with wires. Once the
electron/positron pair has been created in one of the converter
layers, they traverse the chamber, ionizing the
gas. Triggering the detector electrifies the wires, attracting the
free electrons and providing the detected signal. The trail of sparks
provides a three-dimensional picture of the the e+/e- paths. Another
type of tracking material is silicon strip detectors,
which consists of two planes of silicon. In one plane the strips are
oriented in the "x"-direction, while the other plane has strips in the
"y"-direction. The position of a particle passing through these two
silicon
planes can be determined more precisely than in a spark chamber.
By reconstructing the tracks of the charged pair as it passes
through the vertical series of trackers, the gamma-ray direction, and
therefore its origin on the sky, are calculated. In addition, through
the analysis of the scattering of the pair (which is an
energy-dependent phenomenon) or through the absorption of the pair by a
scintillator detector or a calorimeter after they exit the spark
chamber, the total energy
of the initial gamma-ray is determined.
It is very important to keep the chamber from triggering on the
overwhelming flux of cosmic rays.
To this end, anti-coincidence shields are used which cover the entire
telescope with a charged particle detector. If the anti-coincidence
shield has detected a charged particle, it won't allow the chamber to
trigger to prevent detecting cosmic rays. In addition, it is common to
have a so-called time-of-flight system, which are detectors which
determine the relative times at which the pair travel through the
chamber. In this way, it can be determined whether the pair came from
the correct direction. The EGRET instrument, shown above, was a
successful pair telescope on the Compton Gamma Ray
Observatory, utilizing the technology of a spark chamber.
Detector characteristics
Given the scarcity of photons at higher gamma-ray energies, it is
important to make these detectors as large as possible. Like an optical
telescope's mirror, the horizontal cross section of the telescope is a
measure of its ability to collect photons. For EGRET, the peak collection
area was about 1600 cm2, much smaller than air Cerenkov
detectors, but as large as many low-energy scintillator experiments.
Pair telescopes operate much like optical telescopes in that they can
take a "picture" of the region being viewed. The direction of each
photon is measured, which allows scientists to image the sky.
A view of the galactic anti-center taken
by EGRET
The
energy resolution of spark chamber experiments is only about 20%.
Losses in the chamber and the intrinsic resolution of the scintillator
are relevant factors here. At the lowest energies, the resolution
worsens, because the pair particles lose energy through multiple
scattering as they move across the detector. These losses are difficult
to account for. At high energies, the pair energy may be incompletely
absorbed in the
calorimeter which can also limit energy resolution.
The use of silicon strip detectors improves the energy resolution to
about 10%.
Recent developments
Larger is better, especially for gamma-ray sources above 30 MeV. All
sources of gamma rays emit fewer photons at higher energies. Therefore,
at the highest energies, photons are the most scarce, so larger
telescopes are needed. Spark chamber instruments are wide field-of-view
instruments with collecting area that extends to 30 or 40 degrees from
the center of the field-of-view. Instruments such as the Fermi
Gamma-ray Space Telescope have made these even larger, enabling a
scan of the entire sky in only two orbits of the satellite
(approximately 3 hours).
The task of trying to expand the energy range over which the
telescope is sensitive is related to larger collecting areas. At lower
energies,
one is limited to going down to about 20 MeV, which is the pair
production threshold for most converter materials. Techniques that
improve the
sensitivity of telescopes from about 20 to 100 MeV,
to merge with Compton scatter telescopes, are being researched. At the
highest
energies, larger collection areas will possibly allow space-based
gamma-ray detectors to detect sources up to around 100 GeV, which will
merge nicely with ground-based air Cerenkov detectors.
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The
Fermi Gamma-ray Space Telescope
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The Fermi Gamma-ray Space Telescope, launched in June 2008, uses
solid-state detectors as the tracking material instead of the
gas-filled chamber. This allows for improved energy and spatial
resolution. Improvements in energy resolution (10% resolution) and
spatial location (0.3-2.0 arcminutes) by Fermi have contributed to the
understanding of source behavior. In addition, a replenishable supply
of chamber gas is no longer needed, which makes longer mission
lifetimes possible. Fermi is fulfilling its potential, with the
detection of nearly 1,500 sources in its first year operation.
Updated: December 2010
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