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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.

How a gamma-ray photon creates an electron/positron pair
A view of the basic interaction in a pair production telescope.

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.

Diagram of the EGRET Spark Chamber

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.

EGRET's view of the galactic anti-center
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.

Artist's impression of the Fermi satellite
The Fermi Gamma-ray Space Telescope

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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