The second advantage is that although the efficiency of the instrument is low (two scatters are required and one pays the price in effective area), it efficiently rejects background events. These background events can arise in single element detectors through activation. Finally, by measuring the energy deposits in the two detectors one not only has a measure of the total incident photon energy but also a measure of the Compton scatter angle that can later be used to create an image. Present day Compton telescopes only employ the technologies that allow them to measure the location of the photon interactions, the energies of the interactions and the time-of-flight. These data are not sufficient to assign a unique incoming direction to the incident photon. One also needs the direction of the scattered electron in the forward detector. This is not easily accomplished. MeV electrons scatter efficiently and unless the material in the forward detector is tenuous and of low-Z composition, the direction information of the electron is quickly lost. The problem of tracking the scattered electrons is now being attacked with solid-state silicon or liquid Argon particle detectors. If silicon detectors are used as the forward detector and the detectors are kept thin the electrons do not scatter much. This then allows one to greatly constrain the incident photon direction, thereby improving its imaging properties and also its signal-to-noise ratio. An angular resolution of a few arc minutes is attainable with technology currently under development. The next major step in Compton telescope design will come in the form of an electron-tracking forward detector and the high density electronics necessary to support the large number of data channels.

5.1.4 PAIR PRODUCTION TELESCOPES

The observation of high-energy gamma-rays is done indirectly, by detecting the electron and positron produced when the gamma-ray undergoes pair production in the presence of a nucleus. A high-energy gamma-ray telescope consists of high-Z pair-production material (metal foils) interleaved with position sensitive charged particle detectors. The direction and energy of the incident gamma-ray is reconstructed from the direction and energy of the electron and positron.  Thus, a gamma-ray telescope is, in effect, a track imaging charged particle detector and calorimeter. An anticoincidence is needed to screen the track imaging detector from the charged particle cosmic rays which outnumber the gamma rays by a factor of approximately 104. Since the electron and positron emanate from a common vertex, giving the inverted V signature of the gamma-ray conversion to an electron/positron pair, the two-track resolution of the track imaging detector is important for both event recognition and subsequent direction determination. The instruments which define high-energy gamma-ray astronomy - SAS-2, COS-B, and EGRET - all used this same basic design. The great advance possible in this area comes from the application of recent developments in particle tracking detectors (see below), along with improvements in on-board processing and telemetry bandwidth.

5.1.5 ATMOSPHERIC CERENKOV TELESCOPES

At sufficiently high gamma-ray energies, typically above 100 GeV, the Earth's atmosphere itself can be used as part of a gamma-ray telescope. As these high-energy photons collide with the upper atmosphere, they convert to electron-positron pairs just as 100 MeV photons do, but these particles are sufficiently energetic to produce a cascade of secondary particles traveling fast enough through the air to produce a flash of Cerenkov radiation. This radiation is then detected by large-area optical collectors on the ground. The Whipple Observatory, HEGRA, and CANGAROO telescopes are all active now, and new telescopes (e.g., CELESTE, MILAGRO, VERITAS) are either under construction or proposed.