(Image from Imagine! the Universe page on gamma-ray detectors)
Like X-ray detectors, gamma-ray detectors depend on a photon's interaction with a medium. One new form of interaction with detector material which comes into play for gamma rays (at least those with energies larger than 30 MeV) is pair production (defined below). Pair production telescopes are just one type of gamma-ray detectors; these, along with a few others, are described below.
Gamma-ray scintillators work by the interaction of a gamma ray with the scintillator material (a crystal of some sort). This interaction produces some form of low-energy light (typically visual) which is then collected by a photomultiplier tube. To make images with a scintillator, a coded aperture or Compton scattering configuration (described below) must be used. Scintillators have been used on CGRO, HEAO-1 and RXTE for hard X-ray/soft gamma-ray detection.
(Image from Imagine! the Universe page on gamma-ray detectors)
Solid state detectors are usually made from germanium or cadmium-zinc-telluride (CdZnTe, or CZT) semiconducting material. An incoming gamma ray causes photoelectric ionization of the material (the gamma ray causes an electron to get knocked out of its place), so an electric current will be formed if a voltage is applied to the material. As with scintillators, a coded aperture mask or Compton scattering configuration must be used to make a solid state detector into an imager. The upcoming INTEGRAL mission will use such a detector.
Compton scattering happens when a photon interacts with an electron - the photon leaves the interaction with a lower energy and the electron has a higher energy. The energies of the outgoing photon and electron along with the angle at which these two leave the interaction allow determination of the energy and direction of the original photon.
A Compton scattering telescope consists of two levels. Both levels are composed of scintillator materials. The incoming gamma ray Compton scatters off an electron in the top level. The scattered photon travels into the lower level where it is absorbed. Phototubes viewing both levels determine the interaction points in the two layers and the energy deposited in both layers. From this, the energy and direction of the incoming photon can be determined. Such a telescope was used on CGRO.
Image from Imagine! the Universe webpages on Compton scatter telescopes.
Gamma rays with energies greater than 30 MeV tend to pair produce when they interact with materials. (Pair production occurs when a photon interacts with "something" to produce an electron/positron pair. That "something" could be another photon, matter or even a magnetic field. A positron is an anti-electron - a particle with all the same properties of an electron, such as mass, but with opposite charge.)
This image is a diagram of the EGRET telescope on the CGRO satellite; from Imagine! the Universe page on pair production telescopes.)
A pair production telescope is composed of layers of converter material interleaved with tracking material. An incoming gamma ray will interact with one of the converter layers to produce a pair. The trackers, usually gas-filled regions criss-crossed with wires, become ionized as the pair passes through them, leaving a trail of sparks. By recording the trails through the tracker, we can get a 3-dimensional picture of the pair as it travels through the chamber. The paths of the pair through the tracker allows determination of the direction of the incoming photon and the energy of the electron and positron give the energy of the incoming photon. The EGRET telescope on CGRO was such a telescope. The future GLAST mission will also use a pair production telescope.
One of the challenges of high energy astronomy was the fact that the Earth's atmosphere absorbed incoming X-rays and gamma rays. Well, for the highest energy gamma rays, we can actually use the atmosphere itself as a detector.
You may have heard that the speed of light, 300,000,000 meters per second or about 700,000,000 miles per hour, is the fastest anything can go. Well, the truth is that the speed of light I just quoted is the fastest anything can go in a vacuum. The speed of light through a medium, such as the Earth's atmosphere, is less than the speed quoted above.
Now, when a high-energy gamma ray interacts with the atmosphere, it produces an air shower (the gamma ray produces a pair of particles which then interact with other particles in the atmosphere to produce more particles; thus, it produces a "shower" of particles). The particles in this air shower are traveling faster than the speed of light should be in the atmosphere. When something is traveling faster than the local speed of light, it produces what is called Cerenkov radiation. In the case of the air shower, the Cerenkov radiation is in the form of a bluish "pancake" of light about 200 meters in diameter. Observations of such light require a dark, moonless night.
Image from Imagine! the Universe webpages on Cerenkov gamma-ray telescopes.
