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Solid State Gamma-ray Detectors

Solid state detectors are those using advanced materials such as semiconductors. These detectors are generally used in the same manner as scintillator-based detectors. Advanced materials such as germanium or the recently popular cadmium zinc telluride (CdZnTe) offer better energy resolution, less noise, and better spatial resolution than the standard scintillators. This will allow scientists to more carefully measure gamma-ray line emission. Some materials, such as germanium, require more care than scintillators, such as cooling them to low operating temperatures. They also tend to be more expensive.

Basic operating principles

As with scintillators, these detectors mainly rely on a photoelectric ionization of the material by the gamma-ray, but in this case electron/hole pairs are created in the semiconductor material rather than electron/ion pairs as in a scintillator.

Using these materials as an imager requires the used of coded aperture masks or Compton scatter type configurations. This is another feature these detectors have in common with scintillators.

Detector characteristics

Most of the advanced materials being considered for future missions have the problem that the crystals are small. It requires large arrays of these crystals to achieve collecting areas acceptable for gamma-ray astronomy. For instance, the imaging spectrometer on the future INTEGRAL mission will use an array of 19 germanium detectors. Other instrument concepts require even larger arrays. Nevertheless, collection areas on the order of thousands of cm2 are achievable.

High energy resolutions, E/dE of 500, can be achieved with germanium detectors. Other materials can routinely match or improve on the resolution available with scintillators.

Future developments

New materials offer the promise of better, cheaper instrumentation. The European Space Agency's INTEGRAL mission will incorporate some of this new technology to survey the gamma-ray sky up to 10 MeV. Future missions will continue to employ advanced materials for sensitive gamma-ray imaging.

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