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.
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.
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.