4.3.3 Semiconductor detectors

Semiconductor detectors use the electronic carriers (electron-ion and electron-hole pairs) created by the absorption of gamma ray photons in the semiconductor material. These collect directly on the detector electrodes, causing a flow of electric current through the semiconductor and produce an output voltage pulse of amplitude proportional to the energy of the incident gamma ray photon. The detector consists of a semiconductor crystal mounted in a vacuum cryostat cooled to -196 °C.

Cooling is made by insertion of the cryostat in a Dewar vessel filled with liquid nitrogen, or by electrically powered cryogenic refrigerators. The detectors are generally of small volume and used in in-situ gamma ray spectrometry. The energy resolution of these detectors is very high, but because of their small volume, their sensitivity is low and it may take tens of minutes to record a spectrum.

The main semiconductor materials used for gamma radiation measurements are High Purity Germanium (HPGe) and Cadmium Zinc Telluride (CZT) or Cadmium Telluride (CT), whereas for alpha spectrometry. Silicon surface barrier detectors are used.