3.2.6.1 Intrinsic semiconductors: Si(Li) and Ge detectors

The Si(Li) detector consists of a Silicon crystal (p-i-n devices formed by lithium compensation or drifting of p-type silicon) that is mounted in a vacuum cryostat cooled to -196 °C to achieve the semiconducting behaviour. Cooling is made by insertion of the cryostat in a Dewar vessel filled with liquid nitrogen. The entrance window is often made from Beryllium foils, but sometimes other special light materials are used. The energy resolution of these detectors is very high, since the high number of electron-hole pairs n created.

Low absorbing entrance window Ge detectors function based on the same principle, only having a p-i-n structure. The high-energy efficiency intrinsic to germanium detectors because of the high atomic number (Z), combined with a relatively high thickness (5-10 mm), allows covering an extremely wide range of energies (see figure in 3.2.1), and the analysis of heavy elements by using their K-lines emission.

Si(Li) detectors were extensively used in the first years of deployment of EDXRF spectrometry. Ge ultra-light window detectors were incorporated later with the aim of extending EDXRS to the analysis of heavy elements by K lines. However, the need for constant liquid nitrogen cooling makes them less comfortable than the thermoelectrically cooled PIN and SDD detectors.