The germanium detector detects gamma rays with energies ranging from 10 kev to 10 Mev. It is a useful detection method because of the high resolution produced in each spectrum. The setup essentially consists of a diode, various electronics, and a liquid nitrogen dewer.
In essence, the gamma ray detector detects electrons. By keeping the detector cold using liquid nitrogen the electrons from the atoms are prevented from continually loosing valence electrons. The only time an electron-hole pair is broken apart then is when a gamma ray breaks it apart. Then the electron can travel through a resistor and the voltage can be determined. By keeping it cold the electrons do not continually leave the atoms and travel through the resistor. If this happened we would not know if the electron had come from a gamma ray or had simply escaped from the valence shell of an atom.