Germanium detector

A germanium detector is a semiconductor detector whose active material is germanium. Radiation deposits energy that produces electron-hole pairs; an applied electric field collects charge and the electronics convert it into a pulse. High-purity germanium instruments are useful for energy spectroscopy because a single absorbed photon can produce many charge carriers. Cooling reduces thermally generated carriers and electronic noise.^[ref]

For a simple charge inventory, let deposited energy be $E$ and the mean energy per electron-hole pair be $w$. With complete charge collection,Using $w=2.96\mathrm{eV}$, the manufacturer's value at77K, an illustrative $E=59.2\mathrm{keV}$ deposit produces about20,000 pairs, corresponding to about $3.20\times 10^{-15}\mathrm{C}$ of collected pulse charge. This is an average conversion, not a claim that every event produces exactly the same carrier count.^[ref]

The apparatus therefore needs more than a piece of germanium: contacts establish a collecting field, a cooled enclosure controls the environment, and a charge-sensitive preamplifier measures the small pulse. A photopeak corresponds to events depositing the full photon energy. Escaping radiation or incomplete charge collection can produce smaller pulses, so a pulse-height distribution must be interpreted with the detector geometry and response.^[ref]