radiation measurement

Semiconductor detectors also can be used in gamma-ray spectroscopy. In this case, however, it is advantageous to choose germanium rather than silicon as the detector material. With an atomic number of 32, germanium has a much higher photoelectric cross section than silicon (atomic number, Z, of 14), as the probability of photoelectron absorption varies approximately as Z^4.5. Therefore, it is far more probable for an incident gamma ray to lose all its energy in germanium than in silicon, and the intrinsic peak efficiency for germanium will be many times larger. In gamma-ray spectroscopy, there is an advantage in using detectors with a large active volume. The depletion region in germanium can be made several centimetres thick if ultrapure material is used. Advances in germanium purification processes in the 1970s have led to the commercial availability of material in which the residual impurity concentration is about one part in 10¹².

The most common type of germanium gamma-ray spectrometer consists of a high-purity (mildly p-type) crystal fitted with electrodes in a coaxial configuration. Normal sizes correspond to germanium volumes of several hundred cubic centimetres. Because of their excellent energy resolution of a few tenths of a percent, germanium coaxial detectors have become the workhorse of modern-day high-resolution gamma-ray spectroscopy. The band gap in germanium is smaller than that in silicon, so thermally generated charge carriers are even more of a potential problem. As a result, virtually all germanium detectors, even those with relatively small volume, are cooled to liquid-nitrogen temperature during their use. Typically, the germanium crystal is sealed inside a vacuum enclosure, or cryostat, that provides thermal contact with a storage dewar of liquid nitrogen. Mechanical refrigerators are also available to cool the detector for use in remote locations where a supply of liquid nitrogen may not be available.

Although semiconductor detectors can be operated in current mode, the vast majority of applications are best served by operating the device in pulse mode to take advantage of its excellent energy resolution. The time required to collect the electrons and holes formed along a particle track is typically tens to hundreds of nanoseconds, depending on detector thickness. The rise time of the output pulse is therefore of the same order, and relatively precise timing measurements are possible, especially for thin detectors.