In ionizing radiation detection physics, a semiconductor detector is a device that uses a semiconductor (usually silicon or germanium) to measure the effect of incident charged particles or photons. Semiconductor detectors find broad application for radiation protection, gamma and X-ray spectrometry, and as particle detectors.
Detection mechanism In semiconductor detectors, ionizing radiation is measured by the number of charge carriers set free in the detector material which is arranged between two electrodes, by the radiation. Ionizing radiation produces free electrons and electron holes. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit, as described by the Shockley-Ramo theorem. The holes travel in the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be determined. The energy required to produce electron-hole-pairs is very low compared to the energy required to produce paired ions in a gas detector. Consequently, in semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon rise time. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
Detector types
Silicon detectors
Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) silicon strips to turn them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source). They also suffer degradation over time from radiation, however, this can be greatly reduced thanks to the Lazarus effect.
Diamond detectors Diamond detectors have many similarities with silicon detectors but offer significant advantages – such as solar blindness, high temperature operation, fast response, and radiation hardness. They are also well suited for neutron detection, and are one option being explored for use in harsh environments where other detector methods are not feasible. This development is motivated by the need for such monitoring in nuclear reactor containment vessels, especially in the wake of the accident at the Fukushima Nuclear Power Plant. Further improvements have been made with respect to radiation tolerance with the development of three-dimensional structures for these devices. Diamond semiconductor detectors have also seen uses in research of nuclear fusion, high energy physics, and medical physics. At present, however, they have not seen as widespread use as silicon and germanium detectors because of cost and challenges in the manufacturing process.
Germanium detectors
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