Semiconductor detector
A semiconductor detector is a radiation or particle detector in which the special electrical properties of semiconductors are used to detect ionizing radiation . The radiation generates free charge carriers in the semiconductor that migrate to metal electrodes. This current signal is amplified and evaluated. Semiconductor detectors are used, for example, in spectroscopy , nuclear physics and particle physics .
Working principle
Put simply, the detector is a diode to which a DC voltage is applied in the reverse direction, so that normally no current flows. If the incident radiation generates electron-hole pairs in the material , i.e. free charge carriers, these migrate in the electric field to the electrodes and can be measured as a current pulse.
How many electron-hole pairs a particle or quantum of the incident radiation releases depends, in addition to its energy, largely on the band gap energy of the material used. Depending on the type of ionizing radiation, the charge clouds generated in the detector arise in different ways and are distributed differently in the volume. A charged particle creates an ionization track along its path. A photon , on the other hand, can use the photo effect to release the entire charge corresponding to its energy practically at one point by releasing it to a secondary electron . In competition with the photo effect , the Compton effect occurs at higher photon energy , in which only part of the energy is transferred to the electron and is deposited in the detector.
application
Semiconductor detectors are used because of their high energy resolution and - with appropriate structuring - their location sensitivity (position-sensitive detectors). They are used z. B. in X-ray fluorescence analysis , gamma spectroscopy , alpha spectroscopy and particle physics . An example of the latter is the Semiconductor Tracker (SCT) of the ATLAS detector .
Electromagnetic radiation
With the absorption of high-energy ultraviolet radiation ( vacuum UV , extreme UV ) as well as X-ray and gamma radiation , a primary electron is first lifted from the valence band to the conduction band . Its kinetic energy is very high, which is why numerous secondary electrons and phonons are formed. The generation of secondary particles is a statistical process. With the same initial energy, the same number of charge carriers does not always arise. The range of the secondary particles is relatively short. Compared with the ionization processes that are caused by charged particles, the charge carriers are generated in a very small area.
In order to achieve a high probability of detection, semiconductors with a high atomic number such as germanium , gallium arsenide or cadmium telluride are used for gamma radiation . In addition, a relatively large thickness of the single crystal is necessary. Semiconductor detectors made of germanium, such as the HP-Ge detector shown , have to be cooled to the temperature of liquid nitrogen (77 K) because they have a very high leakage current at room temperature, which would destroy the detector at the required operating voltage. The previously used lithium- drifted germanium detectors (common name: Ge (Li) detector ) as well as the lithium-drifted silicon detectors (Si (Li) detector) that are still common today even have to be constantly cooled, because storage at room temperature means that lithium Would destroy diffusion . Cooling also reduces inherent noise.
See also X-ray image sensor .
Alpha radiation
The penetration depth of alpha particles is relatively small at approx. 25 µm, as their ionization capacity is very high. According to the Bethe-Bloch equation , the ionization loss of charged particles depends on Z ² / v ², so it increases with a higher atomic number and lower speed . The density of the electron-hole pairs therefore increases with depth, because the speed of the alpha particle decreases when it penetrates. It has a clear maximum at the end point ( Bragg curve ).
Beta radiation
Compared to alpha particles, electrons have an order of magnitude less mass and half the electrical charge . Their ionization capacity is therefore much lower. Relativistic (high-energy) beta radiation therefore penetrates significantly deeper into the detector or penetrates it completely and creates a uniform density of electron-hole pairs along its path. If most of their energy is released, then - similar to alpha particles - a higher ionization occurs at the end point of their orbit. Extremely low-energy electrons no longer generate charge carriers and primarily interact with phonons .
Other types of particles
Charged particles with high energy ( pions , kaons , etc.) penetrate the detector at an almost constant speed and generate electron-hole pairs with a uniform density along their path. This density is almost independent of the energy of the particles and proportional to the square of their electrical charge. In contrast, protons and (charged) nuclei generate an ionization density that is also proportional to the square of their charge, but inversely proportional to their energy.
Neutrons or very fast protons can also generate signals in semiconductor detectors by e.g. B. collide with an atomic nucleus, which in turn generates electron-hole pairs. However, the likelihood of this is low. For this reason, semiconductor detectors are less suitable for detecting these particles.
See also
literature
- Gerhard Lutz: Semiconductor Radiation Detectors . Springer-Verlag Berlin Heidelberg, 1999, ISBN 978-3-540-71678-5 .
- Glenn F. Knoll: Radiation detection and measurement. John Wiley & Sons, New York 1979, ISBN 0-471-49545-X .
Web links
- Semiconductor detector (fundamentals of particle physics)
- Martin zur Nedden: Chapter 6: Semiconductor detectors. (Lecture notes in the compulsory elective subject particle physics, Humboldt-Universität zu Berlin, summer semester 2006; PDF; 561 kB) In: Detectors in elementary particle physics. 2006, accessed May 26, 2009 .
- Comparison of the energy resolution of a Ge (Li) semiconductor detector and a NaI (Tl) scintillator
Individual evidence
- ↑ See also RD50 (Radiation hard semiconductor devices for very high luminosity colliders), an international research association at CERN that is developing radiation hard semiconductor detectors for future experiments on accelerators with the highest luminosities.
- ↑ Rudolf Nicoletti, Michael Oberladstätter and Franz König: Metrology and Instrumentation in Nuclear Medicine: An Introduction . facultas.wuv Universitätsverlag, 2010, ISBN 978-3-7089-0619-5 , p. 69 ( limited preview in Google Book search).