Semiconductor detector

A semiconductor detector is a radiation or particle, which makes special electrical properties of semiconductors advantage to detect ionizing radiation. The radiation generates free charge carriers migrate to the electrode metal in the semiconductor. This current signal is amplified and evaluated. Semiconductor detectors are used for example in spectroscopy, nuclear physics and particle physics.

Principle of operation

Put simply, the detector is a diode to which a DC voltage is applied in the reverse direction, so that normally no current flows. Now generates the incident radiation in the material, electron-hole pairs, ie free charge carriers, this migration in the electric field to the electrodes and are measurable current pulse.

How many electron -hole pairs releases a particle or quantum of the incident radiation depends not only on its energy mainly on the band gap energy of the material used. Depending on the type of ionizing radiation incurred the charge clouds generated in the detector in different ways and are differentially distributed in the volume. A charged particle generated along its path a ionisation, while a photon interaction via the photoelectric effect in the entire load may be substantially free contact at a point. In competition with the photoelectric effect occurs at a higher photon energy at the Compton effect, in which usually only one part of the energy is deposited in the detector.

Application

Semiconductor detectors due to their high energy resolution and - ( position sensitive detectors ) used their local sensitivity - with the appropriate structuring. They are used, for example, 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

In the absorption of high-energy ultraviolet ( vacuum UV, extreme UV ) and X-ray and gamma radiation, first a primary electron is raised from the valence into the conduction band. Its kinetic energy is very high, and as a result numerous secondary electrons and phonons arise. The generation of secondary particles is a statistical process. With the same initial energy and therefore not always the same number of charge carriers produced. The range of the secondary particles is relatively short. Compared to the ionization processes, which are caused by charged particles, the charge carriers are generated in a very small spatial region.

To achieve a high detection probability, semiconductors with high atomic number such as germanium, gallium arsenide or cadmium telluride may be used for gamma rays. In addition, a relatively large thickness of the single crystal necessary. Semiconductor detectors of germanium, such as the illustrated HP Ge detector must be cooled to the temperature of liquid nitrogen (77 K), because they have a very high leakage current at room temperature that would result in the necessary operating voltage to the destruction of the detector. The lithium - drifted germanium detectors previously used ( usually called Ge ( Li) detector ) as well as the still usual lithium - drifted silicon detectors ( Si ( Li) detector ) even need to be refrigerated because storage at room temperature it by Lithium would destroy diffusion. Cooling also reduces the inherent noise.

Alpha radiation

The depth of penetration of alpha particles is relatively low at about 25 microns, since their ionization capacity is very high. According to the Bethe- Bloch equation of Ionisationsverlust charged particles of Z ² / v ² depends, therefore takes at higher nuclear charge and smaller speed. Therefore, the density of electron -hole pairs increases with the depth of the penetration decreases as the speed of the alpha particle. She has a clear maximum at the end point ( Bragg curve).

Beta radiation

Have electrons compared to alpha particles is lower by orders of magnitude mass and a half as much electrical charge. Your ionization capacity is so much lower. Therefore relativistic ( high-energy ) beta radiation penetrates much deeper into the detector or penetrates it completely and generated along its path a uniform density of electron- hole pairs. Is their energy emitted mostly, the result is - similar to alpha particles - a higher ionization at the end point of its orbit. Extremely low energy electrons do not generate more charge carriers and interact primarily with phonons.

Other types of particles

Charged particles with high energy ( pions, kaons, etc. ) penetrate the detector with a nearly constant velocity along its orbit and generate electron -hole pairs with a uniform density. This density is approximately independent of the energy of the particles and proportional to square of electric charge. On the other hand produce protons and ( charged) nuclei one ionization density, which is also proportional to the square of their charge, but inversely proportional to their energy.

Neutrons or protons can also generate very fast in semiconductor detectors signals by, for example, an atomic nucleus trigger, which in turn electron-hole pairs are generated. However, the probability is low. For this reason, semiconductor detectors are less suitable for the detection of these particles.