Scintillators-interest

Wikipedia

The first device which used a scintillator was built in 1903 by Sir William Crookes and used a ZnS screen.[3][4] The scintillations produced by the screen were visible to the naked eye if viewed by a microscope in a darkened room; the device was known as a spinthariscope. The technique led to a number of important discoveries but was obviously tedious. Scintillators gained additional attention in 1944, when Curran and Baker replaced the naked eye measurement with the newly developed PMT. This was the birth of the modern scintillation detector.[3]

The scintillation process in inorganic materials is due to the electronic band structure found in crystals and is not molecular in nature as is the case with organic scintillators.[16] An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band (located just below the conduction band and separated from the valence band by an energy gap; see picture). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron-hole pairs which wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). The activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states again results in scintillation light (slow component).

BGO is a pure inorganic scintillator without any activator impurity. There, the scintillation process is due to an optical transition of the Bi 3+ ion, a major constituent of the crystal.[6] A similar process exists in CdWO4.

1. The following figure which closely resembles to Wikipedia description, was taken from Google [[PDF] 

Kein Folientitelhttp://hsag.physik.uni-bonn.de/docs/lectures/StudentExperimentSS08/chap_03_detectors_1.pdf

hsag.physik.uni-bonn.de/docs/lectures/.../chap_03_detectors_1.pdf

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According to Tsoulfanidis, the, production of a scintillation is the result of the occurrence of these events:

1. Ionizing radiation passes through the crystal. 2. Electrons are raised to the conduction band. 3. Holes are created in the valence band.4. Excitons are formed. 5. Activation centers are raised to the excited states by absorbing electrons, holes, and excitons. 6. Deexcitation is followed by the emission of a photon.

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Semiconductors

According to Modern Physics by Taylor, et.al., here is another way of understanding why donor states lie just below

the conduction band and acceptor states lie just above the valence band. Consider a valence 5 donor impurity, surrounded by four complete covalent

bonds, and now ask what happens when we add the fifth electron. Suppose,for just a moment, that instead of a +5 impurity, the site contained an ordinary

+4 Si ion. Then the extra electron would go into the lowest availableelectronic state of a pure Si lattice, namely, a state at the bottom of the con-duction band. However, instead of a +4 Si, the site contains a +5 impurity, so

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the extra electron is a little more strongly bound by the Coulomb attractionof the extra charge. This lowers the energy of the state below the bottom ofthe conduction band.We can repeat the argument for a trivalent acceptor site. Consider a valence 3

acceptor impurity, surrounded by four completed covalent bonds. An extra electron is required to complete the last bond. We. now remove the extra electron and ask, "What is the energy of the resulting hole state?" If instead of a+3 impurity, there was a +4 Si at the site, then we would simply have a pure Si lattice with one missing electron; the lowest energy state of such a system has a hole at the top of the valence band. However, the site has a +3 impurity, lot a +4 Si, so the binding energy of the extra electron is less. The energy of the state is thus a little higher than expected and is raised above the top of the valence band.

Very small concentrations of impurities have a large effect on the carri-er concentration in semiconductors. At room temperature and above, eachlonor impurity adds a conduction electron and each acceptor impurity adds aralence hole. Because the thermally activated carrier concentration in intrin-sic semiconductors is very small, even low concentrations of impurities canlave a huge effect. For instance, adding just one donor impurity for everymillion Si atoms increases the room-temperature concentration of mobilecharge carriers by a factor of 106, from the intrinsic value of lO^rrT3 to1022 mT3. Furthermore, the relatively large concentration of conduction elec-trons in n-type semiconductors greatly suppresses the concentration of holesbecause the electrons drop down into the valence band and fill the holes.Likewise, the large concentration of holes in p-type semiconductors greatlyiBppresses the conduction electron concentration. Thus, impurity doping con-tols both the concentration and type of charge carriers. The suppression ofa-called minority carriers — holes in /z-type and electrons in p-type — andhe dominance of the majority carriers — electrons in n-type, holes in p-type-play an important role in the behavior of semiconductor devices, as we willee in the following sections.Because of the extreme sensitivity of semiconductor properties to im-lurity levels, the manufacture of semiconductor devices must begin with theiroduction of exceedingly pure single crystals of silicon. Impurity levels musttart below 1 part in 1010, so that donors or acceptors can then be added in aontrolled manner (see Problem 14.12). This level of sample purity was prac-ically unknown prior to the ris^of the semiconductor industry, and specialiiirification techniques had to be developed to produce the necessary semi-onductor purity.

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