Question

For an incident gamma-ray interacting with an inorganic scintillator, describe the interaction of the gamma-ray, how scintillation light is generated, how it is converted into an electrical signal, and how it is amplified.

Answer 1

Gamma rays

High-Z materials, e.g. inorganic crystals, are best suited for the detection of gamma rays. The three basic ways that a gamma ray interacts with matter are: the photoelectric effect, Compton scattering, and pair production. The photon is completely absorbed in photoelectric effect and pair production, while only partial energy is deposited in any given Compton scattering. The cross section for the photoelectric process is proportional to Z⁵, that for pair production proportional to Z², whereas Compton scattering goes roughly as Z. A high-Z material therefore favors the former two processes, enabling the detection of the full energy of the gamma ray.^[19] If the gamma rays are at higher energies (>5 MeV), pair production dominates.

Inorganic scintillators

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. 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 Bi³⁺ ion, a major constituent of the crystal.^[6] A similar process exists in CdWO₄

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Principle of operation

A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as a photomultiplier tube (PMT), photodiode, or silicon photomultiplier. PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on the other hand, detect incoming photons by the excitation of charge carriers directly in the silicon. Silicon photomultipliers consist of an array of photodiodes which are reverse-biased with sufficient voltage to operate in avalanche mode, enabling each pixel of the array to be sensitive to single photons.

inorganic scintillators

The following is a list of commonly used inorganic crystals:

• BaF₂ or barium fluoride: BaF₂ contains a very fast and a slow component. The fast scintillation light is emitted in the UV band (220 nm) and has a 0.7 ns decay time (smallest decay time for any scintillator), while the slow scintillation light is emitted at longer wavelengths (310 nm) and has a 630 ns decay time. It is used in for fast timing applications, as well as applications for which pulse shape discrimination is needed. The light yield of BaF₂ is about 12 photons/keV.^[21] BaF₂ is not hygroscopic.
• CaF₂(Eu) or calcium fluoride doped with europium: The material is not hygroscopic, has a 940 ns decay time, and is relatively low-Z. The latter property makes it ideal for detection of low energy β particles because of low backscattering, but not very suitable for γ detection. Thin layers of CaF₂(Eu) have also been used with a thicker slab of NaI(Tl) to make phoswiches capable of discriminating between α, β, and γ particles.
• BGO or bismuth germanate: Bismuth germanate has a higher stopping power, but a lower optical yield than NaI(Tl). It is often used in coincidence detectors for detecting back-to-back gamma rays emitted upon positron annihilation in positron emission tomography machines.
• CdWO₄ or cadmium tungstate: a high density, high atomic number scintillator with a very long decay time (14 μs), and relatively high light output (about 1/3 of that of NaI(Tl)). CdWO₄ is routinely used for X-ray detection (CT scanners). Having very little ²²⁸Th and ²²⁶Ra contamination, it is also suitable for low activity counting applications.
• CaWO₄ or calcium tungstate: has one of the shortest wavelength maximum in its broad band emission after high-energy excitation at 420 nm.^[22]
• CsI(Tl) or cesium iodide doped with thallium: these crystals are one of the brightest scintillators. Its maximum wavelength of light emission is in the green region at 550 nm. CsI(Tl) is only slightly hygroscopic and does not usually require an air-tight enclosure.
• CsI(Na) or cesium iodide doped with sodium: the crystal is less bright than CsI(Tl), but comparable in light output to NaI(Tl). The wavelength of maximum emission is at 420 nm, well matched to the photocathode sensitivity of bialkali PMTs. It has a slightly shorter decay time than CsI(Tl) (630 ns versus 1000 ns for CsI(Tl)). CsI(Na) is hygroscopic and needs an air-tight enclosure for protection against moisture.
• CsI: undoped cesium iodide emits predominantly at 315 nm, is only slightly hygroscopic, and has a very short decay time (16 ns), making it suitable for fast timing applications. The light output is quite low, however, and very sensitive to variations in temperature.
• Gd₂O₂S or Gadolinium oxysulfide has a high stopping power due to its relatively high density (7.32 g/cm³) and the high atomic number of gadolinium. The light output is also good, making it useful as a scintillator for x-ray imaging applications.
• LaBr₃(Ce) (or lanthanum bromide doped with cerium): a better (novel) alternative to NaI(Tl); denser, more efficient, much faster(having a decay time about ~20ns), offers superior energy resolution due to its very high light output. Moreover, the light output is very stable and quite high over a very wide range of temperatures, making it particularly attractive for high temperature applications. Depending on the application, the intrinsic activity of ¹³⁸La can be a disadvantage. LaBr₃(Ce) is very hygroscopic.
• LaCl₃(Ce) (or lanthanum chloride doped with cerium): very fast, high light output. LaCl₃(Ce) is a cheaper alternative to LaBr₃(Ce). It is also quite hygroscopic.
• PbWO₄ or Lead tungstate: due to its high-Z, PbWO₄ is suitable for applications where a high stopping power is required (e.g. γ ray detection).
• LuI₃ or lutetium iodide
• LSO or lutetium oxyorthosilicate (Lu₂SiO₅): used in positron emission tomography because it exhibits properties similar to bismuth germanate (BGO), but with a higher light yield. Its only disadvantage is the intrinsic background from the beta decay of natural ¹⁷⁶Lu.
• LYSO (Lu_1.8Y_0.2SiO₅(Ce)): comparable in density to BGO, but much faster and with much higher light output; excellent for medical imaging applications. LYSO is non-hygroscopic.
• NaI(Tl) or sodium iodide doped with thallium: NaI(Tl) is by far the most widely used scintillator material. It is available in single crystal form or the more rugged polycrystalline form (used in high vibration environments, e.g. wireline logging in the oil industry). Other applications include nuclear medicine, basic research, environmental monitoring, and aerial surveys. NaI(Tl) is very hygroscopic and needs to be housed in an air-tight enclosure.
• YAG(Ce) or yttrium aluminum garnet: YAG(Ce) is non-hygroscopic. The wavelength of maximum emission is at 550 nm, well-matched to red-resistive PMTs or photo-diodes. It is relatively fast (70 ns decay time). Its light output is about 1/3 of that of NaI(Tl). The material exhibits some properties that make it particularly attractive for electron microscopy applications (e.g. high electron conversion efficiency, good resolution, mechanical ruggedness and long lifetime).
• ZnS(Ag) or zinc sulfide: ZnS(Ag) is one of the older inorganic scintillators (the first experiment making use of a scintillator by Sir William Crookes (1903) involved a ZnS screen). It is only available as a polycrystalline powder, however. Its use is therefore limited to thin screens used primarily for α particle detection.
• ZnWO₄ or zinc tungstate