Photon detector crystal

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

Figure 1 Schematic of an EDS system on an electron column. The incident electron interacts with the specimen with the emission of X rays. These X rays pass through the window protecting the Si (Li) and are absorbed by the detector crystal. The X-ray energy is transferred to the Si (Li) and processed into a dig-itai signal that is displayed as a histogram of number of photons versus energy.

Here, the first factor at the right-hand-side, which represents the fraction of gamma photons intercepted by the scintillating crystal, is equal to the solid angle extended by the detector crystal to the source, with dc denoting the crystal diameter. The efficiency of the crystal t depends on both the material and the size of the crystal. 

Escape peaks , which occur when a detected X-ray photon fluoresces silicon atoms in the detector crystal. A small proportion of the resultant Si K-line photons are likely to escape out of the detector rather than be absorbed within the crystal and so represent an apparent loss in detected energy from the parent photon. This phenomenon causes an artifact peak to appear at an energy of 1.78 keV (the energy of the Si K line) below the parent event. 

Fig. 7 The central component of a high resolution emission spectrometer is a Bragg crystal that spectrally analyzes the fluorescence from the sample and reflects it onto a photon detector. This particular example utilises a spherically bent Johann type crystal in a one-to-one focusing Rowland geometry in connection with a solid state detector. The solid angle of collection can be increased by increasing the number of analyzer crystals, all aligned to intersect at the two focal points. ...

Nonequilibrium methods are those that change the carrier distribution by elevating the effective temperature of carriers over the crystal lattice temperamre. Thus, obtained nonequilibrium causes spatial redistribution of charge carriers, and can be thus utilized to decrease the carrier concentration in the desired part (the active area) of a photonic detector. This decreases carrier concentration-dependent g-r process rates, causes thermally-induced noise drop and as a result produce effects similar to those of cryogenic cooling. 

The influence of a PBG structure to the detectivity of a photonic detector may be considered in a manner analogous to that presented in Sect. 2.12. Actually a photonic crystal may be considered the ideal case of a radiative shields, describing... 

One limitation is that these detectors must, in general, be relatively thick. The dopant concentration must be low to preserve the semiconductor behavior - but with low concentrations the absorption is poor. If the detector thickness is less than 2 or 3 times the absorption length, the detectors will not absorb an appreciable fraction of the incoming photons - so the quantum efficiency (QE) will be low. Practical limits of the absorption constant for optimized IR detectors are l-10cm for Ge and 10-50 cm for Si. Thus, to maximize QE, the thickness of the detector crystal should be at least 0.5 cm for doped Ge and about 0.1 cm for doped Si. This makes the doped detectors impractical for use in an array. Si As is an exception - it can be used with detectors thin enough for arrays. 

If the detection system is an electronic, area detector, the crystal may be mounted with a convenient crystal direction parallel to an axis about which it may be rotated under tlie control of a computer that also records the diffracted intensities. Because tlie orientation of the crystal is known at the time an x-ray photon or neutron is detected at a particular point on the detector, the indices of the crystal planes causing the diffraction are uniquely detemiined. If... 

Scintillation detectors are substances which fluoresce when stmck by x-radiation. Scintillation can, therefore, serve to convert x-ray photons into visible or ultraviolet light. Scintillation materials include thaUium-activatedcrystals of sodium iodide, NaI(Tl), potassium iodide, KI(T1), or cesium iodide, CsI(Tl) crystals of stilbene (a, P-diphenylethylene) [588-59-0] and anthracene [120-12-7] bismuth germanium oxide [12233-56-6] ... 

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