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International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. C. ch. 7.1, pp. 630-632
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Much development has gone into quantitative measurements with area detectors in which the diffraction pattern is formed on an external phosphor fibre-optically coupled to a low-light-level television camera. Mostly the television camera embodies a demagnifying image intensifier coupled via demagnifying optics to a sensitive television camera tube (Arndt & Gilmore, 1979![[link]](/graphics/greenarr.gif)
; Arndt, 1982, 1985; Arndt & In't Veld, 1988; Kalata, 1982, 1985; Gruner, Milch & Reynolds, 1982) or to a CCD or an array of CCD's (Strauss, Naday, Sherman, Kraimer & Westbrook, 1987; Strauss, Westbrook, Naday, Coleman, Westbrook, Travis, Sweet, Pflugrath & Stanton, 1990; Templer, Gruner & Eikenberry, 1988; Widom & Feng, 1989; Fuchs, Wu & Chu, 1990; Karellas, Liu, Harris & D'Orsi, 1992). Camera tubes were frequently read at commercial television rates (625 lines with a field repetition rate of 25 Hz or 525 lines with a field repetition rate of 30 Hz in Europe and in North America and Japan, respectively), leading to pixel rates of about 10 MHz. Successive images were then digitized and their sums stored in memory (see Fig. 7.1.6.7
). Milch, Gruner & Reynolds (1982) developed a slow-scan method for a silicon-intensifier-target (SIT) tube. This latter method has been facilitated by the development of very large scale integration memory circuits, which have made it possible to construct economical image stores into which the camera can write at a slow rate and which can then be read at normal television rates for display purposes. CCD's are usually operated in this fashion, so that the read–out circuits can have a narrow band-width and produce an excellent signal-to-noise ratio.
![[Figure 7.1.6.7]](/figures/Cbfig7o1o6o7thm.gif) | Fast-scanning television X-ray detector (after Arndt, 1985 |
For very low X-ray intensities in which the probability of the arrival of a photon per pixel per frame period is much less than one, the camera can be operated at normal frame frequencies in a digital mode. Specially designed circuits detect the charge image produced by a single X-ray photon and find the centroid of this image (Kalata, 1982); the events are `counted' in a histogramming memory. The method is capable of some energy discrimination and has a high spatial resolution because the centroid of the image can be found to a high precision.
The incoming X-rays are converted to light in a phosphor that is coupled to the first photocathode of the system. Both polycrystalline and monocrystalline phosphors are used for X-ray detection. The former give a higher light output but have a limited resolution; the latter tend to have a poorer light-conversion efficiency but have the best resolution. The most useful phosphors are shown in Table 7.1.6.2.
Many attempts have been made to improve the spatial resolution of phosphor screens by constructing them in the form of scintillating fibres that are optically isolated from one another so that the scintillation does not spread. This can be achieved by growing columnar scintillating crystals (Oba, Ito, Yamaguchi & Tanaka, 1988), by intagliating polycrystalline phosphors (Fouassier, Duchenois, Dietz, Guillemet & Lemonnier, 1988) and by using arrays of scintillating fibres (Bigler, Polack & Lowenthal, 1986; Ikhlef & Skowronek, 1993, 1994).
Image intensifiers designed for radiography with relatively hard radiation usually have an X-ray-transparent window – which may be up to 300 mm in diameter – and an internal phosphor–photocathode sandwich deposited on an X-ray-transparent substrate. Problems of compatibility of phosphor and photocathode have restricted the phosphor used, but CsI works well with multialkali photocathodes. Moy and his collaborators have constructed a large-diameter television detector in which the image intensifierhas been modified by using beryllium for the window and for the sandwich substrate; this intensifier is coupled to a slow-scan CCD camera (Moy, 1994).
Each incident X-ray photon should give rise to several photoelectrons from the first photocathode in order to achieve a high DQE (Arndt & Gilmore, 1979). The best photocathodes have a yield of about 0.2 electrons per light photon; only fibre-optics coupling between the phosphor and the photo-cathode can give an adequate light-collection efficiency (in excess of 80% for 1:1 imaging). With demagnifying fibre optics, the light loss is considerable for purely geometrical reasons. Fibre-optic cones, in which each individual glass fibre is conical, are available for magnification or demagnification up to about 5:1. It should be noted that image intensifiers and TV tubes with electrostatic focusing are normally made with fibre-optics face-plates and that some CCD's are also available with fibre-optics windows.
Where lens optics must be employed, it is best to use two infinity-corrected objectives of the same diameter, but not necessarily of the same focal length, back to back. In practice, the best light-collection efficiency that can be achieved at a demagnification of M is about ![[(2M)^{-2}]](/teximages/cbch7o1/cbch7o1fi19.svg)
. It is for this reason that there are limitations on the maximum size of image that can be projected onto a CCD without the use of an image intensifier (but see Naday, Westbrook, Westbrook, Travis, Stanton, Phillips, O'Mara & Xie, 1994; Koch, 1994; Allinson, 1994). The function of this intensifier is to match a relatively large diameter X-ray phosphor to a small-size read-out device. As a result of the high sensitivity of CCD's, especially of slow-scan CCD's, a low photon gain in the intensifier and a low light-coupling efficiency in the coupling between intensifier and read-out device are quite adequate. It is, however, essential to couple the X-ray phosphor as efficiently as possible to the photocathode.
Roehrig et al. (1989) have described a design in which an image intensifier with 150 mm diameter input and output face plates is coupled by means of six demagnifying fibre-optic cones to six CCD's. Allinson (1994) has examined the need for image intensification and has shown that it is possible to construct a 150 mm square detector that has a performance approaching that of an ideal detector without using an image intensifier. Different methods of light coupling and format alteration have been discussed by Deckman & Gruner (1986).
In an image intensifier, the photoelectrons from a photocathode are made to produce a visible intensified image on an output phosphor. In so-called first-generation tubes, the intensification is produced by subjecting the electrons to accelerating voltages of up to about 15 keV: the number of visible-light photons at the output per keV of electron energy is about 80. The photon gain of the devices is typically about 100; there may be a brightness gain factor of M2 if the electrostatic electron-optical system produces a demagnification of M. Standard first-generation image intensifiers of this type are made with input field diameters up to 80 mm, they always have fibre-optics input face plates on which X-ray phosphor may be deposited, they are stable and robust, and have a good resolution of better than 100 µm at the input. Their low gain requires the use of a low-light-level TV camera tube in the next stage (Arndt & Gilmore, 1979) or of two or more intensifiers of this type in tandem (Kalata, 1982) or of an intrinsically more sensitive slow-scan read out (Eikenberry, Gruner & Lowrance, 1986).
For military and civilian night-vision applications, first-generation image intensifiers have largely been replaced by devices embodying one or two microchannel plates (MCP's) that produce an electron gain of up to 1000 per stage (see, for example, Emberson & Holmshaw, 1972; Garfield, Wilson, Goodson & Butler, 1976). Commercial second- and third-generation intensifiers (Pollehn, 1985) are less suitable for quantitative scientific purposes than the first-generation devices: their GaAs photocathodes are less well matched to most X-ray phosphors, the gain of MCP's decreases with time, and the tubes have a slightly lower resolution than diode types of comparable diameter. Most third-generation intensifiers have plain rather than fibre-optics face plates and none appear to be available with a diameter greater than 50 mm (Airey & Morgan, 1985). Nevertheless, these high-gain intensifiers do make it possible to construct relatively cheap moderate-performance X-ray detectors using standard-sensitivity TV pick-up devices, including CCD's (Dalglish, James & Tubbenhauer, 1984), instead of the low-light-level camera tubes necessary with a lower pre-amplification.
An intensifier can, in principle, employ a variety of read-out methods, e.g. by substituting a resistive disc anode, a coded anode or a CCD for the output phosphor. However, the only way of employing standard modules is to couple them to a TV pick-up device.
Vacuum-tube television cameras have been largely replaced by semiconductor devices but of the former the preferred tube for use in an X-ray detector is still the silicon-intensifier-target (SIT) tube (Santilli & Conger, 1972). It has an adequate resolution for images up to 512 × 512 pixels and a linear transfer function (unity gamma) and its sensitivity is well matched for use with a single-stage image intensifier with a gain of 100. When cooled, this tube can be used for long exposures in an integrated slow read-out mode (Milch, Gruner & Reynolds, 1982).
When better high-gain image intensifiers become available, the preferred choice may be tubes like the Saticon (Goto, Isozaki, Shidara, Maruyama, Hirai & Fujita, 1974) whose diode gun gives them a superior resolution (Izosaki, Kumada, Okude, Oguso & Goto, 1981) and which have superior `lag' or `sticking' performance, that is a short `memory' for previous high-intensity patterns to which they have been exposed (Shidara, Tanioka, Hirai & Nonaka, 1985).
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