International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 7.1, p. 632

Section 7.1.6.5.2. Light coupling

U. W. Arndtb

7.1.6.5.2. Light coupling

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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[link]). 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}]. 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[link]; Koch, 1994[link]; Allinson, 1994[link]). 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[link]) 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[link]) 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[link]).

References

Allinson, N. M. (1994). Development of non-intensified charge-coupled device area X-ray detectors. J. Synchrotron Rad. 1, 54–62.
Arndt, U. W. & Gilmore, D. J. (1979). X-ray television area detectors for macromolecular structural studes with synchrotron radiation sources. J. Appl. Cryst. 12, 1–9.
Deckman, H. W. & Gruner, S. M. (1986). Formal alterations in CCD-based electro-optic X-ray detectors. Nucl. Instrum. Methods, A246, 527–533.
Koch, A. (1994). Lens-coupled scintillating screen CCD X-ray area detector with a high DQE. Nucl. Instrum. Methods, A348, 654–658.
Naday, I., Westbrook, E. M., Westbrook, M. L., Travis, D. J., Stanton, M., Phillips, W. C., O'Mara, D. & Xie, J. (1994). Nucl. Instrum. Methods, A348, 635–640.
Roehrig, H., Dallas, W. J., Ovitt, T. W., Lamoreaux, R. D., Vercillo, R. & McNeill, K. M. (1989). A high-resolution X-ray imaging device. Proc SPIE, 1072, 88–99.








































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