X-ray topography with scintillators coupled to image intensifiers or camera tubes

The possibility of imaging topographic figures in real time by using a thin scintillator coupled to either a high-gain image intensifier or a camera tube is investigated. The camera tube must have a high gain because of the low photon fluxes that are encountered in practice, and because of the relatively low quantum yield of thin phosphors. With conventional X-ray generators, the resolution is photon-noise limited. With more powerful generators like synchrotrons, real-time imaging appears possible, and the resolution is limited by the modulation transfer function of the image tube. Higher resolution can be reached by increasing the magnification between the screen and the image tube. When doing so, the input field is reduced and thinner phosphor screens must be used, resulting in a lower yield. Each time the magnification is doubled, the minimum required photon flux is multiplier by about 8, so that the advantages of increasing the magnification are rapidly limited, so far as real-time imaging is concerned. Because image tube resolution is mainly limited by the modulation transfer function of the phosphor for image intensifiers, and by that of the target for camera tubes, improvement of photocathode resolution can be obtained by magnifying electron optics. A zooming electron optic would permit the field and the resolution of the tube to be adapted to the observed subject. Unfortunately such tubes do not exist at present for this type of application, and in the required size