Abstract :
Further progress has been made with a direct recording system of X-ray diffraction patterns from single crystals. In this system the pattern is formed on a phosphor screen and viewed by a T.V. camera tube whose video output is digitised and presented to a computer. Earlier work had shown that with a sufficiently high degree of light amplification between the phosphor and and the T.V. camera, individual 8keV quantum scintillations could be detected and moderately weak patterns could be displayed on a T.V. monitor. However, for quantitative work on weak patterns the limit is set by the quantum noise due to the low X-ray flux; consequently, in the present system the camera tube, which is now an Image Isocon, is operated in an integrating mode. After each ´exposure´, during which the reading beam is switched off, the integrated charge pattern on the target of the Isocon is read out in a strobing fashion: the target is scanned at normal speeds but the beam is switched on only at those points where diffraction spots are known to be situated. The charge image of a diffraction spot is neutralised when the spot has been scanned, i.e. the read-out from the camera is destructive. It has been found that, owing to slight crystal misorientation and to aberrations in the light-and electron-optical systems, the positions of the diffraction spots can only be predicted approximately from the lattice spacings and initial orientation of the crystal: a computer-controlled search for the spots is, therefore, necessary.
