A small-angle X-ray camera using a two-dimensional multiwire proportional chamber

The construction and performance of a small-angle X-ray camera equipped with a two-dimensional multiwire proportional chamber (two-dimensional MWPC) are described. The system has been used for studies of the structure of biological samples with a conventional rotating-anode X-ray source. The small-angle camera is of Franks type with two orthogonal platinum-coated bent optical flats placed in a helium-tight box. The specimen-to-detector distance is variable between 60 and 260 cm. The sensitive area of the two-dimensional MWPC is 256 × 256 mm with 256 × 256 resolution elements. The camera covers high angles up to about 5 Å, although diffraction line widths in this region are highly broadened owing to parallax. The electronic system comprises NIM modules, a hand-made module and a commercially available compact two-dimensional multichannel analyser (two-dimensional MCA). The two-dimensional MCA has a memory size of 256 × 256 channels (24 bits/channel). The estimated dead time of this system is approximately 5 μs, which permits a maximum count rate of 20 kHz at 10% loss. Diffraction patterns of a Langmuir–Blodgett film of arachidic acid and stacked purple membranes and a solution scattering of horse spleen apoferritin are demonstrated as examples.