Method and apparatus for the elimination of the distortions of a x-rays - diffraction image of substances with crystals with a preferred orientation

摘要

1,119,511. Diffraction apparatus. ESSO RESEARCH & ENG. CO. 24 Aug., 1965 [12 Oct., 1964], No. 36333/65. Heading H5R. A method of eliminating the distortions in the diffraction pattern of a fibrous or non- fibrous material having crystallites of a preferred orientation involves rotating a sample of the material so that an axis # through the sample rotates end over end through angles # in a plane ZY normal to the plane determined by the beams incident on, and diffracted by, the sample and bisecting the angle formed by these beams. If the sample is non-fibrous, it is also spun about the # axis. If the sample is fibrous, e.g. of cylindrical symmetry, the spinning about the # axis is unnecessary and the # motion only is carried out. This is the case with all films or plane sheets whose preferred orientation is known to have such symmetry. The distortions take the form of inaccuracies in the intensities of the peaks of the diffraction patterns (Figs. 9A-9F, not shown) and are compensated for in a number of ways depending on the physical form of the material. With very small samples, the X-ray beam completely bathes the sample with radiation so that all parts of the sample are irradiated equally. A more massive sample is shaped into a cylinder or sphere and absorption of radiation is reduced by the use of an aperture diaphragm to ensure irradiation of a selected portion of the sample. A translational oscillatory motion in the plane normal to that determined by the incident and diffracted beams in a direction parallel to the bisector of these beams (the Z axis) is given to such a sample and the aperture (A), Fig. 4 (not shown), ensuring equal irradiation of every crystallite within the aperture limits. All the motions (#, # and oscillatory) have periods much shorter than that of the diffractometer recorder, whose motion, through angles 2#, is in the plane of incident and diffracted beams. With all forms of sample, a factor sin # has to be generated for combining with the intensity measurement to complete the compensation for distortions. In the described embodiment, this is carried out using a rotating filter disc 1 in the incident beam, although it may alternatively be in the diffracted beam, and synchronized with the # rotation of the sample. The disc comprises an apertured aluminium plate having aluminium and plastics absorbing plates of thicknesses varying such that the relative intensities transmitted through a sector of the disc located at angular position # approximates to sin #. The sin # factor may also be introduced by varying the X-ray source intensity. The application of the method to diffraction analysis of a sample of polypropylene, wound round a thin metal plate, is described. The record is a diffraction curve recorded on a strip chart, the distance along the chart, corresponding to angle 2#, and that vertically, to the intensity signal.