Patent GB-1067430-A (GB)
Title: Improvements in or relating to x-ray analysis
Inventor: BOYCE IAN STANLEY
IPC: G01N23/203
Filed: 1964-09-11
Abstract: 1,067,430. Photo-electric X-ray analysis. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Sept. 1, 1965 (Sept. 11, 1964 (2)]. Nos. 37234/64 and 37235/64. Heading G1A. In X-ray analysis apparatus, using backscattering geometry, the amount of a wanted component or element in a sample containing an interfering component or element is determined by a method which compensates for the effect of the interfering element on the analysis and involves first passing primary X-rays into the sample to produce both secondary radiation (comprising backscattered rays from wanted and interfering elements and possibly some characteristic radiation from the wanted element) and fluorescent radiation from the interfering element, such that the fluorescent radiation is relatively inefficiently excited and the energy of the secondary X-rays is greater than that of the fluorescent rays, and then measuring the intensity of the secondary and fluorescent X-rays using a detector adapted to be more sensitive to the fluorescent X-rays. The effect of an interfering element on the radiation obtained from the sample (matrix absorption effect) is to reduce the intensity of the secondary X-rays by increasing amounts as the concentration of the interfering element increases. By making the detector more sensitive to the fluorescent rays due to the interfering element, this reduction is compensated for.  The method can be applied to the determination of ash content in coal or the analysis of tin ores, in both of which cases iron is the interfering element. In ash content determination, the coal sample 4 Fig. 1 (not shown), in particulate form, is mounted in a test cell or on a moving belt etc., opposite a radiation source mounted in a gold  holder (2). The source may be Cd-109 whose radiation produces backscattered X-rays, of the same energy, and fluorescent X-rays from the iron, of lower energy and comparatively lower intensity, which radiation passes through a protective plastics window (8) and towards the detector, a photo-multiplier. The detector sensitivity to the backscattered radiation is reduced by a plastics phosphor located after the window (8), the thickness of the phosphor being so designed that its efficiency for the fluorescent radiation with respect to its efficiency for the backscattered radiation is at a maximum. If the phosphor is directly coupled to the photo-multiplier cathode direct excitation by X-rays undetected by the phosphor, occurs, so that an attenuating light guide (7) is interposed. The combined effect of phosphor and light guide thus acts to compensate for varying amounts of interfering element. The count rate of the detector (5) can be referred to a calibration curve of ash content independent of iron content or the detector output may be calibrated directly. In the analysis of tin, the source used is Pm-147/A. bremsstrahlung and the sample is of a certain  required thickness. The light guide (7) is not required but an aluminium filter window (8) is  associated with the phosphor, to obtain compensation. In the tin analysis, an Am-241 source may be used, this having two distinct energy components, one exciting the secondary radiation, and the other, the interfering element fluorescent radiation. Figures 2-4 (not shown) illustrate the operation of the compensating apparatus for the two applications.