A cumulative method of particle size analysis is described wherein the mass deposited from an homogeneous suspension by gravitational or centrifugal force is determined by a p-back-scatter thickness measurement technique. The method is applicable to compounds having an effective atomic number greater than 16, and comparisons of results obtained by this and other methods for thoria, urania, zirconia and lithopone powders are given.J. appl. Chem., 9, October, 1959
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COIVNOR et al.-PAHTICLE SIZE DETERMINATION
IntroductionIn a previous paper1 it was mentioned that the application of p-back-scattering to particle size analysis was being studied, and the results of this work are now presented.Apart from optical microscopy most methods of particle size analysis involve the measurement of the rate of particle sedimentation in liquid or gaseous media. The use of @-backscattering for measuring the cumulative deposition of solid particles does not appear to have been investigated although its application in the measurement of the thickness of materials and films is well known.The scattering of p-particles is a complex process and is described elsewhere.a I t may be said to depend upon the energy of the p-particles and the thickness and mean atomic number of the scattering material. The way in which the back-scattered radiation from a aoSr-gOY source increases to a limiting value as the thickness of scattering material is increased is shown in Fig. 1 for water, for dry thoria and for a sedimented thoria slurry on the thin base of a Perspex tube. The shape of the curve depends on the geometry and type of detector used.3 It is apparent that when such a tube contains a dilute aqueous suspension of, for example, thoria, the scattering is almost entirely from the plastic base and water and reaches its saturation value at a very small distance from the bottom. If the thoria is allowed to settle, back-scattering will increase as the thorium atoms make their contribution and the mass of thoria settled can be deduced from the increased count rate, after a suitable calibration, using the initial linear portion of the curve. Only the thoria on, or very near, the bottom of the tube will be effective in scattering as the rest is shielded by the water.The sensitivity of the technique will depend on the relative scattering efficiencies of the solid and the liquid in which it is suspended. This is illustrated in Fig. 2, which shows that the relative scattering of titania (TiO,) and water is barely sufficient for a size analysis. The effective atomic number, z, for back-scattering from compounds is defined by Male+ as fl(AbZb) f m(AcZc) mol. wt. of B,Cm where Ab and Ac are the atomic weights of B and C for the compound B, C, .For titaniax is approximately 16 and the method cannot be used for powders with z lower than this. However, the range of application can be extended by sedimentation in gaseous, rather than liquid, media, since back-scatter from a gas is extrcmcly small. While this possibility has been demonstrated, only work relating...