Neutron activation analysis for the determination of gold and the platinum group elements in large samples

Abstract

The method of activation analysis is extended to the analysis of very large samples (up to 1kg) of ores and minerals. The use of such large samples has two advantages: first, the sensitivity of the method is greatly increased, and secondly, inhomogeneity of distribution and hence sampling problems are greatly reduced. Samples are used without any kind of treatment and the method is non - destructive. There are also disadvantages associated with the use of large samples. Absorption effects (both of neutron and y - rays) become important and as a result the calibration curves are influenced by the bulk density and composition of the samples. In certain cases, the calibration curves may also be non - linear. These effects have been studied in detail for the determination of gold, through the reaction 198Au(n,y)l98Au. After irradiating various samples of known concentrations of gold, calibration curves for gold in gold ore and carbon samples have been established. The effect of y -ray and neutron absorption has been studied in carbon samples using either thermal or epithermal neutrons and the optimum sample thickness has been established. In addition, the curve for the optimum decay time has been obtained for gold in ore samples. Based on the experimental results, a theoretical model has been applied to study the variation of the statistical error with the various factors involv~d, i.e. irradiation, decay, counting times, source strength and sample size. It has been also used to analyse theoretically the results for the optimum thickness and the variation of the background under the 411. 8 keV peak of Au-198. Further work has been undertaken to investigate whether the method can be used to determine elements from the platinum group which are much less active than the gold. Gamma ray peaks from Ir - 192, Au - 199, Pt - 199 Rh - 104m and Os-193 could be identified and possibly from Pd-109. Further work is necessary to study each element separately. In the case of gold, very satisfactory results were obtained. It is too early to state that it provides a replacement for the fire - essay method for gold, but the potential definitely exists.