ABSTRACT

Self absorption is a significant source of error in the assay of Pu-bearing waste by gamma-ray spectrometry. Differential absorption of the 129keV and 414keV lines can be used as a means of determining self-absorption and applying a correction. With a view to refining this technique, the behaviour of the 129keV and 414keV lines from ²³⁹Pu as they emerge from lumps of Pu has been studied using a point-kernel point-detector theoretical model for a range of lump shapes, densities and masses. The shapes include right circular cylinders, rods and rotating square plates and the densities range from those typical of PuO₂ to pure metal. The mass ranged studied covers 1μg to 350g.

Variation in shape and density for Pu lumps may be expected to complicate any correction method. However, from inspection of 3D plots of total lump mass against apparent 129keV and 414keV masses it is observed that points on a plot of apparent masses of the 129keV against the 414keV line of constant total mass lie on continuous curves, irrespective of the shape or density of the lump. This observation has been used to derive a lump correction approach by a 3D fitting algorithm to the computed apparent mass data.

A model has been devised to determine the self-absorption corrected mass from the observed apparent 129keV and 414keV line masses. The paper will present a description of the model, results in the form of 3D plots, a description of the fitting method and results obtained for the corrected mass for theoretical samples and for real samples of known total Pu mass for which the 129keV and 414keV lines have been measured. A discussion of the applicability of this method for multiple lumps is presented.

Keywords: NDA, Plutonium, Self-Absorption