HOMOGENEITY TESTING OF SPIKED REFERENCE MATERIALS

As part of a EUROMET project to produce a range of gamma-ray emitting radioactive spiked reference materials, a technique, based on analysis of variance, was developed to evaluate the uncertainties in character ising such materials. A particular concern was to ensure that users of these materials are properly informed regarding the consequences of a ny remaining inhomogeneities in them. A minimum of five random samples of each reference material were counted six times on a suitable gamma -ray spectrometer. Once all of the data were collected and normalised, between-sample (MS(BS)) and within-sample (MS(WS)) variances were cal culated. The quantity MS(WS) includes factors associated with sample m easurement such as counting statistics, background stability, detector response stability, source-to-detector geometry and electronic stabil ity. The quantity MS(WB) includes not only all of the terms in MS(WS) but also includes terms for sample inhomogeneity, mass determination a nd density determination. These quantities enable the following calcul ation to be performed; (1) sigma(mat)(2) = sigma(tot)(2) - sigma(meas) (2) where sigma(tot) is the overall uncertainty, sigma(meas) is the me asurement uncertainty and sigma(mat) is the uncertainty arising from s ample inhomogeneity; it is assumed here that the effects arising from mass and density determinations are negligible. The value of sigma(mat ) is only valid at the sample weight at which it was measured, and dat a are presented to show the variation of the uncertainty arising from sample inhomogeneity with changing sample weight.