Abstract
The 240Pu-effective mass fraction is a linear combination of 238Pu, 240Pu and 242Pu. It is used in the interpretation of passive neutron coincidence and multiplicity counting measurements for the assay of elemental Pu-mass. In this article we compute the values, along with the associated uncertainty structure, of the coefficients that appear in the linear combination. We do this using the best currently available evaluated nuclear data – spontaneous fission half-life and prompt neutron multiplicity distributions. Comparisons are made against commonly used values and also direct determination. Uncertainty in the relative weighting factors translate into an uncertainty in the Pu mass estimate when the measured items relative isotopic composition differs from that of the reference materials used for calibration.