Calculation of the 240Pu-Effective Coefficients for Neutron Correlation Counting?

The 240Pu-effective mass fraction of a plutonium item is a linear combination of the 238Pu, 240Pu, and 242Pu mass fractions. It is a parameter used in the interpretation of passive neutron coincidence, and also multiplicity counting measurements for the assay of total elemental plutonium mass. In this paper, we first define the traditional 240Pu-effective mass equation and then compute the values of the coefficients that appear in the linear combination, along with the associated uncertainty structure. 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 book values (e.g., PANDA Manual) and also against a direct experimental determination. Uncertainty in the relative weighting factors translates into an uncertainty in the plutonium mass estimate when the relative isotopic composition of the measured items differs from that of the reference materials used for calibration. Quantifying this uncertainty is one motivation for the current work, but another goal is to improve firstprinciples neutron metrology and, in particular, the use of other spontaneously fissile materials as a