Uncertainty analysis for determination of plutonium mass by neutron multiplicity counting

This paper describes an uncertainty analysis carried out in association with the use of neutron multiplicity counting to collect data, and assign a total plutonium mass. During 1997, the Los Alamos Safeguards Science and Technology Group carried out careful calorimetry and neutron multiplicity certification measurements on two {sup 239}Pu metal foils used as reference standards at the Idaho National Environmental Engineering Laboratory (INEEL). The foils were measured using a five ring neutron multiplicity counter designed for neutron measurement control activities. This multiplicity counter is well characterized, and the detector parameters were reaffirmed before the measurements were made using several well-known Los Alamos standards. Then, the {sup 240}Pu effective mass of the foils was determined directly from the multiplicity analysis without a conventional calibration curve based on representative standards. Finally, the {sup 240}Pu effective mass fraction and the total plutonium mass was calculated using gamma ray isotopics. Errors from statistical data collection, background subtraction, cosmic ray interaction, dead time corrections, calibration constants, sample geometry, and sample position were carefully estimated and propagated. The authors describe these error sources, the final calculated relative error in the foil assay, and the comparison with very accurate calorimetry measurements