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Abstract  

The International Organization for Standardization (ISO) Guide to the expression of Uncertainty in Measurement (GUM) was developed to meet the demand for a standardized way of evaluating and expressing uncertainties. The Davies and Gray (D&G) titrimetry method is routinely used in nuclear safeguards for uranium accountability measurement and a statement of the uncertainty that can reasonably be attributed to the measured assay value is therefore of importance. A mathematical model for an uncertainty evaluation of D&G measurements in compliance with ISO GUM is presented. This is illustrated by a numerical example and the utilization of the uncertainty budget is explored.

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Abstract  

Nondestructive measurements of γ-ray and X-ray emissions are often made to characterize special nuclear materials. Various computer codes are available to determine the relative isotopic composition of uranium or plutonium (along with certain other associated nuclides) from analysis of the spectra resulting from such measurements. MGA (Gunnink, Proceedings of the 9th ESARDA symposium on safeguards and nuclear management 167, 1987) and MGAU (Gunnink et al., Proceedings of the IAEA symposium on international safeguards 541, 1994) are among the major isotopic codes. The purpose of this study was to investigate MGA and MGAU performance versus energy resolution of the counting system.

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Abstract  

Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides. Historically, most of these measurements have been performed using shift register based electronics. The shift register data acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay) must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently, the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC) for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register and a commercially available list mode acquisition module.

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