In this module, we will use the Canberra InSpector-2000 Multichannel Analyzer with a high-purity germanium detector (HPGe) and the MGA isotopic anlysis software to assay a variety of plutonium samples. The module provides an understanding of the MGA method, its attributes and limitations. You will assess the system performance by measuring a range of materials similar to those you may assay in your work. During the final verification exercise, the results from MGA will be combined with the 240Pueff results from neutron coincidence or multiplicity counters so that measurements of the plutonium mass can be compared with the operator-declared (certified) values.

The isotopic composition of plutonium and uranium is needed for purposes of sample confirmation, or for interpreting results from calorimeters or neutron-coincidence measurement instruments to determine nuclear material mass. The authors have developed measurement methods and computer codes utilizing high-resolution gamma-ray spectrometry to measure the relative isotopic abundances of plutonium and uranium in various forms nondestructively. The computer codes, known as MGA and MGAU, have unique analysis methodologies that the authors briefly describe in this paper.

A method was developed for the determination of plutonium isotopic composition at the sub-picogram level. Investigations of mass spectral backgrounds and of means of concentrating and purifying sample plutonium were conducted. Isotopic measurements were made by alpha energy analysis and mass spectrometric analysis. A technique for evaluating mass spectral background with a plutonium spike enriched in the mass 239 isotope to 99.98 atom percent is described. A combined alpha energy-mass analysis procedure for the determination of isotopic composition on a single sample mount is reported. The data presented demonstrate that sources of environmentally occurring plutonium could be differentiated by isotopic composition analysis with the developed method even at sub-picogram levels.

We discuss the general approach, computerized data analysis methods, and results of measurements used to determine the isotopic composition of plutonium by gamma-ray spectroscopy. The simple techniques are designed to be applicable to samples of arbitrary size, geometry, age, chemical, and isotopic composition. The combination of the gamma spectroscopic measurement of isotopic composition coupled with calorimetric measurement of total sample power is shown to give a totally nondestructive determination of sample plutonium mass with a precision of 0.6% for 1000-g samples of PuO2 with 12% 24°Pu content. The precision of isotopic measurements depends upon many factors, including sample size, sample geometry, and isotopic content. Typical ranges are found to be 238Pu, 1 to 10%; 239Pu, 0.1 to 0.5%; 24°Pu, 2 to 5%; 241Pu, 0.3 to 0.7%; 242Pu (determined by isotopic correlation); and 241Am, 0.2 to 10%.

We report results of gamma-ray spectroscopy measurements of the isotopic distributions of plutonium in the reference-material set CBNM NRM 271 as analyzed by the FRAM and MGA plutonium isotopic codes. We acquired high-quality spectral data under measurement conditions approximating field-use conditions recommended by the code developers. Bias and precision results from these measurements are presented for both codes. Both codes performed very well for these measurements. These standards have proven to be very useful for testing the Los Alamos FRAM code in the high-burnup region where well-characterized materials have been unavailable at Los Alamos.

A measurement control (MC) program should be an integral part of every nondestructive assay measurement system used for the assay of special nuclear materials. This report describes an MC program for plutonium isotopic composition measurements using high-resolution gamma-ray spectroscopy. This MC program emphasizes the standardization of data collection procedures along with the implementation of internal and external measurement control checks to provide the requisite measurement quality assurance. This report also describes the implementation of the MC program in the isotopic analysis code GRPAUT. Recommendations are given concerning the importance and frequency of the various MC checks in order to ensure a successful implementation of the MC procedures for the user's application.