At many points in the safe and transparent handling of plutonium materials the relative isotopic composition of the principle isotopes needs to be known. Sometimes this information may be of primary interest — such as in the verification of safeguard declarations or in the confirmation of the reactivity of mixed oxide fuel. At other times, e.g., for radioactive waste characterization, the isotopic composition may be needed to calculate specific thermal power or specific spontaneous fission rates for the item under study, which can subsequently be combined with calorimetric and correlated neutron counting measurements, respectively, in order to make quantitative assessments of the mass of Pu and associated nuclides that are present in an item. The Multi-Group Analysis code MGA is a highly regarded and widely used computer code for the analysis of high resolution gamma ray spectra in order to extract the relative isotopic composition of plutonium for a diversity of items with minimal prior information. It has been honed over many years to give reliable results for a broad range of measurement scenarios commonly encountered in the fuel cycle. The nuclear industry is not dormant however and the demands on such codes continue to shift as a combination of technology and necessity open up new application areas. For example, while MGA had its origins in the analysis of clean spectra on product material principally for nuclear safeguards applications taken with germanium detectors having good low-energy resolution, it is now widely applied to the characterization of drummed waste forms and the complex spectra from such items acquired with much larger volume and poorer resolution detectors often used in such applications for the dual use of quantitative assay of the many gamma-emitters. This new domain of operational experience resulted in the need to enhance MGA to deal with spectra of poor statistical quality and also to cope with some of the complications that arise in the analysis of unusual spectra. Together with some additional changes made to incorporate feedback since the release of version 9.63 (which had minor revisions denoted by the letters A through H) of the code this has resulted in the creation of MGA v10. In this paper we shall outline the main changes to the code explaining why they were conceived and implemented. We illustrate what kinds of measurement problems can now be addressed over and above the previous capabilities which have been preserved and verified by the same set of regression tests that have been applied to previous generation of the code.

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