Optimization of ISOCS Parameters for Quantitative Non-Destructive Analysis of Uranium in Bulk Form

Quantitative calculation of the isotopic masses of fissionable U and Pu is important for forensic analysis of nuclear materials. γ-spectrometry is the most commonly applied tool for qualitative detection and analysis of key radionuclides in nuclear materials. Relative isotopic measurement of U and Pu may be obtained from γ-spectra through application of special software such as MGAU (Multi-Group Analysis for Uranium, LLNL) or FRAM (Fixed-Energy Response Function Analysis with Multiple Efficiency, LANL). If the concentration of U/Pu in the matrix is unknown, however, isotopic masses cannot be calculated. At present, active neutron interrogation is the only practical alternative for non-destructive quantification of fissionable isotopes of U and Pu. An active well coincidence counter (AWCC), an alternative for analyses of uranium materials, has the following disadvantages: 1) The detection of small quantities (≤100 g) of ²³⁵U is not possible in many models; 2) Representative standards that capture the geometry, density and chemical composition of the analyzed unknown are required for precise analysis; and 3) Specimen size is severely restricted by the size of the measuring chamber. These problems may be addressed using modified γ-spectrometry techniques based on a coaxial HPGe-detector and ISOCS software (In Situ Object Counting System software, Canberra). We present data testing a new gamma-spectrometry method uniting actinide detection with commonly utilized software, modified for application in determining the masses of the fissionable isotopes in unknown samples of nuclear materials. The ISOCS software, widely used in radiation monitoring, calculates the detector efficiency curve in a specified geometry and range of photon energies. In describing the geometry of the source-detector, it is necessary to clearly describe the distance between the source and the detector, the material and the thickness of the walls of the container, as well as material, density and chemical composition of the matrix of the specimen. Obviously, not all parameters can be characterized when measuring samples of unknown composition or uranium in bulk form. Because of this, and especially for uranium materials, the IAEA developed an ISOCS optimization procedure. The target values for the optimization are Μ^matrix_fixed, the matrix mass determined by weighing with a known mass container, and Ε_fixed, the ²³⁵U enrichment, determined by MGAU. Target values are fitted by varying the matrix density (ρ), and the concentration of uranium in the matrix of the unknown (w). For each (ρ_i, w_i), an efficiency curve is generated, and the masses of uranium isotopes, Μ^235U_i and Μ^238U_i, determined using spectral activity data and known specific activities for U. Finally, fitted parameters are obtained for Μ^matrix_i = Μ^matrix_fixed ± 1σ, Ε_i = Ε_fixed ± 1σ, as well as important parameters (ρ_i, w_i, Μ^235U_i, Μ^238U_i, Μ^U_i). We examined multiple forms of uranium (powdered, pressed, and scrap UO₂ and U₃O₈) to test this method for its utility in accurately identifying the mass and enrichment of uranium materials, and will present the results of this research.