Compton Suppression System for Nuclear Safeguards Application

Quantification of nuclear material such as uranium and plutonium isotopes, as well as other actinides such as 241Am is required for nuclear material accountancy for nuclear T safeguards. Besides quantitative results, regulatory bodies such as the International Atomic Energy Agency (IAEA) need the isotopic attributes of nuclear material, e.g., $^{\mathbf {235}}\textbf{U}$ enrichment. Prominent gamma ray signatures from nuclear materials are predominantly at low energies. Besides actinides, samples may contain high activities of fission product nuclides, many of which emit higher energy gamma rays ($\ge $ 600 keV). High Purity Germanium (HPGe) detectors are commonly employed in safeguards applications that require the best possible energy resolution. The analysis of low energy gamma rays is rendered difficult because of the high Compton continuum (partial energy deposition in the HPGe) due to high energy gamma rays originating in the sample. This results in the degradation of the sensitivities that can be achieved at low energies of interest. Use of a Compton Suppression Spectrometer (CSS) is explored for enhancing sensitivity of isotopic analysis and for improving the detection limits of radionuclides of interest in safeguards. A CSS, consisting of a an annular NaI(Tl) guard detector, an NaI(Tl) plug detector and a HPGe spectrometer was configured at the neutron activation analysis (NAA) laboratory at the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL). Gamma ray spectra were simulated for challenging scenarios of interest in nuclear safeguards such as assay of $^241$Am in the presence of relatively high activity of $^137$Cs, and assay of $^235U$ in the presence of relatively high activity of $^137$textbfCs. An extended version of the Monte Carlo N-Particle (MCNP) code, called MCNP-CP (where CP stands for Correlated Particles) is used to study the performance of Compton Suppression Spectrometer by progressively increasing the relative activity of the interfering isotope ($^137$Cs) and studying the impact on the sensitivity of the net peak areas at the gamma ray energies of interest. Suppression ratios were established using the simulated spectra. Impact of Compton Suppression on gamma rays emitted in true-coincidence summing was demonstrated using results from $^60$Co simulations. Benchmarking measurements are in progress. The system will be available in the near future for the measurement of samples containing fissile material.