Delayed Neutron and Delayed Gamma Counting To Co-detect and Quantify Trace Quantities of Uranium and Plutonium*

The analytical method involving irradiation of swipe samples potentially containing trace quantities of fissile material in a thermal neutron field, followed by the counting of delayed neutrons, is a well-known technique in the field of safeguards and nonproliferation. It is used for detecting the presence of microscopic amounts of fissile material and quantifying it in terms of the equivalent mass of 235U. The technique is been routinely employed at the High Flux Isotope Reactor (HFIR) facility at Oak Ridge National Laboratory (ORNL). The delayed neutron counting technique is not well suited to the task of distinguishing between individual fissile isotopes. Plutonium has a high safeguards value, but it may be masked by U; therefore, being able to distinguish between U and Pu would augment and strengthen safeguards conclusions that can be drawn by the International Atomic Energy Agency from this approach and provide a detection approach for other kinds of undeclared activities. To determine isotope-specific information (e.g., 235U, 239Pu and 241Pu), a new hybrid experimental approach that uses delayed gamma counting of fission product gamma rays in combination with delayed neutron counting is under investigation at the HFIR. The present work seeks to build on the delayed neutron and delayed gamma methods that have been developed at ORNL. In the present work, it is recognized that the distribution of fission products created following neutron-induced fission of different fissile isotopes are. The distribution profile of heavy fission products remains fairly invariant for the fissile nuclides. However, the distribution of light fission products varies from one isotope to another. That is, the ratio of light fission fragments to heavy fission fragments is different for different fissile species such as 235U, 239Pu, 233U, and so forth. Measurement of the ratio of the net full energy peak (FEP) from high/low mass fission products offers an attractive way to characterize the fraction of fissile materials present in a mixture. By empirically calibrating the ratio of the net FEP as a function of known concentration of the binary mixture, one can determine the fraction of fissile isotopes in an unknown sample. The delayed neutron counter at the HFIR Neutron Activation Analysis Laboratory along with a well-characterized Large- Volume Active Well (neutron) Coincidence Counter and a High-Purity Germanium (HPGe) gamma-ray spectrometer are used to perform test measurements of the irradiated fissile samples. Samples of single