Nuclear resonance fluorescence (NRF), a process by which a nucleus is excited by absorption of a specific quantum of energy and then de-excites via the emission of one or more γ rays, may be applied to non-destructively measure the isotopic composition of a sample. NRF excitations in 240 Pu were identified in the energy range of 2.1 to 2.8 MeV using a 3-MeV bremsstrahlung source. Utilizing high-purity germanium detectors at backward angles, nine resonances in 240 Pu were identified in this energy range. The measured integrated cross sections range from 29 to 104 eV·b. These resonances are of interest to nuclear structure physics and provide unique signatures for the assay of 240 Pu content for nuclear forensics, nuclear safeguards and counter-terrorism applications.

No abstract

No abstract

Executive SummaryIn nuclear resonance fluorescence (NRF) measurements, resonances are excited by an external photon beam leading to the emission of gamma rays with specific energies that are characteristic of the emitting isotope. The promise of NRF as a non-destructive analysis technique (NDA) in safeguards applications lies in its potential to directly quantify a specific isotope in an assay target.This report addresses the assessment of NRF-based methods for safeguards applications in the context of related studies at LBNL, LLNL, and BNL. Our FY10 effort was comprised of three tasks: the study of the non-resonant scattering background and its simulation with MCNPX, analysis of our previously performed NRF transmission experiment, and the assessment of NRF for safeguards applications.While the recent correction of the treatment of Rayleigh scattering in MCNPX resulted in much better agreement with some experimental data, the photonuclear processes, which are important contributors to the elastic scattering background at higher energies, are still not included. Our analysis showed that calculations based on ENDF form factors as currently implemented in MCNPX could underestimate the elastic scattering cross section by as much as a factor of ten for a uranium target at photon energies above 2 MeV. Even larger discrepancies, up to several orders of magnitude, are possible for lighter elements such as zirconium. It would be desirable to at least include nuclear Thomson scattering in MCNPX simulations for NRF studies.The transmission experiment, using a bremsstrahlung beam and a target of comparable thickness to a spent nuclear fuel assembly, demonstrated sensitivity to a 238 U content of 1%. However, the precision was count rate limited. The data obtained in this experiment indicated notch refill that could change the measured NRF rates by up to 5% for the worst case. A correction based on MCNPX modeling has been implemented in the analysis.NRF-based methods were assessed for three potential safeguards applications: the isotopic assay of spent nuclear fuel (SNF), the measurement of 235 U enrichment in UF 6 cylinders, and the determination of 239 Pu in mixed oxide (MOX) fuel. Given the small integrated nuclear resonance cross sections, the main challenge in these application, albeit to a varying degree, lies in accruing of sufficient counting statistics in an acceptable measurement times for achieving the desired uncertainties.Pu isotopic masses in SNF could precisely be determined in transmission measurements using bremsstrahlung sources, but such measurements would require up to 100's of hours, a very intense bremsstrahlung source, and a very large array of fast detectors with high energy resolution. Quasimonoenergetic photon sources such as Laser Compton scattering sources could potentially enable greatly improved performance. As an example, assuming a photon source with a 1 keV energy spread, an intensity of 6x10 8 ph/eV/s, and operating continuously or at very high pulse rates, the measurement time would be on ...

No abstract

No abstract