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A program to measure fission neutron spectra from neutron-induced fission of actinides is underway at the Los Alamos Neutron Science Center (LANSCE) in a collaboration among the CEA laboratory at Bruyères-le-Châtel, Lawrence Livermore National Laboratory and Los Alamos National Laboratory. The spallation source of fast neutrons at LANSCE is used to provide incident neutron energies from less than 1 MeV to 100 MeV or higher. The fission events take place in a gas-ionization fission chamber, and the time of flight from the neutron source to that chamber gives the energy of the incident neutron. Outgoing neutrons are detected by an array of organic liquid scintillator neutron detectors, and their energies are deduced from the time of flight from the fission chamber to the neutron detector. Measurements have been made of the fission neutrons from fission of 235U, 238U, 237Np and 239Pu. The range of outgoing energies measured so far is from 0.7 MeV to approximately 8 MeV. These partial spectra and average fission neutron energies are compared with evaluated data and with models of fission neutron emission. Results to date are summarized in this presentation. Future plans are to make significant improvements in the fission chambers, neutron detectors, signal processing, data acquisition and the experimental environment to provide high fidelity data including measurements of fission neutrons below 0.7 MeV and improvements in the data above 8 MeV. [ABSTRACT FROM AUTHOR]

An accurate knowledge of neutron-induced γ-ray production cross sections is important in enabling the determination of absolute cross sections from relative measurements of γ-ray yields. Such data have a variety of uses, for example, in determining reaction cross sections for activation of materials used as integral neutron flux detectors, and in measuring photon production necessary to evaluate γ-ray heating in nuclear systems. Such data also find application in neutron-induced γ-ray detection and assay systems and in planetary exploration where cosmic-ray-neutron-induced photon production is used to determine elemental composition of soils. In our work using the GEANIE γ-ray spectrometer at the WNR facility of LANSCE, we observed discrepancies outside of the error estimates for evaluations of the natFe(n,n1′γ) cross section for the 847-keV γ ray and for other reactions that are taken as standards. We have performed measurements relative to the natCr(n,n1′γ) 1434-keV γ ray absolute cross section measurements using the 235U(n,f) and 238U(n,f) reactions to determine the neutron fluence. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutron-induced fission of 235U and 239Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Neutron emission spectra from inelastic neutron scattering on natural nickel at the FIGARO facility have been measured by a double time-of-flight technique. The incident neutrons are produced from the spallation source of the Weapons Neutron Research facility, and their energies are determined by time of flight. The emitted neutrons and gamma rays are detected by 16 liquid scintillators and one high-resolution germanium or one barium-fluoride detector, respectively. The results for incident neutron energies from 2 to 10 MeV are compared with predictions of nuclear model calculations performed with the code EMPIRE-II. Finally, the level density parameters “a” and “Δ” are extracted. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Neutron capture cross section measurements on many of the actinides are complicated by low-energy neutron-induced fission, which competes with neutron capture to varying degrees depending on the nuclide of interest. Measurements of neutron capture on 235U using the Detector for Advanced Neutron Capture Experiments (DANCE) have shown that we can partially resolve capture from fission events based on total photon calorimetry (i.e. total γ-ray energy and γ-ray multiplicity per event). The addition of a fission-tagging detector to the DANCE array will greatly improve our ability to separate these two competing processes so that improved neutron capture and (n,γ)/(n,fission) cross section ratio measurements can be obtained. The addition of a fission-tagging detector to the DANCE array will also provide a means to study several important issues associated with neutron-induced fission, including (n,fission) cross sections as a function of incident neutron energy, and total energy and multiplicity of prompt fission photons. We have focused on two detector designs with complementary capabilities, a parallel-plate avalanche counter and an array of solar cells. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]