Mechanical, Aerospace, and Nuclear Engineering

A description is given of a lead slowing-down spectrometer (LSDS) installed at the 800-MeV proton accelerator of the Los Alamos Neutron Science Center (LANSCE). The LSDS is designed to study neutron-induced fission on actinides that can only be obtained or used in small quantities. The characteristics of this LSDS (energy-time relation, energy resolution, neutron flux) are presented through simulations with MCNPX and measurements with several different methods. Results on neutron-induced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, additional MCNPX calculations have been performed to simulate the measurement of the cross-section for 235m Uðn; fÞ using different target quantities and different initial isomer-to-ground state compositions. r ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ -see front matter r (D. Rochman), haight@lanl.gov (R.C. Haight), odonnell@lanl.gov (J.M. O' Donnell), michaudon@lanl.gov (A. Michaudon), wender@lanl.gov (S.A. Wender), vieira@lanl.gov (D.J. Vieira), bond@lanl.gov (E.M. Bond), toddb@lanl.gov (T.A. Bredeweg), kronenberga@mail.phy.ornl.gov (A. Kronenberg), j_wilhelmy@lanl.gov (J.B. Wilhelmy),

The ENDF/B-VII.1 library is our latest recommended evaluated nuclear data file for use in nuclear science and technology applications, and incorporates advances made in the five years since the release of ENDF/B-VII.0. These advances focus on neutron cross sections, covariances, fission product yields and decay data, and represent work by the US Cross Section Evaluation Working Group (CSEWG) in nuclear data evaluation that utilizes developments in nuclear theory, modeling, simulation, and experiment. The principal advances in the new library are: (1) An increase in the breadth of neutron reaction cross section coverage, extending from 393 nuclides to 423 nuclides; (2) Covariance uncertainty data for 190 of the most important nuclides, as documented in companion papers in this edition; (3) R-matrix analyses of neutron reactions on light nuclei, including isotopes of He, Li, and Be; (4) Resonance parameter analyses at lower energies and statistical high energy reactions for isotopes of Cl, K, Ti, V, Mn, Cr, Ni, Zr and W; (5) Modifications to thermal neutron reactions on fission products (isotopes of Mo, Tc, Rh, Ag, Cs, Nd, Sm, Eu) and neutron absorber materials (Cd, Gd); (6) Improved minor actinide evaluations for isotopes of U, Np, Pu, and Am (we are not making changes to the major actinides 235,238 U and 239 Pu at this point, except for delayed neutron data and covariances, and instead we intend to update them after a further period of research in experiment and theory), and our adoption of JENDL-4.0 evaluations for isotopes of Cm, Bk, Cf, Es, Fm, and some other minor actinides; (7) Fission energy release evaluations; (8) Fission product yield advances for fission-spectrum neutrons and 14 MeV neutrons incident on 239 Pu; and (9) A new decay data sublibrary. Integral validation testing of the ENDF/B-VII.1 library is provided for a variety of quantities: For nuclear criticality, the VII.1 library maintains the generally-good performance seen for VII.0 for a wide range of MCNP simulations of criticality benchmarks, with improved performance coming from new structural material evaluations, especially for Ti, Mn, Cr, Zr and W. For Be we see some improvements although the fast assembly data appear to be mutually inconsistent. Actinide cross section updates are also assessed through comparisons of fission and capture reaction rate measurements in critical assemblies and fast reactors, and improvements are evident. Maxwellian-averaged capture cross sections at 30 keV are also provided for astrophysics applications. We describe the cross section evaluations that have been updated for ENDF/B-VII.1 and the measured data and calculations that motivated the changes, and therefore this paper augments the ENDF/B-VII.0 publication [1].

We report on the design, fabrication, and performance of solid-state neutron detector based on three-dimensional honeycomb-like silicon micro-structures. The fabricated detectors use boron filled deep holes with aspect ratio of over 12 and showed a very low leakage current density of $7 Â 10 À7 A/cm 2 at À1 V for device sizes varying from 2 Â 2 to 5 Â 5 mm 2 . A thermal neutron detection efficiency of 4.5% 6 0.5% with discrimination setting of 500 keV and gamma to neutron sensitivity of (1.1 6 0.1) Â 10 À5 for single layer was measured without external bias for these devices. Monte-Carlo simulation predicts a maximum efficiency of 45% for such devices filled with 95% enriched 10 boron. V C 2012 American Institute of Physics.

A Lead Slowing Down Spectrometer (LSDS) provides a high neutron flux environment that enables measurements of small samples (∼µg) or samples with small cross sections (tens of µb). The LSDS at Rensselaer Polytechnic Institute (RPI) was previously used for fission cross section measurements and for studies of methods for assay of used nuclear fuel. The effective energy range for the LSDS is 0.1 eV to 10 keV with energy resolution of about 35%. Two new LSDS applications were recently developed at RPI; the first enables simultaneous measurements of the fission cross section and fission fragment mass and energy distributions as a function of the incident neutron energy. The second enables measurements of the (n,α) and (n, p) cross sections for materials with a positive Q value for these reactions. Fission measurements of 252 Cf, 235 U, and 239 Pu were completed and provide information on fission fragment and energy distributions in resonance clusters. Measurements of the (n,α) cross section for 147,149 Sm were completed and compared to previously measured data. The new data indicate that the existing evaluations need to be adjusted.