[en] In support of the Advanced Fuel Cycle Initiative, cross sections for hydrogen and helium production by neutrons are being investigated on structural materials from threshold to 100 MeV with the continuous-in-energy spallation neutron source at the Los Alamos Neutron Science Center (LANSCE). The present measurements are for elemental iron. The results are compared with values from the ENDF/B-VI library and its extension with LA150 evaluations. For designs in the Advanced Fuel Cycle Initiative, structural materials will be subjected to very large fluences of neutrons, and the selection of these materials will be guided by their resistance to radiation damage. The macroscopic effects of radiation damage result both from displacement of atoms in the materials as well as nuclear transmutation. We are studying the production of hydrogen and helium by neutrons, because these gases can lead to significant changes in materials properties such as embrittlement and swelling. Our experiments span the full range from threshold to 100 MeV. The lower neutron energies are those characteristic of fission neutrons, whereas the higher energies are relevant for accelerator-based irradiation test facilities. Results for the nickel isotopes, ^58,60Ni, have been reported previously. The present studies are on natural iron.

[en] This consultant was asked to look into the possibility of so-called “scission neutrons”, that is neutrons emitted in the fission process before full acceleration of the two large fragments. Results of new measurements that measure neutron emission relative to the direction of the fragments are available, and the quantification of scission neutron has been derived from these data. More detailed models of the fission process are also new. It is however the conclusion of this consultant that the existence of scission neutrons has not been proven from experimental data. Further, the possibility of some pre-equilibrium process producing high energy neutrons in spontaneous fission or in fission induced by low energy neutrons is also not confirmed. Recommendations are made, with a principal one being that detailed modelling of neutron scattering in the analysis of experimental data is of utmost importance. The data base that pertains to scission neutrons and pre-equilibrium neutrons from the fission process is limited, although the recent experimental data could be mined for more information. (author)

[en] Data on the spectrum of neutrons emittcd from neutron-induced reactions are important in basic nuclear physics and in applications. Our program studies neutron emission from inelastic scattering as well as fission neutron spectra. A ''white'' neutron source (continuous in energy) allows measurements over a wide range of neutron energies all in one experiment. We use the tast neutron source at the Los Alamos Neutron Science Center for incident neutron energies from 0.5 MeV to 200 MeV These experiments are based on double time-of-flight techniques to determine the energies of the incident and emitted neutrons. For the fission neutron measurements, parallel-plate ionization or avalanche detectors identify fission in actinide samples and give the required fast timing pulse. For inelastic scattering, gamma-ray detectors provide the timing and energy spectroscopy. A large neutron-detector array detects the emitted neutrons. Time-of-flight techniques are used to measure the energies of both the incident and emitted neutrons. Design considerations for the array include neutron-gamma discrimination, neutron energy resolution, angular coverage, segmentation, detector efficiency calibration and data acquisition. We have made preliminary measurements of the fission neutron spectra from ²³⁵U, ²³⁸U, ²³⁷Np and ²³⁹Pu. Neutron emission spectra from inelastic scattering on iron and nickel have also been investigated. The results obtained will be compared with evaluated data.

No abstract available

[en] This Workshop was held on September 3--4, 1999, following the 10th International Symposium on Capture Gamma-Ray Spectroscopy. Presentations were made by 14 speakers, 6 from the US and 8 from other countries on topics relevant to s-, r- and rp-process nucleosynthesis. Laboratory experiments, both present and planned, and astrophysical observations were represented as were astrophysical models. Approximately 50 scientists participated in this Workshop. These Proceedings consist of copies of vu-graphs presented at the Workshop. For further information, the interested readers are referred to the authors

[en] Prompt-fission-neutron multiplicities were measured at a 'white' neutron source for the fission of ²³⁸U and ²³⁵U up to 200 MeV. The data are of great importance in the connection of accelerator-coupled nuclear reactor systems incinerating actinides, with uranium considered as a prototype actinide. We report that the fission induced by 200 MeV neutrons produces ∼10 more prompt neutrons than the fission induced by reactor neutrons. In conclusion, new data on (bar ν)_p are reported with errors ≤ 15%. (bar ν)_p grows steadily in the fission of ^238,235U induced by neutrons up to 200 MeV. At this energy, it exceeds by ∼10 the number of neutrons from fission induced by reactor neutrons. Uranium 238 shows enhanced neutron emission compared to uranium 235 due to the higher mass and lower binding energy of the neutrons. In future experiments, we plan to investigate on a much broader range the energy and the angular distributions of the emitted neutrons to improve the precision of the data.

[en] The Los Alamos Neutrons Science Center produces intense pulses of neutrons over 16 orders of magnitude in energy from ultra-cold neutrons to neutrons with energies up to 800 MeV. The facility is based on an 800-MeV proton linear accelerator with high beam intensity and very flexible pulse structures. Neutrons are produced by spallation reactions of the proton beams with tungsten targets. Four separate neutron production areas, their neutron energy ranges, and the number of flight paths that can be used simultaneously are as follows: (1) the Weapons Neutron Research facility (0.1 to 800 MeV) (6 flight paths); (2) the Lujan Center with moderated neutrons (cold - 500 keV) (3 flight paths for nuclear data measurements); (3) the Lead Slowing-Down Spectrometer (0.1 eV to 200 keV) (several channels); and (4) the Ultracold Neutron Research facility (ultracold) (1 flight path). Nuclear data measurements are conducted with the first three of these sources. (author)

[en] Measurements using radioactive targets are important for the determination of key reaction path ways associated with the synthesis of the elements in nuclear astrophysics (sprocess), advanced fuel cycle initiative (transmutation of radioactive waste), and stockpile stewardship. High precision capture cross-section measurements are needed to interpret observations, predict elemental or isotopical ratios, and unobserved abundances. There are two new detector systems that are presently being commissioned at Los Alamos National Laboratory for very precise measurements of (n,γ) and (n,f) cross-sections using small quantities of radioactive samples. DANCE (Detector for Advanced Neutron-Capture Experiments), a 4 π gamma array made up of 160 BaF₂ detectors, is designed to measure neutron capture cross-sections of unstable nuclei in the low-energy range (thermal to ∼500 keV). The high granularity and high detection efficiency of DANCE, combined with the high TOF-neutron flux available at the Lujan Center provides a versatile tool for measuring many important cross section data using radioactive and isotopically enriched targets of about 1 milligram. Another powerful instrument is the Lead-slowing down spectrometer (LSDS), which will enable the measurement of neutron-induced fission cross-section of U-235m and other short-lived actinides in a energy range from 1-200 keV with sample sizes down to 10 nanograms. Due to the short half-life of the U-235m isomer (T_1/2 = 26 minutes), the samples must be rapidly and repeatedly extracted from its ²³⁹Pu parent. Since ²³⁹Pu is itself highly fissile, the separation must not only be rapid, but must also be of very high purity (the Pu must be removed from the U with a decontamination factor >10¹²). Once extracted and purified, the ^235mU isomer would be electrodeposited on solar cells as a fission detector and placed within the LSDS for direct (n,f) cross section measurements. The production of radioactive targets of a few milligrams will be described as well as the containment for safe handling of these targets at the Lujan Center at LANSCE. To avoid any contamination, the targets are electrochemically fixed onto thin Ti foils and two foils are placed back to back to contain the radioactive material within. This target sandwich is placed in a cylinder made of aluminum with thin translucent windows made of Kapton. Actinides targets, such as ^{234,235,236,238}U, ²³⁷Np, and ²³⁹Pu are prepared by electrodeposition or molecular plating techniques. Target thicknesses of 1-2 mg/cm² with sizes of 1 cm² or more have been made. Other targets will be fabricated from separation of irradiated isotopically enriched targets, such as ¹⁵⁵Eu from ¹⁵⁴Sm,¹⁷¹Tm from ¹⁷⁰Er, and ¹⁴⁷Pm from ¹⁴⁶Nd, which has been irradiated in the high flux reactor at ILL, Grenoble. A radioactive sample isotope separator (RSIS) is in the process of being commissioned for the preparation of other radioactive targets. A brief summary of these experiments and the radioactive target preparation technique will be given.

[en] Relative measurements of the cross section for scattering of neutrons by protons have been made at 10 MeV neutron energy for center-of-mass neutron scattering angles from 60' to 180'. The measurements were made using the Ohio University Accelerator Laboratory's tandem Van de Graaff accelerator with the D(d,n) reaction as the neutron source. The data are in good agreement with predictions from the phase shift analyses of Arndt, the groups of Nijmegen and Bonn, and the ENDF/B-V evaluation. The ENDF/B-VI evaluation does not appear to have the same angular dependence as the data. KEYWORDS: hydrogen cross section, neutron cross section standard, hydrogen angular distribution standard

[en] We report the current results of a large effort to accurately measure the Prompt Fission Neutron Spectra (PFNS) for neutron-induced fission of ²³⁵U and ²³⁹Pu for incident neutrons with energies from 1 to 20 MeV. The Chi-Nu experiment at the Los Alamos Neutron Science Center used an unmoderated, white spectrum of neutrons to induce fission in actinide samples that were placed inside a parallel plate avalanche counter to provide a fast fission signal. A double time-of-flight technique was used to determine the incoming and outgoing neutron energies. Two neutron detector arrays, one with 54 liquid scintillators and another with 22 lithium glass detectors, were used to detect the outgoing neutrons and measure the PFNS distributions over a wide range in outgoing neutron energy, from below 100 keV to 10 MeV. Extensive Monte Carlo modeling was used to understand the experiment response and extract the PFNS. Systematic errors and uncertainties in the method have been examined and quantified. A summary of these results for incoming energies from 1 to 5 MeV is presented here.