FY07 LDRD Final Report Neutron Capture Cross-Section Measurements at DANCE

Abstract

We have measured neutron capture cross sections intended to address defense science problems including mix and the Quantification of Margins and Uncertainties (QMU), and provide details about statistical decay of excited nuclei. A major part of this project included developing the ability to produce radioactive targets. The cross-section measurements were made using the white neutron source at the Los Alamos Neutron Science Center, the detector array called DANCE (The Detector for Advanced Neutron Capture Experiments) and targets important for astrophysics and stockpile stewardship. DANCE is at the leading edge of neutron capture physics and represents a major leap forward in capability. The detector array was recently built with LDRD money. Our measurements are a significant part of the early results from the new experimental DANCE facility. Neutron capture reactions are important for basic nuclear science, including astrophysics and the statistics of the {gamma}-ray cascades, and for applied science, including stockpile science and technology. We were most interested in neutron capture with neutron energies in the range between 1 eV and a few hundred keV, with targets important to basic science, and the s-process in particular. Of particular interest were neutron capture cross-section measurements of rare isotopes, especially radioactive isotopes. A more » strong collaboration between universities and Los Alamos due to the Academic Alliance was in place at the start of our project. Our project gave Livermore leverage in focusing on Livermore interests. The Lawrence Livermore Laboratory did not have a resident expert in cross-section measurements; this project allowed us to develop this expertise. For many radionuclides, the cross sections for destruction, especially (n,{gamma}), are not well known, and there is no adequate model that describes neutron capture. The modeling problem is significant because, at low energies where capture reactions are important, the neutron reaction cross sections show resonance behavior or follow 1/v of the incident neutrons. In the case of odd-odd nuclei, the modeling problem is particularly difficult because degenerate states (rotational bands) present in even-even nuclei have separated in energy. Our work included interpretation of the {gamma}-ray spectra to compare with the Statistical Model and provides information on level density and statistical decay. Neutron capture cross sections are of programmatic interest to defense sciences because many elements were added to nuclear devices in order to determine various details of the nuclear detonation, including fission yields, fusion yields, and mix. Both product nuclei created by (n,2n) reactions and reactant nuclei are transmuted by neutron capture during the explosion. Very few of the (n,{gamma}) cross sections for reactions that create products measured by radiochemists have ever been experimentally determined; most are calculated by radiochemical equivalences. Our new experimentally measured capture cross sections directly impact our knowledge about the uncertainties in device performances, which enhances our capability of carrying out our stockpile stewardship program. Europium and gadolinium cross sections are important for both astrophysics and defense programs. Measurements made prior to this project on stable europium targets differ by 30-40%, which was considered to be significantly disparate. Of the gadolinium isotopes, {sup 151}Gd is important for stockpile stewardship, and {sup 153}Gd is of high interest to astrophysics, and nether of these (radioactive) gadolinium (n,{gamma}) cross sections have been measured. Additional stable gadolinium isotopes, including {sup 157,160}Gd are of interest to astrophysics. Historical measurements of gadolinium isotopes, including {sup 152,154}Gd, had disagreements similar to the 30-40% disagreements found in the historical europium data. Actinide capture cross section measurements are important for both Stockpile Stewardship and for nuclear forensics. We focused on the {sup 242m}Am(n,{gamma}) measurement, as there was no existing capture measurement for this isotope. The cross-section measurements (cross section vs. E{sub n}) were made at the Detector for Advanced Neutron Capture Experiments. DANCE is comprised of a highly segmented array of barium fluoride (BaF{sub 2}) crystals specifically designed for neutron capture-gamma measurements, using small radioactive targets (less than one milligram). A picture of half the array, along with a photo of one crystal, is shown in Fig. 1. DANCE provides the world's leading capability for measurements of neutron capture cross sections with radioactive targets. The DANCE is a 4{pi} calorimeter and uses the intense spallation neutron source the Lujan Center at the Los Alamos National Laboratory. The detector array consists of 159 barium fluoride crystals arranged in a sphere around the target. « less