Re-evaluation of the ¹⁶O(n, γ)¹⁷O cross section at astrophysical energies and its role as a neutron poison in the s-process

The doubly magic nucleus ¹⁶O has a small neutron-capture cross section of just a few tens of microbarns in the astrophysical energy region. Despite this, ¹⁶O plays an important role as a neutron poison in the astrophysical slow neutron capture (s) process due to its high abundance. We present in this paper a re-evaluation of the available experimental data for ¹⁶O(n, γ)¹⁷O and derive a new recommendation for the Maxwellian-averaged cross sections between kT = 5 and 100 keV. Our new recommendations are lower up to kT = 60 keV compared to the previously recommended values but up to 14% higher at kT = 100 keV. We explore the impact of this different energy dependence on the weak s-process during core helium burning (kT = 26 keV) and shell carbon burning (kT = 90 keV) in massive stars where ¹⁶O is the most abundant isotope

Detector for advanced neutron capture experiments at LANSCE

The Detector for Advanced Neutron Capture Experiments (DANCE) is a 159-element 4x barium fluoride array designed to study neutron capture on small quantities, 1 mg or less, of radioactive nuclides. It is being built on a 20 m neutron flight path which views the 'upper tier' water moderator at the Manuel J. Lujan Jr. Neutron Scattering Center at the Los Alamos Neutron Science Center. The detector design is based on Monte Carlo calculations which have suggested ways to minimize backgrounds due to neutron scattering events. A data acquisition system based on fast transient digitizers is bcing implemente

Workshop on the origin of the heavy elements: Astrophysical models and experimental challenges, Santa Fe, New Mexico, September 3-4, 1999

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

An Update of the Nuclear Data Standards Activities

To allow new experimental data and improvements in evaluation procedure to be used in a timely manner in producing standards evaluations, an IAEA Data Development Project was initiated. For this project, it was decided that, in addition to the traditional activities related to standards, studies should be done on the possibility of extending the energy ranges of the standards and whether to include ¿reference data¿ that are not as well known as the standards but can be very useful in the measurements of certain types of cross sections. Activities include; an update of the experimental database for the traditional standards; improvements in smoothing procedures for capture cross sections, a review of the status of 252Cf spontaneous fission neutron spectrum measurements; new work on the 235U thermal neutron induced fission neutron spectrum that is not a standard but is important for reactor applications; improved calculations for fission neutron spectra; consideration of the 197Au capture reaction as a reference cross section for capture cross-section measurements at energies of importance to astrophysics; and a study of reference cross sections for prompt gamma-ray production in fast neutron-induced reactions. A review of the progress made by this project will be given.JRC.DG.D.5-Nuclear physic

New work on updating and extending the Nuclear Data Standards

An IAEA Data Development Project was initiated to provide a mechanism for allowing new experimental data and improvements in evaluation procedure to be incorporated into new evaluations of the standards. The work on this project is ongoing. In the past very long periods sometimes occurred between evaluations of the standards. Through the use of this Project, such long periods should not occur. Work on the cross section standards through this Project has included an update of the experimental data to be used in the cross section standards evaluations, a study of the uncertainties obtained in the international standards evaluation; and improvements in the smoothing procedure for capture cross sections. It was decided that this Project should have a broader range of activities than just the cross section standards and thus encompass standards related activities. The following are being investigated: improvements in the gold cross section at energies below where it is considered a standard; work on certain cross sections which are not as well known as the cross section standards but could be very useful as reference cross sections relative to which certain types of cross section measurements can be made. This work includes prompt gamma-ray production in fast neutron-induced reactions; and work on the 252Cf spontaneous fission neutron spectrum and the 235U thermal neutron fission spectrum. Most of the data investigated through this Project are used in dosimetry applications.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

DANCE : Device for Measurement of (n.g.) Reactions on radioactive Species /

DANCE (Device for Advanced Neutron Capture Experiments) is a 4{pi} 162 element BaF{sub 2} array under development at Los Alamos National Laboratory. It is designed to provide high granularity, fast timing and high photon detection efficiency. It will be located at the Los Alamos Neutron Scattering Center where neutrons are produced using 800 MeV proton induced spallation reactions on heavy element production targets. Using the pulsed high neutron fluence available at this facility combined with time of flight techniques it will be possible to make neutron capture measurements in the neutron energy range from eV to 100's of keV on rare and radioactive target material at the milligram and below level. These measurements will provide critically needed data for the interpretation of the astrophysical s-process 'branching point' nuclei as well as information for reactions needed in understanding transmutation processes of radioactive species