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A new prompt gamma-ray database for cold and thermal neutron capture

  • Author(s): Firestone, R.B.
  • Molnar, G.L.
  • Revay, Zs.
  • Kasztovszky, Zs.
  • et al.
Abstract

Comprehensive prompt neutron capture gamma-ray data are important for basic nuclear structure studies, reaction mechanisms, sorr-process nucleosynthesis, and applications like Prompt Gamma Ray Activation Analysis (PGAA). The only readily available prompt neutron capture gamma ray library is the compilation by Lone et al1. These data are elemental measurements without anyisotopic assignments reported. They were measured with early, small Ge(Li) detectors and have numerous contaminant peaks, missing transitions, and limited uncertainty information. The Evaluated Nuclear Structure Data File (ENSDF)2 has more up-to-date information and is organized by isotope. The ENSDF neutron capture intensities are not normalized to absolute cross sections, but instead to per 100 decays of the parent. These normalizations are seldom more precise than +-10% and are not useful for applications like PGAA. Until now no completely satisfactory library of prompt neutron gamma rays has been produced. New prompt gamma-ray data have emerged from a series of experiments at the 10-MW Budapest Research Reactor. Accurate gamma-ray energies and production cross sections have been measured for nearly all elements from hydrogen to uranium. The measurements were performed with natural targets using a subthermal guided neutron beam and a Compton-suppressed HPGespectrometer. Energies were measured with respect to well-known 35Cl neutron-capture gamma rays. Gamma-ray production cross sections were determined with respect to the 1H capture cross section using internal standards, mostly stoichiometric compounds, to avoid corrections for neutron absorption and scattering. Precision of a few percent was attained for the strongest gamma rays from most elements. The Budapest data is precise, but the complexity of elemental measurements often makes it difficult to resolve weak transitions. To remedy this situation we are combining the isotopic data from ENSDF with the Budapest elemental data as part of an International Atomic Energy Agency Coordinated Research Project. The ENSDF file often contains precise energies and relative transition intensities that were measured with separated isotope targets. We are using ENSDF to assign the gamma rays in the Budapest datasets to the correct isotopes and place them in the level scheme. The level energies are then least-squares fit to the gamma-ray energies todetermine the self-consistency of the data. Outliers are a possible indication of unrecognized multiplets or errors in the level schemes. The ENSDF gamma-ray intensities are then renormalized to the identical scale as the Budapest intensities. The two datasets are then averaged and discrepancies are noted and corrected. The resulting prompt gamma-ray neutron capture database will be more completeand self-consistent then either of the component databases. In special cases additional corrections for non-unity Westcott g-factors will be provided.A first pass through the Budapest data has been completed, and we have assigned gamma rays for each element to the level schemefor the appropriate isotope based on ENSDF. This provides a database for identifying impurities in the spectra. We have also compared the observed cross section yields, based on the level scheme or the decay gamma rays, with those compiled by Mughabghabet al3. Although our values should be less than or equal to the compiled values, depending on the completeness of the level scheme, we found that many values differed well beyond the error bars. The final analysis of the prompt gamma-ray neutron capture database is in progress and we will report on our results in this paper in this presentation.

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