Neutron capture cross section measurements

Abstract

Neutron capture is an important nuclide production mechanism in nuclear reactors, spent fuel burning and processes of astrophysical interest. The activation cross section values are required to evaluate the number of available neutrons for new fissions and the overall radioactivity balance for further handling of spent fuel. The cross sections of radioactive nuclei are needed in calculations related to accelerator driven energy systems. In astrophysical studies, nucleosynthesis, isotopic abundance and other quantities of interest demand the knowledge of cross sections for some nuclides in the stellar medium characteristic energy range.The thermal neutron capture cross section of a specific nuclide cannot be predicted by theoretical calculations. For energies near the first neutron absorption resonance, the order of magnitude of the energy separation between bound states embedded in the continuum is 1 keV, and the thermal cross-section can be sensitive to changes of a few eV in the position of the first resonance. The currently available nuclear models cannot predict neither which will be the first level above the neutron separation energy nor the position of a level with such precision. An analogous comment can be made on the resonance integral cross-section, which is the nuclide average cross-section in a 1/E neutron kinetic energy spectrum, because the resonances in the first few keV represent the major part of its value. Therefore, the thermal and resonance integral cross-sections can be assessed only by experimental methods. After these quantities were measured, nuclear model calculations can predict at least partially the cross-section energy dependence.We are planning experimental measurements of neutron absorption cross sections by the activation method, in a procedure similar to that used in our previous measurements of 57Co, 137Cs, and 241Am. In order to obtain accurate results, the available computing resources will be used to quantify the detection phenomena using Monte Carlo techniques. The experiment consists in observation of the residual activity of the reaction product after irradiation in the research reactor of IPEN/CNEN-SP, by gamma- and beta-ray spectroscopy in singles or coincidence spectra. The neutron flux will be monitored with wires of Au-Al or Co-Al alloys, or pure gold foils. The thermal and epithermal fluxes in the irradiation position are calculated from the activity of irradiated monitors, covered or not with Cd, employing the Westcott formalism.In several cases, the target will show relatively strong radioactivity, but will yield low activity products. In these cases, the only way to get a proper signal from the reaction product consists in observing high counting rate spectra from the irradiated sample and using the good energy resolution of the HPGe detector to discriminate between target and product. Since conventional gamma-ray spectroscopy systems have troublesome response at counting rates above ten thousand counts per second (cps), we are asking funds for buying a digital spectrometer, which can reach up to one hundred thousand cps with acceptable energy resolution, dead time and deformation of the photon total energy absorption peaks, therefore improving the reaction cross-section measurement threshold by an order of magnitude. (AU)