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The neutron capture cross sections for 207, 208Pb have been measured in the neutron energy region from 10 to 110 keV. The γ-rays cascaded from a capture state to the ground state or low-lying states of 208, 209Pb were observed for the first time, using an anti-Compton NaI(Tl) spectrometer and a TOF method. The observed discrete γ-ray energy spectra enabled us to determine neutron capture cross sections for 207, 208Pb with small systematic errors, since we could distinguish γ-ray of 207, 208Pb(n, γ) reactions from background γ-ray with use of the γ-ray spectra. The obtained cross sections include both contributions of resonance and direct capture components different from the previous TOF measurements. [ABSTRACT FROM AUTHOR]

We are developing a 4π Ge-spectrometer for the measurement of neutron capture cross sections of minor actinides. The 4π Ge-spectrometer consists of Ge detectors that cover a large solid angle of a sample; the spectrometer has high efficiency and good energy resolution for γ-ray detection. This feature enables us to discriminate desired events from backgrounds based on the measurement of total excitation energy following the neutron capture reaction. In this work, we determined an optimal configuration and evaluated performances of 4π Ge-spectrometer using a Geant4 simulation. From comparison of performances, two configurations that have uniform count rates or a high efficiency were determined. Therefore one of the two configurations is used properly to reduce an experimental error for the neutron capture cross section. Further simulations were made with a full geometry including BGO shields; absolute efficiency for the high-efficiency configuration was estimated to be 15% for a 1-MeV γ ray. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Most of nuclei emit two or more prompt gamma rays in the neutron capture reaction. In multiple prompt gamma-ray analysis (MPGA), the gamma rays are simultaneously measured by two or more sets of gamma-ray detectors; these gamma rays are reduced to a pair of two prompt gamma rays, and it is added to the two-dimensional spectrum, which sets two energy axes. A gamma-ray peak corresponding to the pair of gamma rays that are emitted simultaneously will appear on the two-dimensional spectrum. By analyzing this two-dimensional peak, resolution can be raised by leaps and bounds. MPGA needs not only the knowledge of energies of capture gamma rays, but also nuclear level structure data. While the energies are usually accurate, many of the excited level structures are poorly known. Complete sets of these nuclear data are of great importance. A new MPGA system will be constructed at the guide-hall of JRR-3M in Japan Atomic Energy Research Institute. This system consists of three clover Ge detectors with anti-Compton suppressors. The absolute efficiency of the system is about 10% for a 1.3-MeV gamma ray. It will be widely used in the fields of agriculture, engineering, geosciences, environmental science, and measurement of neutron capture cross sections of minor actinides. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]