Abstract

The purpose of this work is the precise determination of the /sup 238/U neutron capture yield (i.e., the probability of neutron absorption) as a function of neutron energy with the highest available neutron energy resolution. The motivation for this undertaking arises from the central role played by the /sup 238/U neutron capture process in the calculation of the neutron balance of both thermal reactors and fast breeder reactors. The present measurement was performed using the Oak Ridge Electron Linear Accelerator (ORELA) facility. The pulsed beam of neutrons from the ORELA facility is collimated on a sample of /sup 238/U. The neutron capture rate in the sample is measured, as a function of neutron time-of-flight (TOF), by detecting the gamma rays from the /sup 238/U(n,..gamma..)/sup 239/U reaction with a large gamma-ray detector surrounding the /sup 238/U sample. At each energy, the capture yield is proportional to the observed capture rate divided by the measured intensity of the neutron beam. The constant of proportionality (the normalization constant) is obtained as the ratio of theoretical to experimentally measured areas under small /sup 238/U resonances where the resonance parameters have been determined from high resolution /sup 238/U transmission measurements. The cross section for the reaction /sup 238/U(n,..gamma..)/sup 239/U can be derived from the measured capture yield if one applies appropriate corrections for multiple scattering and resonance self-shielding. The present work will allow the extension of the resolved resonance region in /sup 238/U from its current limit of 4 keV up to 20 keV. In addition, some 200 /sup 238/U neutron resonances in the energy range from 250 eV to 10 keV have been observed which had not been detected in previous measurements. These results are relevant for both reactor design and for the understanding of the structure of the (/sup 238/U + n) compound nucleus. 33 refs., 20 figs., 9 tabs.

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