Absolute Measurements of the Fast Neutron Capture Cross-Section of Indium-115.

Abstract

The In('115)(n,(gamma))In('116ml) cross section has been absolutely determined at neutron energies of 23, 265, 770 and 964 keV. These energies are the median neutron energies of the four photoneutron sources, Sb-Be, Na-D(,2)C, La-Be and Na-Be, utilized in this work. The measurements are independent of other cross section data except for corrections amounting to less than 10%. Independent determinations of the reaction rate, detector efficiency, neutron source strength, scalar flux and target masses were performed. Reaction rates were determined by beta counting of the In('116ml) decay activity using a 4(pi) gas flow proportional counter. The detector efficiency was measured using 4(pi)(beta)-(gamma) coincidence counting techniques. A correction factor for non-ideal detector behavior and the complex decay scheme effects was performed using the foil absorber method of efficiency extrapolation. Photoneutron source emission rates were determined by intercomparison with a Cf('252) spontaneous fission neutron source in The University of Michigan Manganese Bath. The Cf('252) source was itself calibrated against NBS-II, the secondary national neutron st and ard. The normalized scalar flux was calculated from the neutron emission angular distribution results of the Monte Carlo computer program used to model neutron and gamma transport in the source. Target mass determinations were made with a microbalance. Correction factors were applied for competing reaction activities, neutron scattering from experiment components, room-return induced activities, spectral effects in the manganese bath and the neutron energy spectra of the photoneutron sources. The neutron energy spectra were also determined with the Monte Carlo program. The experimental cross section results were normalized to the source median energy using the energy spectra and cross section shape data. The absolute cross sections obtained for the In('115)(n,(gamma))In('116ml) reaction were 588 (+OR-) 12, 196 (+OR-) 4, 288 (+OR-) 10 and 203 (+OR-) 3 millibarns at 23, 265, 770 and 964 keV, respectively.