Absolute Measurements of the ¹¹⁵In(n,γ)^116m₁In Cross Section for Fast Neutrons

The neutron capture cross section in ¹¹⁵In leading to the 54.12 min isomeric state (m₁) in ¹¹⁶In has been absolutely determined at neutron energies of 23, 265, and 964 keV. These energies are the median neutron energies of the three photoneutron sources, Sb-Be, Na-D, and Na-Be, applied in this work. The measurements are independent of other cross-section data except for corrections amounting to <10%. Reaction rates were determined by beta counting of the ^116m₁In decay activity using a 4π gas flow proportional counter. Detector efficiency was measured using 4π beta-gamma coincidence counting techniques, incorporating the foil absorber method of efficiency extrapolation for correction of complex decay scheme effects. Photoneutron source emission rates were determined by indirect comparison with the U.S. National Bureau of Standards NBS-II standard source in the University of Michigan Manganese Bath. The normalized scalar flux was calculated from the neutron emission angular distribution results of a Monte Carlo computer program used to model neutron and gamma transport in the source. Correction factors were applied related to competing reactions, neutron scattering from experiment components, background from room-return neutrons, and differences in the energy spectra of the neutron sources. The absolute cross-section values obtained for the ¹¹⁵In(n, γ)^116m₁In reaction were 588 ± 11, 196 ± 4, and 203 ± 4 mb at 23, 265, and 964 keV, respectively.