Speaker
Description
Accurate gamma production in capture reactions is critical for simulation of nuclear reactor applications. This includes modeling of strength functions and level densities and calculating cross sections from resonance parameters. To improve this work, the Rensselaer Polytechnic Institute (RPI) 16-segment gamma-multiplicity NaI(Tl) detector at the Gaerttner Linear Accelerator Center (LINAC) has been upgraded by implementing a digitizing data acquisition system. The new digitized system can measure the gamma energy distribution in each individual detector, and gamma-multiplicity values as a function of neutron time-of-flight. With the new capabilities, high precision capture (and fission) yield measurements can be made, and the accuracy of simulation tools used to predict the capture gamma cascades can be tested. To validate the yield measurements completed with the updated system, an experiment was performed using a natural Ta sample to measure $^{181}$Ta and $^{180m}$Ta resonance capture yield as a function of neutron energy by detecting prompt gammas emitted from neutron capture interactions using the time-of-flight method. Capture yield was also calculated as a function of the measured gamma multiplicity of each capture event. The Ta results confirm earlier measurements and agree with theoretical yield in the low energy resonance region from 1 to 20-eV. This measurement also includes the 0.01-eV thermal region, where there is currently little experimental data. In addition to the yield validation, a measurement of $^{238}$U was performed to generate gamma emission spectra for observed multiplicities. For capture gamma cascades where the total gamma energy deposition is close to the neutron binding energy, gamma emission spectra were measured for individual resonance energies for the $^{238}$U(n, γ) reaction. The results were compared to a recent measurement done using the Detector for Advanced Neutron Capture Experiments (DANCE) array at Los Alamos Neutron Science Center (LANSCE), and Monte-Carlo n-particle simulations. The measured gamma emission spectra for observed two step cascades had general agreement with the LANSCE shape; however, there were noticeable differences between the current measurement (RPI) and previous work (LANSCE).