New Capabilities of the RPI gamma-Multiplicity Detector to Measure y-Production

Accurate modeling of gamma-production in neutron capture reactions is critical for many applications including non-proliferation, safeguards and modeling nuclear reactors. To improve this work, the Rensselaer Polytechnic Institute (RPI) 16 -segment gamma-multiplicity NaI(T1) detector at the Gaerttner Linear Accelerator (LINAC) Center has been upgraded by implementing a digital data acquisition system. The new digitized system records the gamma-energy deposition distribution in each individual detector, and gamma-multiplicity values as a function of neutron time -of-flight (TOF). With the new capabilities, high precision capture (and fission) yield measurements can be made, and the accuracy of simulation tools used to predict capture gamma-cascades can be tested. To validate the updated system, an experiment was performed using a natural Ta sample to measure Ta-181 and (180m) Ta resonance capture yield by detecting prompt gamma-rays emitted from neutron capture interactions as a function of both neutron energy and measured gamma-multiplicity of each capture event. The results confirm earlier measurements and agree with theoretical yield in the low energy resonance region from 1 to 20 eV. A U-238(n, gamma) measurement was also performed to generate gamma-spectra. For capture gamma-cascades where the total gamma-energy deposition is close to the neutron binding energy, gamma-spectra were measured for individual resonance energies and observed gamma-multiplicities. The results are comparable in shape to a recent measurement done using the Detector for Advanced Neutron Capture Experiments (DANCE) array at Los Alamos Neutron Science Center (LANSCE); however, differences need to be compared to Monte-Carlo n -particle simulations.