New Capabilities of the RPI γ-Multiplicity Detector to Measure γ-Production

Accurate modeling of γ-production in neutron capture reactions is critical for many applications including on-proliferation, safeguards and modeling nuclear reactors. To improve this work, the Rensselaer Polytechnic Institute (RPI) 16-segment γ-multiplicity NaI(Tl) detector at the Gaerttner Linear Accelerator (LINAC) Center has been upgraded by implementing a digital data acquisition system. The new digitized system records the γ-energy deposition distribution in each individual detector, and γ-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 γ-cascades can be tested. To validate the updated system, an experiment was performed using a natural Ta sample to measure 181Ta and 180mTa resonance capture yield by detecting prompt γ-rays emitted from neutron capture interactions as a function of both neutron energy and measured γ-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 238U(n, γ) measurement was also performed to generate γ-spectra. For capture γ-cascades where the total γ-energy deposition is close to the neutron binding energy, γ-spectra were measured for individual resonance energies and observed γ-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