Measuring and Simulating Neutron Capture <i>γ</i>-Ray Spectra using the RPI <i>γ</i>-Ray Multiplicity Detector

Accurate modeling of neutron induced capture γ -ray production is essential for many applications such as reactor and shielding calculations, understanding γ -ray heating in critical systems, and non-proliferation efforts. To determine the accuracy of nuclear data evaluations and simulation tools used to transport capture γ -cascades, the 16-segment γ -ray multiplicity NaI(T1) detector at the Rensselaer Polytechnic Institute Gaerttner Linear Accelerator Center has been upgraded to measure neutron capture γ -ray spectra and multiplicity as a function of energy. Several samples including Fe, Mn, Co, Ta, and 235,238 U have been measured using the time-of-flight method for incident neutrons in the low-energy region from 0.01 – 100 eV. A new method has been developed to model the event-by-event capture γ -ray cascade energy deposition in the detector array using DICEBOX and a modified version of MCNP-6.2. The method has been validated using 22 Na and 60 Co coincidence sources and the well-studied thermal 56 Fe(n, γ ) γ -ray intensities. Additional measured samples will be used for further validation and analysis. The new modeling capabilities coupled with measured γ -ray spectra can be used to test transport codes and nuclear data evaluations of neutron capture γ -rays used to simulate experimental results.