UC Berkeley

Improved inelastic neutron scattering and neutron-induced gamma ray production data are needed for many of the next generation nuclear technologies, from advanced reactors to space exploration, shielding applications, and detection platforms based on prompt neutron interrogation analysis. The data for these applications come from evaluated libraries like ENDF/B-VIII.0, which are derived from calculations that have been validated against measurements of reaction cross sections and a set of standard benchmarks. Historically, the generation of these libraries has relied on the separate measurements of the outgoing neutrons and $\gamma$-rays from $(n,n'\gamma)$. The Gamma Energy Neutron Energy Spectrometer for Inelastic Scattering (GENESIS) located at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL) is an experimental platform containing organic liquid scintillators for measurements of secondary neutron energy and angle distributions and high-purity germanium (HPGe) detectors for simultaneous measurements of gamma-ray production cross sections. The array attempts to bridge the gap between evaluators and experimentalists and produce nuclear data of maximal utility via the simultaneous measurement and a nuclear reaction modeling based analysis approach.

The establishment of GENESIS as viable experimental platform was accomplished through a series of calibrated source and beam measurements.The characteristics of this array were measured, including the timing resolution and delays of each detector and the energy and light yield resolution and gain of the detectors. The efficiency of the HPGe CLOVER detectors was measured experimentally using a calibrated $^{152}$Eu point source and an elliptical $^{56}$Mn sample. A GEANT4 model of the CLOVERs was developed and validated against the efficiency measurements to within $5\%$ allowing for calculations of single leaf or full-CLOVER efficiency for different target geometries and gamma ray energies. The neutron energy and light yield dependent response of the neutron detectors was investigated using spontaneous fission neutrons from an encapsulated $^{252}$Cf spontaneous fission source. A GEANT4 model of the array, including the CLOVERs, organic scintillators, and support structures was developed to calculate the absolute in-situ efficiency of the neutron detectors. This simulation was validated against the measurements carried out with the $^{252}$Cf source to within $5\%$.

Experiments with a $99.98\%$-enriched $^{56}$Fe target were performed using GENESIS with a broad-energy, collimated, time-resolved neutron beam generated via the break-up of 14 MeV deuterons in a thick carbon target. Two analysis techniques were pursued, including a novel forward-modeling approach enabled by the well validated GEANT4 simulation and a new C++ interface to the nuclear reaction code TALYS. Differential gamma-ray production cross sections for the yrast\footnote{States with the lowest ratio of excitation energy to total angular momentum.} $4\rightarrow2$ and $6\rightarrow4$ transitions, and 8 other off-yrast transitions were obtained using conventional analysis techniques. The yrast $2\rightarrow0$ gamma ray production cross section was determined using the forward modeling approach. Total secondary neutron energy/angular distributions as a function of incident neutron energy were also obtained using the forward-modeling approach. The results of the forward modeling approach were found to be in agreement with the results obtainable through conventional means and to previous measurements found in the literature. This work establishes the forward modeling analysis approach as a valid method to obtain cross sections with GENESIS. Finally, the possibilities for analyses of secondary neutron/gamma coincidences in the $^{56}$Fe data and the prospects for coincidence analysis in future GENESIS experiments is discussed.