Flux Normalization Using Activation Analysis for Cross-Section Measurements at GENESIS

Inelastic neutron scattering is an important mechanism behind neutron spectroscopy, tomography, and down scattering in nuclear reactors. However, the cross-section data are sparse or often lack uncertainties. The Gamma Energy Neutron Energy Spectrometer for Inelastic Scattering (GENESIS) located at the 88-Inch Cyclotron at Lawrence Berkeley National Lab was built to address this issue using a combination of fast neutron detectors and high resolution γ-ray detectors (HPGe). To obtain cross-section data, the neutron flux from the beam must be determined; foil activation analysis was performed to determine the flux. Four activation foils were irradiated for two hours. An EJ-309 organic liquid scintillator-based double time-of-flight detector called sTOF recorded the neutron spectrum. Thick target breakup of 25 MeV deuterons on a carbon target produced the broad energy neutron beam. The irradiated foils were placed 10 cm from the front face of an HPGe detector to record the gamma rays emitted by the activation products. The activity of each foil was calculated using the known branching ratios, detector efficiency, and recorded times. The analysis yielded the neutron fluence of 2.45E10 N/str/μC with 1.57% uncertainty suggesting a high accuracy for the fluence and the cross-section data. This data will be compared against the results obtained from sTOF to validate the modeled sTOF efficiency.