Analysis of In-Situ Fission Rate Measurements Using $^4{\rm He}$ Gas Scintillation Detectors

Active neutron interrogation is a powerful non-destructive assay technique that relies on detecting and analyzing fission neutrons produced in a fissionable sample by an interrogating high neutron flux. ⁴He gas scintillation fast neutron detectors are investigated in this paper for use in a novel fission rate measurement technique. The ⁴He detectors have excellent gamma rejection, a fast response time, and give significant information on incident neutron energy, allowing for energy cuts to be applied to the detected signal. These features are shown in this work to allow for the detection of prompt fission neutrons in-situ during active neutron interrogation of a natural uranium sample. The detected neutron energy spectra from ²⁵²Cf and (d,d) generator neutron sources are measured with the ⁴He detection system and compared against incident and simulated spectra. An partial response matrix for the detector is determined using the kinematic interaction properties of the elastic scattering with the ⁴He.