SUMMARY
A novel broad spectrum neutron spectrometer has been created to extend the useful energy range of existing Bonner Sphere Spectrometers (BSS). Both an active LiI scintillator probe and passive gold foil detectors were utilized to extend the energy response of existing Bonner Sphere Spectrometers. Above 20 MeV the energy resolution for the standard polyethylene Bonner Sphere Spectrometer are poor because the response functions are not unique. MCNPX was used to investigate several modifications to the BSS system which resulted in the Bonner Sphere Extension (BSE). This cost effective extension uses several concentric spheres of copper, lead, and tungsten heavy metal down scatter materials to extend the useful energy range of current BSS from 20 MeV to 1 GeV and above. Using both a 3” and 5” inner polyethylene spheres, aluminum shell sets were made with a 1” cavity and filled with lead, copper and tungsten respectively for a total of six sets of spheres. A 12” and 8” polyethylene sphere were also milled to accept the heavy metal spheres. The system was calibrated using AmBe neutron spectra as well as the spallation neutron source at the Los Alamos National Laboratory. The Los Alamos Neutron Science CEnter (LANSCE) neutron beam delivers up to 600 MeV neutrons down several flight paths. Utilizing target four's fifteen degree right ninety meter flight path, the high energy response was verified. Detailed models in MCNPX were used to create fine group neutron responses to a flat incident neutron spectrum having energies from thermal to 1 GeV. The system utilizes a maximum entropy unfolding program to deconvolve the incident neutron spectra from the individual detector’s responses. Good agreement was seen between the spectrum measured using the BSE and the time-of-flight spectrum obtained at LANSCE.