Energy Response and Angular Dependence of a Bonner Sphere Extension

A Bonner Sphere Spectrometer (BSS) is a widely used neutron spectrometer in the health physics and research arenas. It provides the means to measure neutron spectra over many orders of magnitude in energy with a single instrument, but has decreased response above 20-MeV. To increase the sensitivity of BSS at higher energies, a low cost, easily fabricated Bonner Sphere Extension (BSE) has been designed and constructed. The BSE extends the sensitivity of the BSS to higher energies and utilizes either an active (LiI(Eu) scintillator) or passive (gold foil activation) detector. Two factors which affect the uncertainties in unfolded neutron spectra are the energy dependence arising from the cross sections used to generate the response functions and the directional dependence of the measurement system. We assessed the energy dependence of the BSE by calculating the response functions using two different cross section libraries (ENDF B. VI and ACTL). These response functions were then used to unfold data measured with the BSE at the WNR facility at Los Alamos Neutron Science Center (LANSCE). The spectrum unfolded with the ENDF B.VI response functions was in better agreement with the LANSCE time of flight (TOF) spectrum. We evaluated the angular response of the BSE by unfolding data which were measured with the detector parallel, perpendicular, and at a 45deg angle to the incident neutron beam using response functions computed in three directional orientations (parallel, perpendicular, and at a 45deg angle). Directional dependence was found to be more significant for the passive detector, especially when used with small moderating spheres.