IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 1325 Energy Response and Angular Dependence of a Bonner Sphere Extension Eric A. Burgett, Nolan E. Hertel, and Rebecca M. Howell Abstract—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 func- tions 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 un- folding data which were measured with the detector parallel, per- pendicular, and at a 45 angle to the incident neutron beam using response functions computed in three directional orientations (par- allel, perpendicular, and at a 45 angle). Directional dependence was found to be more significant for the passive detector, especially when used with small moderating spheres. Index Terms—Bonner Sphere Spectrometer (BSS), high-energy, neutron detectors, neutron spectroscopy. I. INTRODUCTION T HE Bonner Sphere Spectrometer (BSS) is a widely used neutron spectrometer for health physics and research purposes because it responds over a very large energy range (thermal energies to hundreds of MeV) and has a nearly isotropic response [1]. The BSS measurement system was first introduced in 1960 by Bramblett, Ewing and Bonner [2]. The system consists of a series of high-density polyethylene (PE) moderating spheres ranging from 2 to 12 in diameter with a thermal neutron detector placed at their centers [2]. Manuscript received June 30, 2008; revised January 05, 2009. Current version published June 17, 2009. This work was supported in part by the Oak Ridge As- sociated Universities Ralph E. Powe Junior Faculty Enhancement Award. Beam time at Los Alamos National Laboratory Neutron Science Center was awarded under proposal for the Run Cycle beginning June 2007. R. M. Howell was sup- ported in part by an NCI K01 Career Development Grant-7K01CA125204-02. E. A. Burgett and N. E. Hertel are with the Georgia Institute of Tech- nology, Atlanta, GA 30332 USA (e-mail: eric.burgett@nre.gatech.edu; nolan.hertel@me.gatech.edu). R. M. Howell is with the U.T.M.D. Anderson Cancer Center, Houston, TX 77030 USA (e-mail: rhowell@mdanderson.org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2009.2019272 Typically, BSS measurements are unfolded to produce a low resolution neutron spectrum. Above approximately 20 MeV however, the sensitivity of BSS decreases dramatically. Several research groups have investigated the use of high atomic number (Z) shells placed within polyethylene spheres to increase the high-energy sensitivity of the system [3]–[6]. In a recent work we described the design and construction of a cost effective Bonner Sphere Extension (BSE) [8]. The BSE makes use of a commercially available BSS system (Ludlum 42-5 neutron ball cart), 1 and concentric shells of copper (Cu), tungsten (W), and lead (Pb) with polyethylene covers to increase its response to higher energies. The BSE design is briefly described here; a full description is provided in [8]. Two designs designated as the small as- sembly and the large assembly incorporate the standard 3 and 5 Bonner spheres, respectively. In the small assembly a 3 Bonner sphere is surrounded by an Al shell (3 inner diameter [ID] and 5 outer diameter [OD]) filled with Cu, Pb, or W and further encased in a PE sphere with an 8 OD. The large assembly has a similar design but uses a 5 Bonner sphere surrounded by a larger Al shell (5 ID and 7 OD) filled with Cu, Pb, or W and encased in a PE sphere with a 12-inch OD. Both assemblies can be used with or without the outer polyeth- ylene sphere. The system can accommodate either the Ludlum LiI(Eu) scintillator or Au activation foils (commercially available from ThermoElectronm 2 1.27-cm diameter and 0.005 cm thickness) placed on a PE holder (with similar dimensions to the Ludlum scintillator). The small and large assemblies with the PE holder are shown in Fig. 1(a) and (b), respectively. II. METHODOLOGY A. Response Function Calculations Response functions were calculated using Monte Carlo N-Particle Code, eXtended (MCNPX), version 2.6e [9] using 500 equally spaced logarithmic energy bins (10 bins per decade) from 0.001 eV to 1 GeV. The response functions were calculated using two different nuclear cross section libraries, ENDF-B.VI [10] and LLDOS (ACTL) [11]. The responses were computed using parallel beams of neutrons. Transport of the neutrons was carried out using the ENDF-B.VI cross section data for transport below 20 MeV, the LA150 cross section up to 150 MeV, and CEM03 physics models [12] above 150 MeV. 1 Ludlum Measurements, Inc., Sweetwater, TX 79556 2 ThermoElectron Corporation, Santa Fe, NM 87507 0018-9499/$25.00 © 2009 IEEE