  Bonner Spheres have been used widely for the measurement of neutron spectra
with neutron energies ranged from thermal up to at least 20 MeV. A Bonner
Sphere neutron spectrometer (BSS) was developed by extending a Berthold LB 6411
neutron-dose-rate meter. The BSS consists of a $^{3}$He thermal-neutron
detector with integrated electronics, a set of eight polyethylene spherical
shells and two optional lead shells of various sizes. The response matrix of
the BSS was calculated with GEANT4 Monte Carlo simulation. The BSS had a
calibration uncertainty of $\pm 8.6\%$ and a detector background rate of $(1.57
\pm 0.04) \times 10^{-3}$ s$^{-1}$. A spectral unfolding code NSUGA was
developed. The NSUGA code utilizes genetic algorithms and has been shown to
perform well in the absence of a priori information.
