The construction and testing of a portable energy-sensitive neutron instrument are described. This instrument has been designed and constructed for the primary purpose of characterizing cosmic-ray neutron fields in the upper atmosphere and in cosmic reference field facilities. The instrument comprises a helium-3 proportional counter surrounded by 15 mm of lead and 140 mm of polyethylene creating a spherical structure with a diameter of 34 cm. The instrument also incorporates 12 boron-coated diodes, six on the outside of the polyethylene layer with six placed within the structure. The dimensions, materials, and arrangement of these in the instrument have previously been optimized with the CNPX Monte Carlo simulation software to provide a compromise between the requirements of portability and spectral response. Testing took place at several locations and experimental data from the instrument's operation at the high-altitude Jungfraujoch laboratory in the Swiss alps are presented.