A field portable neutron spectrometer based on the Bonner Sphere Principles

The Bonner Sphere Spectrometer (BSS) has been used since 1960 for the determination of neutron energy spectra ranging from 25 meV to 20 MeV. Although some researchers have taken full sets of Bonner spheres inside nuclear power plants for field measurements, the volume and mass of the whole device have usually limited its use to laboratories and to calibration and reference facilities. The instrument presented here operates under the same principles of the Bonner spheres, except that different amounts of moderator around a thermal neutron detector are configured by adding or removing cylindrical shells. For this reason it is called the Nested Neutron Spectrometer (NNS). Thus, only the mass and volume equivalent to a single Bonner sphere need to be carried in the field. The reconfiguration of the moderator is very quick, leading to a reduced setup time. The instrument can then easily be used in the workplace and the measured spectra allow the calculation of the ambient dose equivalent rate, without the inherent inaccuracies of REM meters. The focus of this paper is the design principles of the NNS, laboratory test results and field measurements. Measurements were obtained using radionuclide neutron sources, mono-energetic neutrons from accelerators and from neutrons around the containment of a nuclear power reactor. Results are very similar for the NNS and BSS, with the former being much easier to use in the field.