Comparison of methods to determine the fluence of monoenergetic neutrons in the energy range from 30 keV to 14.8 MeV

The primary reference instruments for neutron fluence measurements used at the Physikalisch-Technische Bundesanstalt are based on the primary standard for neutron measurements which is the differential neutron–proton scattering cross section. Such instruments require considerable effort for their operation and analysis. Therefore, routine measurements are carried out using a transfer instrument to facilitate the efficient provision of services to customers. A series of measurements was conducted to compare the transfer device to the primary reference instruments and ensure the traceability of neutron fluence measurements. This resulted in an improved characterisation of the instrument and new analysis procedures. For neutron energies between 144 keV and 14.8 MeV, the ratio of neutron fluence values measured with the primary reference instruments and the transfer instrument deviates from unity by less than the estimated standard measurement uncertainties of 2.6% to 3.2%. At neutron energies between 30 keV and 100 keV, however, the experimental fluence ratios deviate from unity by about 4% which exceeds the estimated uncertainties of 2.5% to 2.9%. At present, the reason for this inconsistency remains unresolved.