Comparing active and passive Bonner Sphere Spectrometers in the 2.5 MeV quasi mono-energetic neutron field of the ENEA Frascati Neutron Generator (FNG)

Bonner Sphere Spectrometer (BSS) equipped with passive detectors are used to replace active BSS in radiation environment characterized by high fluence rate, large photon background and pulsed time structure as those encountered near particle accelerators. In this work a newly developed passive Bonner Sphere Spectrometer, using Dysprosium activation foils as central detectors (Dy-BSS), was tested through comparison with a well-established active BSS. As a suitable neutron field, where both systems can correctly operate, the 2.5 MeV quasi mono-energetic beam of the ENEA Frascati Neutron Generator (FNG) was chosen. The two spectrometers are based on substantially different operation principles, therefore their response matrix are very different. In addition, the BSS are independently calibrated in different reference neutron fields. The exercise took place at 90° and at a fixed distance from the neutron emitting deuterated target. As reference data, the results obtained by unfolding the active BSS data were used. The FRUIT unfolding code, ver. 5 was used.The results of the Dy-BSS are fully comparable with those of the active BSS, in terms of both total fluence and shape of the neutron spectra. For the energy range studied in this exercise, the expected level of accuracy of the Dy-BSS and its suitability for operational neutron monitoring are fully confirmed.

► A Bonner Sphere Spectrometer based on Dy activation foils (Dy-BSS) was developed.
► The Dy-BSS was compared with a well-established active BSS.
► The test was performed in a 2.5 MeV mono-energetic field.
► The comparison focused on the measurement of the spectrum and of the total fluence.
► The Dy-BSS determined the requested quantities within very limited uncertainties.