An active Bonner sphere spectrometer for intense neutron fields

Neutron producing installations can be characterized by the presence of intense pulsed fields, possibly accompanied by large photon background. This is the case of particle accelerators and, more recently, laser-based radiation sources. Determining neutron spectra in these fields is particularly complex, as active detectors may likely be affected by dead-time and saturation effects, and their parasitic response to photons is generally non zero. Passive Bonner Spheres Spectrometers (BSS) equipped with activation foils have been used in these cases. Owing on a new type of active thermal neutron detector (TNRD, Thermal Neutron Rate Detector), this work presents a BSS able to operate in intense, pulsed fields, with large photon background. The response matrix of this spectrometer was experimentally verified by comparison with a well-established BSS based on a 11 mm (diameter) × 3 mm (thickness) 6LiI(Eu) scintillator. The comparison was performed in the 14 MeV and 2.9 MeV neutron fields produced at the Frascati Neutron Generator. The proposed spectrometer can constitute a valuable tool for operational neutron dosimetry.