Single moderator active neutron spectrometer for therapeutic beam characterization in Boron Neutron Capture Therapy facilities

In the framework of the ENTER_BNCT project (INFN-Italy), a directional neutron spectrometer, tailored for BNCT beams QA/QC measurements, has been designed and constructed. This device, which arises from the optimization of previously developed single moderator spectrometers, condenses the functionality of a Bonner Spheres Spectrometer into a single moderator, embedding multiple thermal neutron detectors. The geometry of the spectrometer has been optimized through MCNP6 simulations and consists in a cylindrical collimator followed by a cylindrical core which contains, along his axis, 6 silicon detectors made sensitive to thermal neutrons through a 6LiF deposited layer. The position of the detector has been studied in order to optimize the spectrometer response in the epithermal range. As the internal thermal neutron sensors are simultaneously acquired, the instrument is suited to operate as a real-time neutron spectrometer. The simulation model of the spectrometer has been experimentally verified by exposing the prototype in the reference neutron field of 241 Am-Be of the Politecnico di Milano. The count rate in the spectrometer internal detectors, as calculated from the simulation model, coincided with the experimental ones within about ±2%, confirming the high degree of accuracy of the simulation model. The results are shown in this communication together with aspects related to the future use of the device in therapeutic neutron beams. (author)