A time-of-flight detector for thermal neutrons from radiotherapy Linacs

Boron Neutron Capture Therapy (BNCT) is a therapeutic technique exploiting the release of dose inside the tumour cell after a fission of a 10B nucleus following the capture of a thermal neutron. BNCT could be the treatment for extended tumors (liver, stomach, lung), radio-resistant ones (melanoma) or tumours surrounded by vital organs (brain). The application of BNCT requires a high thermal neutron flux >45 x 10^8 ncm ^-2 s ^-1 with the correct energy spectrum (neutron energy <10 keV), two requirements that for the moment
are fulfilled only by nuclear reactors. The INFN PhoNeS (Photo Neutron Source) project is trying to produce such a neutron beam with
standard radiotherapy Linacs, maximizing with a dedicated photo-neutron converter the neutrons produced by Giant Dipole Resonance
by a high energy (48 MeV) photon beam. In this framework, we have developed a real-time detector to measure the thermal neutron
time-of -flight to compute the flux and the energy spectrum. Given the pulsed nature of Linac beams, the detector is a single neutron
counting system made of a scintillator detecting the photon emitted after the neutron capture by the hydrogen nuclei. The scintillator signal is sampled by a dedicated FPGA clock thus obtaining the exact arrival time of the neutron itself.
The paper will present the detector and its electronics, the feasibility measurements with a Varian Clinac 1800/2100CD and
comparison with a Monte Carlo simulation.