Development of an instrument for non-destructive identification of Unexploded Ordnance using tagged neutrons - a proof of concept study

This work reports on the efficacy of using 14 MeV neutrons tagged by the associated particle neutron time-of-flight technique (APnTOF) to extract neutron induced characteristic γ-rays from an object-of-interest with high signal-to-noise ratio (SNR) and without interference from nearby clutter. A small portable API120 neutron generator was operated at a continuous output of ~ 1×10⁷ n/s while the γ-rays were detected using a 12.7×12.7 cm diameter NaI(Tl) detector. The α-particle detection system comprised a 5-cm diameter fast Amperex XP2020 photomultiplier tube which was mated with the 6.5 cm fiber-optic face-plate of the neutron generator´s ZnO(Ga) α-detector. Standard NIM electronics was used for the α-γ coincidence spectroscopy. A graphite cube of dimension 15.2 cm (6 kg) was used as the object-of-interest while a half-gallon bottle of water and a 5-mm thick iron slab were used as clutter items in non-overlapping and overlapping modes. In all cases, carbon (C) signals from the graphite were successfully extracted without interference from the oxygen (O) signals from water. The SNR at the peak energies of 4.43 (C) and 6.13 MeV (O) were found to be 1.3 when spectra were acquired without the time coincidence mode. The SNRs were vastly improved; 22 and 10 for C and O respectively in the coincidence mode.