Using a fast-neutron spectrometer system to candle luggage for hidden explosives

A continuous spectrum of neutrons with energies up to 8.2 MeV is produced by a 4.2-MeV nanosecond-pulsed deuteron beam slowing down in a thick beryllium target. The spectrum from the locally shielded target is collimated to a horizontal fan-beam and delivered to a row of 16, 6-cm square plastic scintillators located 4 m from the neutron source. The scintillators are coupled to 12-stage photomultiplier tubes, constant-fraction discriminators, time-to-amplitude converters, analog-to-digital converters, and digital memories. Unattenuated neutron-source spectra and background spectra are recorded. Luggage is stepped through the fan beam by an automated lift located 2 m from the neutron source. Transmission spectra are measured, and are transferred to a computer while the location is advanced one pixel width (3 cm). As the next set of spectra is being measured, the computer calculates neutron attenuations for the previous set, deconvolutes attenuations into projected elemental number densities, and determines the explosive likelihood for each pixel. With a time-averaged deuteron beam current of 1 mu A, a suitcase 60-cm long (20 pixels) can be automatically imaged in 1600 s. We will suggest that time can be reduced to 8 s or less with straight-forward improvements. The following paper describes the explosives recognition algorithm and presents the results of tests with explosives.