Improved contraband detection using an experimentally optimized neutron active interrogation pulse structure

Neutron active interrogation is an approach that can identify concealed contraband in cargo, for which chlorine detection can be indicative. We used a pulsed deuterium-deuterium neutron generator to induce chlorine capture gamma rays and measured this signature with a NaI(Tl) inorganic scintillator and organic scintillators analyzed with maximum likelihood estimation-maximization-based spectral reconstruction. The time profile of neutron emission from the neutron generator was also measured using an organic scintillator. We evaluate the signal intensity and signal-to-background ratio for the chlorine gamma-ray signatures for various neutron generator pulse lengths. We demonstrate that decreasing the pulse length can increase the chlorine signal intensity, thereby enhancing sensitivity to chlorine-containing contraband. For example, we show that by decreasing the neutron generator duty factor from 50% to 5%, the chlorine signal intensity measured with NaI(Tl) during the time region between neutron pulses increases from 44.7 ± 0.3% of the signal intensity without time gating to 99.1 ± 0.3% of the signal intensity without time gating. We also show that the signal-to-background ratio for chlorine signatures does not decrease by reducing pulse width.