Pile-Up Correction for Inelastic Gamma-Ray Detection in Pulsed Fast Neutron Analysis

Neutron active interrogation is a method that can be used to detect the smuggling of illicit materials, such as explosives, drugs, and special nuclear material. A pulsed fast neutron analysis (PFNA) can be used to measure the relative concentration of key constituent elements of common objects, such as C, H, O, and N, via inelastic neutron scattering. However, a frequent challenge that must be addressed is the excessive pile-up in PFNA measurements, particularly for intense sources, such as pulsed neutron generators (NGs). The pile-up can lead to a reduction in the number of detected events and an overestimation of deposited energy in an event, leading to a loss of energy resolution. We propose and experimentally demonstrate a method for pile-up correction in PFNA that makes use of time gating to select the gamma-ray detection events coincident with NG pulses. The number of pile-up gamma events and their times of arrival (ToA) are estimated by modified phase-only correlation (MPOC), whereas the amplitudes of individual pulses are estimated by maximum likelihood estimation (MLE). In experiments, carbon and sugar samples were activated using a deuterium-tritium NG, and gamma rays were detected with NaI(Tl) and BGO detectors. The peak-to-background ratio (PBR) for the 4.44-MeV photopeak, which corresponds to the inelastic signature of carbon, increases by a factor of 3.88 for NaI(Tl) and 2.63 for BGO, when prompt time-gated. When comparing the pile-up corrected spectrum with the conventional charge integral spectrum in a measurement of graphite, in the region of the 4.44-MeV peak, the net counts increase by the factors of 2.42 and 1.44 for NaI(Tl) and BGO, respectively, along with an improvement in energy resolution. This approach enables the use of slower scintillators, such as NaI(Tl) and BGO, in high-count-rate scenarios, such as in a typical PFNA environment. This can, in turn, reduce the cost of the PFNA system or allow for measurements in conditions of high neutron flux, thereby increasing the inspection throughput.