# Development of a Dual-Modality Gamma-ray/Fast Neutron Imaging System for Air Cargo Inspection
## 2 Materials and Methods
### Neutron Generation and Detection  
Fast neutrons were generated in nuclear fusion reactions of deuterium and tritium in a D-T generator that emits an isotropic and mono-energetic neutron beam with an energy of 14.1 MeV. The D-T generator acts as a small particle accelerator composed of an ion source, acceleration stage, target, and pressure stabilizer in a vacuum-sealed case. The ions were extracted from the plasma formed in the ion source using a single-stage acceleration optic system. The extracted beam was then focused on the target shown in Figure 3, which was constantly supplied at 150 kV. To reduce radiation risks, the D-T generator was covered with high-purity polyethylene panels and blocks, and 5 cm slits were open on the neutron shielding cover to emit the neutron beam. During operation, the D-T neutron generator emits highly penetrating radiation, including neutrons and gamma-rays. Among the radiation, only neutron property data for air cargo must be collected. The radiation was detected using a plastic scintillator on the neutron detector. The neutron detector module in Figure 4 shows the composition of the plastic scintillator (PS), silicon photomultiplier (SiPM), analog-to-digital converter (ADC) board, and visible shielding case. A polyvinyl toluene-based plastic scintillator (PS, EJ-276G from Eljen Technology [15]) with a size of 10 \(\times\) 10 \(\times\) 30 mm\({}^{3}\) was selected to develop a fast neutron detector with PS arranged in a 2 \(\times\) 8 array. PS converts the high-penetrating radiation to the visible region at a wavelength of approximately 490 nm. This scintillator has a scintillation efficiency of 8000 photons/MeV. A silicon photomultiplier (SiPM, S13360-6025 CS, Hamamatsu) with a photosensitive area of 6 \(\times\) 6 mm\({}^{2}\) senses visible light. The photon detection efficiency of this SiPM is 25%. The contact surface between the PS and SiPM is bonded to titanium oxide grease with a refractive index similar to that of PS to reduce optical losses. The SiPM counts a single photon generated from the PS and generates electrical output pulses. The ADC board measuring the electrical pulse was designed to sample it at a frequency of 125 MHz and a 14-bit resolution. Figure 3: D-T generator. The red line in the center of the D-T generator indicates the position of the target. The electrical pulse shape is characterized by recoil protons and electrons excited by radiation. In general, electrical pulses arising from neutron signals tend to have higher peaks and longer tails than those arising from photon signals, as shown in Figure 5. The peak means the highest intensity of the pulse, and the tail is the integral of the post-peak time and the pulse end time. Using this feature, the pulse shape discrimination (PSD) technique differentiates neutron signals from various highly penetrating radiation signals [17; 18]. With over 100,000 detections, the distribution of the electrical signals can be spread over a spot plot with peak and tail coordinates. Under stable measurement conditions, the PSD spot plot showed two regions: neutrons and gamma-rays. This result can also be displayed as a histogram of the ratio between the pulse peak and tail. The PSD histogram shows two Gaussian distributions, neutrons, and gamma-rays. In the PSD histogram, the valley of the two Gaussian distributions is determined by the area separating the neutron and gamma-ray signals. The neutron detector modules were vertically arranged to create an image frame similar to the gamma-ray detector tower. Vertically arranged neutron detector modules were placed in front of the neutron generator. The neutron detector modules were connected to the data acquisition boards, and they supplied high-voltage power and transmitted the electrical pulse data. Figure 4: Neutron detector module: (**a**) side view of the neutron detector module, (**b**) block diagram of the neutron detector module.