# Development of a Dual-Modality Gamma-ray/Fast Neutron Imaging System for Air Cargo Inspection
## 3 Results  
The detected radiation is of neutrons and gamma-rays of various energies. Figure 6a shows the representative single pulses of gamma-rays and neutrons. The tail regions of normalized electrical pulses of neutrons have higher values. A distribution trend of the main pulse peak intensity and tail region integration was obtained by exposing the neutron detector with the D-T generator activated for 60 s, as shown in Figure 6b. The scatter plot in Figure 6 shows two groups with different ratios of peak intensity and tail integration, and these two groups are separated by a linear line. Electrical pulses with lower intensities were considered as noise signals and were excluded from the scatter plot. The linear line in Figure 6b is the reference value for separating the neutron signals. This reference value can be calculated from the histogram plot of the ratio between the tail and peak in Figure 6c. The tail and peak ratio histograms can be fitted to two Gaussian fits, and the minimum in the valley of the Gaussian fitted plots is the reference value. In addition, the figure of merit (FOM) can be calculated from the two Gaussian fitted plots. The FOM is a value that establishes the ability to discriminate neutrons and other radiation and is defined as [14; 19]  
\[\text{FOM}=\text{Dpp}/(\text{GFWHM}+\text{NFWHM}),\]  
where Dpp is the distance between the peaks of the Gaussian fitted plots, and GFWHM and NFWHM are the full widths at half maximum (FWHM) of the gamma and neutron peaks, respectively. The FOM of the neutron detector is approximately 1.12. The number of electrical pulses attributed to neutrons or gamma-rays corresponds to the image pixel value. The image pixel values are transformed to the gray color map to display the radiation image. The radiation image is obtained with the horizontal translation of the cargo container. Several experiments were conducted using different combinations of sample materials to acquire radiation images. The translation speed was adjusted from 16 mm/s to 200 mm/s. The radiation dose rates were 1.25 Gy/min for gamma-rays and 10\({}^{8}\) n/s for neutrons. The radiation images were captured for 30 s and 90 s for the gamma-ray and neutron images, \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Item** & **Density (g/cm\({}^{3}\))** & **Depth (mm)** \\ \hline
7 & Polyurethane & 1.05 & 240 \\
8 & Nylon & 1.15 & 280 \\
9 & Acetamide & 1.16 & 150 \\
10 & Ammonium Acetate & 1.17 & 200 \\
11 & Lucite & 1.19 & 150 \\
12 & Rubber & 1.20 & 170 \\
13 & Rice & 1.29 & 200 \\
14 & Polyester & 1.36 & 330 \\
15 & Green Tea Powder & 1.37 & 120 \\
16 & Rayon & 1.53 & 430 \\
17 & Melamin & 1.57 & 365 \\
18 & Sugar & 1.59 & 220 \\
19 & Animal Bone & 1.90 & 220 \\
20 & Teflon & 2.20 & 150 \\
21 & Graphite & 2.25 & 150 \\
22 & Glass & 2.50 & 120 \\
23 & Concrete & 2.50 & 120 \\
24 & Aluminum & 2.70 & 150 \\
25 & Ceramic Powder & 4.00 & 70 \\
26 & Titanium & 4.50 & 80 \\
27 & Iron & 7.87 & 60 \\
28 & Bronze & 8.89 & 45 \\
29 & Lead & 11.34 & 40 \\
30 & Tungsten & 19.25 & 17 \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._respectively. Figure 7a,b show one of the results of the gamma-ray and neutron images. These images were corrected with reference and gain signals, and they presented the absorption properties. Figure 7c shows a visible image of the material sample inside the air cargo container. Using the gamma-ray and neutron images and the absorption properties of the radiation, the R-values of the 30 sample materials were calculated, as shown in Figure 8. The R-values were in the range of 0.26 and 1.26 for the prepared sample materials listed in Table 1. The R-values of the metals were mainly below 0.6, but their values were sufficiently different. Plastics have higher R-values at lower densities, and there were large differences between the R-values for materials of different atomic compositions. Organics had R-values greater than 0.6, except for animal bones, rubber, and melamine, most of which had values near 1.