# Development of a Dual-Modality Gamma-ray/Fast Neutron Imaging System for Air Cargo Inspection
## 2 Materials and Methods
### Imaging Process  
The gamma-ray and neutron detectors were connected to the data acquisition (DAQ) system to record the radiation image and communicate the machine control commands with a user computer. The number of electrical pulses obtained from the radiation detector during the measurement time represents the image pixel value. For instance, when the gamma-ray detector is activated for 1 s and the image pixel value is 1, and when the detector functions for longer, the image pixel value increases. The radiation detector modules and image pixel values were arranged vertically. The vertically arranged image pixels formed an image frame. By repeating the measurements at regular intervals, the image frames are horizontally accumulated, and the radiation image is expanded in two dimensions. Under this measurement condition, the container is displayed in the radiation image when the cargo container moves between the radiation generator and the radiation detector. The radiation imaging system in the air cargo inspection system produces gamma-ray and neutron images with transmission properties. In the transmission characteristics of the radiation image, various information on the air, container, and beam, as well as the sample material, is present. However, security inspection requires only the intrinsic properties of the sample material. The reference and gain signals are used to extract the intrinsic properties of the sample material. Because the reference signal is measured before the radiation passes through the cargo container, it only has information about the pure beam intensity of radiation. Image correction using the reference signal can remove beam intensity fluctuations in the radiation image. The gain signal indicates the radiation signals passing through the air or container with no information of the sample material. This image correction using the gain signal separates the background transmission characteristics from the mixed transmission characteristics of the sample material. When used in an optical system, the gain correction removes the background information and characterizes the absorption of the sample material. The absorption coefficient is an intrinsic property of a  
Figure 5: Example plot of normalized electrical pulses measured from the detectors. The peak is the highest intensity of the electrical pulse, and the tail is the integral value after the peak and the delay time. sample material. The optical path length is required to calculate the absorption coefficient from the transmission data. However, because security inspection proceeds in an unopened state, the optical path length cannot be measured, and another method is required. The unique information of the sample material inside the air cargo container can be extracted from the ratio of radiation absorption properties: the neutron and gamma-ray absorption values of the sample material. The ratio of the neutron and gamma-ray attenuation coefficients is:  
\[\mathrm{R}=\frac{\ln\left(I_{n}/I_{n}^{0}\right)}{\ln\left(I_{g}/I_{g}^{0} \right)} \tag{1}\]  
where \(I_{n}\) is the measured neutron intensity through the object, \(I_{n}\)\({}^{0}\) is the measured neutron intensity with the object removed, \(I_{g}\) is the measured gamma-ray intensity through the object, and \(I_{g}\)\({}^{0}\) is the measured gamma-ray intensity with the object removed. The R-value does not require the calculation of the optical path length because the radiation has the same sample material optical path. Hence, calculating the R-values of the sample materials made material classification using the air cargo inspection system possible. The R-value is defined as the ratio of neutron absorption to gamma-ray absorption. The R-values of the sample materials extracted from the radiation images were re-painted onto the radiation image as a color map. The R-value image shows the differentiation of the materials.