# Development of a Dual-Modality Gamma-ray/Fast Neutron Imaging System for Air Cargo Inspection
## 1 Introduction  
Cargo inspection systems have been developed in the security area using various radiation sources, such as radioactive materials [1], electron linear accelerators [2], and neutron generators [3]. Their high penetration power is important for examining the inner container without opening the gate [4]. The first cargo inspection system was developed using a gamma-ray source for the cargo inspection of the carrier and container. The gamma-ray inspection system detects metallic weapons such as guns [5], knives [6], and bombs [7]. Non-invasive measurements that use gamma-rays are convenient and safe for cargo users as much as for criminal investigators. However, gamma-ray systems are limited in detecting objects made of low-density materials, such as drugs or plastic explosives. Neutron sources have been used to analyze low-density materials in the fields of physics [8], nuclear sciences [9], and the battery industries [10]. Neutrons with high energies of several mega-electron volts have a higher penetrating power, similar to gamma-rays. Therefore, the combined use of a gamma-ray and neutrons provides a powerful effect to extend the material discrimination. Neutrons and gamma-rays each have their advantages and disadvantages for inspecting cargo, but combining them into one system compensates for these shortcomings. Recently, a cargo inspection system using a Deuterium-Tritium (D-T) neutron generator and Co-60 irradiators was developed by the Australia group in China [3], and it can differentiate 16 materials. The D-T generator radiates neutrons with an energy of 14.1 MeV in all directions (4\(\pi\)), using a fusion reaction with deuterium and tritium. The Co-60 emits characteristic gamma-rays of 1.2 and 1.3 MeV, which are lighter and smaller than electronically generated gamma-rays. However, the radioactive matter has a limited use time andlow dose rates. A gamma-ray source with higher energy and a higher dose rate can be generated for a linear electron accelerator. The electron accelerator produces gamma-rays with energies of a few MeV by the bremsstrahlung effect. This energy can be higher than that of Co-60. An air cargo inspection system requires high-energy radiation sources. The radiations with higher energy have the higher penetrating power. For instance, the penetrating lengths of iron with \(5.995\times 10^{-2}\) cm\({}^{2}\)/g at 1 MeV and \(3.057\times 10^{-2}\) cm\({}^{2}\)/g at 6 MeV of a gamma-ray are calculated to about 2.122 cm and 4.162 cm, respectively [11]. The high penetrating power of radiation adversely affects the detector owing to its low measurement efficiency. Plastic scintillators and silicon photomultiplier-based radiation readout circuit boards are well-known radiation detectors for high-energy radiation measurements [12; 13]. Plastic scintillators have been widely used to convert high-energy radiation to visible light. It is very attractive because of its nontoxicity, stability at varying temperatures, and simpler geometrical control, unlike liquid scintillators and others [14; 15; 16]. A silicon photomultiplier was used to produce electrical pulses from the photon generated in the scintillator. The radiation detector modules were vertically arranged, and the moving cargo container was scanned. The neutron and gamma-ray system outputs the neutron and gamma-ray transmittance images of the cargo container. The transmittance of the radiation has multiple pieces of information related to the material density, material thickness, and radiation beam conditions. Transmittance data should be calculated to extract the radiation absorption of materials and obtain their intrinsic characteristics for material discrimination. In this paper, we report the development of an air cargo inspection system using fast neutron and gamma-ray sources to investigate the ability of material classification for security inspection.