In this article we examine technologies currently in use and emerging technologies that have potential for use in the primary screening of carry-on baggage, checked baggage and air cargo. We start the article by surveying the trend followed by terrorist attacks and attack attempts on civil aviation in the 20th and 21st centuries. In the most recent trend, malicious agents have attempted, successfully or unsuccessfully, to smuggle explosive materials into both types of baggage and cargo. Thus, current screening systems focus on the detection of these threats and in helping operators in preventing that these threats be carried or loaded into aircraft. The main body of the paper consists of an examination of screening technologies that can raise an automated alarm if a suspicious item such as a potential explosive or explosive precursor is present in baggage or cargo. Highlights Aviation security's focus has evolved from preventing aircraft high jacking to stopping the smuggling of explosives and explosives precursors into aircraft. Attacks and attempted attacks against commercial aircraft have happened using explosives hidden in checked baggage, carry-on baggage, and air cargo. Security organizations around the world install and maintain primary screening systems, capable of raising automated alarms when potential explosives or explosive precursors are detected in baggage or cargo. Secondary screening is used to resolve these alarms and is typically performed by human operators, aided by explosive trace detectors, bottle liquid scanners and other instruments. Primary screening systems are mainly based on X-ray imaging technology and Computed Tomography (CT), both of which have heavily leveraged technological advances driven by the much larger medical imaging market. Specialized algorithms in these primary screeners extract from the X-ray data one or two physical parameters - such as density and effective atomic number- for each material inside a bag or cargo. These parameters are used for discriminating chemical compounds - including explosives - in a one- or two-dimensional Parameter Space. Unfortunately, there is often overlap of the physical parameters of explosives with those of innocuous materials in Parameter Space, which leads to misidentifications and false alarms. Several emerging technologies are promising for improving primary screening. These are X-Ray Diffraction (XRD), Phase Contrast (PC) and Differential PC (DPC), Nuclear Quadrupole Resonance (NQR), and Neutron Scanning. Even if a particular technology may not become the basis for a primary screener, it can still provide the means to automatically resolve alarms generated by the primary screener.
