GIANTS IN SCREENING – Special Report

15 Feb 2011

THE GLOSSARY

The primary method of ensuring that aircraft, and those who fly in them, are free of hijackers and those who, either wittingly or unwittingly, may be about to commit an act of sabotage is the processing of passengers, crew, baggage and cargo through security checkpoints. State-of-the-art technologies, complemented by trained, motivated individuals are the keys to our success in preventing the next atrocity in the skies. Some of the world’s most well known brands in the aviation security industry are associated with the screening checkpoint and their scientists are the brains behind the research and development of technologies that have either proven their effectiveness or show considerable promise of improving our detection capability in the future. Many of the more advanced solutions are hard to comprehend unless one is a physicist or chemist, so Amir Neeman has taken up the challenge to explain the science behind solutions being proffered by those companies who are the Giants in Screening…    

Amplifying Fluorescent Polymer

Amplifying Fluorescent Polymers (AFPs) leverage basic research in nanotechnology, which has created polymers that can detect hazardous substances with extremely high sensitivity. These polymers consist of many monomers – each about one nanometre in length – designed to emit light (fluoresce) when irradiated with light of a particular wavelength. These monomers stop fluorescing if the polymer encounters a minute amount of specific vapours (of a hazardous substance). The loss of light emission is electronically detected and converted it into audio and visual alarms that signal the presence of the explosive material. Because the monomers are chained together, they behave as a nanoscale wire that amplifies the warning signal even when only a small section of the chain actually interacts with the explosive or toxic substance. Hence these polymers are called amplifying fluorescent polymers. 

AFP is a relatively new method in the detection of explosives and rapidly developing; however, it is highly sensitive (similar to canine sensitivity level) and relatively selective with simple instrumentation, especially when detecting explosives with medium to low vapour pressures (e.g., TNT). The system is small and portable and has a relatively low cost.

Fast Neutron Analysis and Pulsed Fast Neutron Analysis

Fast Neutron Analysis (FNA) is a technique that uses fast neutrons (rather than thermal neutrons) to interact with nuclei of hydrogen, oxygen, nitrogen and carbon elements in screened objects.  Fast neutrons are highly penetrating and result in the emission of characteristic gamma rays, which act like an elemental fingerprint. Pulsed Fast Neutron Analysis (PFNA) technology uses a collimated pulsed neutron beam to determine the location and composition of objects in cargo containers or trucks which slowly move through the inspection tunnel while being scanned with the beam. The neutrons cause excitation of the nuclei of carbon, hydrogen, nitrogen, and oxygen within the screened object and those emit gamma rays that are characteristic of the object’s elemental composition. The collimation determines the direction and path of the neutron beam through the container or truck. The position of the object is determined by time-of-flight measurements using very short fast neutron pulses. This method is capable of automatically detecting a relatively broad range of materials by exploiting their specific elemental composition with high specificity.

Key advantages are that information is gathered on elements besides nitrogen, such as carbon and oxygen, which helps in explosives-like material determination; it has high neutron penetration and can be used on large cargo containers; and that 3D threat location information can be determined. Key disadvantages are that the system is highly complex and expensive and that there are the same radiation and shielding concerns as with TNA.

Gamma Ray Inspection

Gamma ray imaging technology provides radiographic images (much like X-ray images) of objects (typically cargo containers). The principle of operation is similar to that of transmission X-ray systems, except the source is a gamma-emitting isotope which is collimated to project a fan-shaped beam onto a linear array of gamma ray detectors. The gamma ray energies depend on the radiation source (typically Cesium-137 or Cobalt-60). Cargo and vehicle screening is one important application of gamma ray inspection (alternatively high-energy X-ray systems can be used for these applications) since it is necessary to achieve penetration of metal parts of vehicles and shipping containers. Often high-efficiency photon-counting technology is integrated with gamma ray imaging as well as Radiation Portal Monitors (RPMs) that consist of large-area gamma ray detectors and neutron detectors to allow the passive detection of nuclear materials or other radioactive materials in cargo containers or trucks. The high detection sensitivity of RPMs allows effective scanning of cargo; however, false positive alarms resulting from cargo that is naturally radioactive (e.g., certain ceramic materials, kitty litter) can reduce throughput.

Key advantages are the ability to effectively penetrate large and dense objects with a relatively low-energy gamma ray source (compared to a high-energy X-ray source) and the ability to effectively screen large containers and trucks.  Key disadvantages are that this technology requires the use of a radioactive gamma ray source and that they are expensive and large.

Gas Chromatography and Chemiluminescence

A Gas Chromatograph (GC) is a chemical analysis instrument for separating chemicals in a complex sample. GC uses a flow-through narrow tube (column), through which different chemical constituents of a sample pass in a gas stream (carrier gas) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling (called the stationary phase). As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate and the temperature.

Often chemiluminescence (CL) detectors are fitted with a front-end GC and the combined GC-CL provides a unique GC retention time and separates materials prior to their CL detection. CL is the production and emission of light that occurs as a product of a chemical reaction(s). Most explosives contain nitrogen, and a common CL reaction for explosives detection involves infrared radiation (IR) light emission from excited-state nitrogen compounds which is proportional to the amount of nitrogen present (related to the amount of the original nitrogen-containing explosive material).

GC as a stand-alone technology is not a good enough explosive detection technique, which is why it is often coupled to other detection methods (CL and MS). GC normally requires a bottled gas, which adds to the complexity of use (and required consumables). GC columns operated in the field are prone to degradation from atmospheric gases and oxidation, as well as bleeding of the stationary phase. MORE ONLINE