Cargo Scanning

Non-intrusive cargo scanning (aka Non-Intrusive Inspection - NII) or more specifically scanning of intermodal freight containers is a technological challenge which have seen a lot of research activity in last decade. In US it is spearheaded by Department of Homeland Security and its Container Security Initiative (CSI) trying to achieve hundred percent cargo scanning by 2012 as required by US Congress and recommended by 9/11 Commission. In US the main purpose of scanning is to detect special nuclear materials (SNMs), with added bonus of detecting other types of suspicious cargo. In other countries the emphasis is on manifest verification, tariff collection and the identification of contraband . As of 2007 less than 5% of US incoming containers are being scanned . In order to bring that number to 100% researchers are evaluating numerous technologies, described in the following sections.

Gamma-ray Radiography
Gamma-ray radiography systems capable of scanning trucks usually use Cobalt-60 or Cesium-137 as a radioactive source and a vertical tower of gamma detectors. This gamma camera is able to produce one column of an image. The horizontal dimension of the image is produced by moving either the truck or the scanning hardware. The Cobalt-60 units use gamma photons with mean energy 1.25 MeV and can penetrate up to 15-18 cm of steel . The systems provide good quality images which can be used for identifying cargo and comparing it with the manifest, in attempt to detect anomalies. It can also identify high-density regions too thick to penetrate, which would be the most likely to hide nuclear threats.

X-ray Radiography
Similar to Gamma-ray radiography but instead of radioactive source, it uses high-energy Bremsstrahlung spectrum with energy in 5-10 MeV range created by linear particle accelerator (LINAC). Such x-ray systems can penetrate up to 30-40 cm of steel in vehicles moving with velocities up to 13 km/h. They provide higher penetration but also cost more to buy and operate. They are more suitable for detection of special nuclear materials than gamma-ray systems. They also deliver about 1000 times higher dose of radiation to potential stowaways.

Neutron Activation Systems
Pulsed Fast Neutron Analysis (PFNA) and Thermal Neutron Activation (TNA) – detect gamma-rays created when neutrons interact with matter.

Gamma Radiation Detectors
Nuclear materials emit large amounts of gamma photons and gamma radiation detectors, also called Radiation Portal Monitors (RPM), are very good at detecting them. Systems currently used in US ports (and steel mills) use several (usually 4) large PVT panels as scintillators and can be used on vehicles moving up to 16 km/h .

They provide very little information on energy of detected photons, and as result, they were criticized for their inability to distinguish gammas originating from nuclear sources from gammas originating from large variety of benign cargo types that naturally emit radioactivity, including bananas, cat litter, granite, porcelain, stoneware, etc. . Those Naturally Occurring Radioactive Materials, called NORMs account for 99% of false alarms . Some radiation, like in case of large loads of bananas is due to potassium and its rarely occurring (0.0117%) radioactive isotope potassium-40, other is due to radium or uranium that occur naturally in earth and rock, and cargo types made out of them, like cat litter or porcelain.

Radiation originating from earth is also a major contributor to so called, background radiation.

Another limitation of gamma radiation detectors is that gamma photons can be easily suppressed by high-density shields made from lead or steel, preventing detection of nuclear sources. Luckily, those types of shields do not stop fission neutrons produced by plutonium sources. As result radiation detectors usually combine gamma and neutron detectors, making shielding only effective for uranium sources.