February 1, 2010 - Ottawa, Ontario
Canadian law-enforcement officials may soon be able to locate hazardous radioactive material more effectively, thanks to new counter-terrorism technology.
Since 2007, scientists at the NRC Institute for National Measurement Standards have worked with Natural Resources Canada (NRCan) and McGill University to build a portable instrument capable of constructing gamma-ray images of radioactive material. The $3 million project is partly funded through Canada's Chemical, Biological, Radiological-Nuclear, and Explosives Research and Technology Initiative (CRTI).
Dr. Patrick Saull is helping to develop a portable gamma-imager for law-enforcement officials.
How the detector works
A typical radioactive source can emit billions of gamma rays per second. The CRTI imager, a radiation detector that contains scintillating material segmented into individual read-out pixels, uses Compton scattering (see box) to determine the location of the radioactive source. Incoming gamma-rays can bounce off an electron in one pixel of the detector, leaving a measurable energy deposit in the scintillating material. The scattered gamma-ray may then be absorbed in another pixel. These essentially simultaneous "hits" form a characteristic "signature" or "golden event."
Did you know?
Gamma rays from radioactive material arise from an atom's nucleus as it "relaxes" during decay. These high energy rays can strip off electrons as they pass through matter, causing potential damage - such as killing living cells.
Compton scattering (named after Arthur Holly Compton, who won the 1927 Nobel Prize in Physics) involves a gamma-ray bouncing off an electron and transferring some energy to the charged particle. Compton scattering was used by the Compton Telescope, which flew aboard NASA's Compton Gamma-Ray Observatory from 1991 to 2000 to create images of the Milky Way.
Using information derived from the hits, the detector software can determine the possible origin of the gamma ray by creating a "cone of directions" whose apex lies at the first hit (see diagram below). The direction of the source can be further narrowed down through two or more golden events by locating the exact position where the cones overlap, says Dr. Patrick Saull, a physicist in the Ionizing Radiation Standards Group at NRC-INMS.
A Compton imager, represented here by the rectangular volume, can be used to locate radioactive sources (star). The wavy line (left) represents a gamma ray emitted by the source that interacts with the detector, producing simultaneous "hits" (yellow dots) that indicate the detected points of scatter and absorption. The detector generates a cone of possible directions from the hit information, and determines the exact position of the radioactive source from the overlap of several cones (right).
He says any radioactive sources emitting gamma rays could be highlighted as "bulls-eyes" within a field of view on a conventional photograph of, say, a typical downtown urban street (see below), which reveals a bulls-eye on a building and another on a car.
A typical street scene, overlaid with fictional data from imager software developed at NRC showing the most likely position of radioactive sources emitting gamma rays.
In the simulated photo, each of the circles in the bulls-eye represents a progressively higher level of confidence of the source's position. For instance, there is a 68 percent probability of the source being situated in the innermost circle; a 96 percent probability that it's located in the next largest circle; and a greater than 99 percent probability that it's within the total area contained by the third and largest circle.
Field applications
Initially, this counter-terrorism tool will likely be designed as a large, portable black box featuring a computer and camera that could easily be transported to a desired location. But the detector may eventually be configured to a smaller size that could be worn on a backpack, says Dr. Saull.
But the size of the detector matters. While a smaller detector could be used to locate radioactive material in clandestine operations, a larger device would be able to determine the position of both weaker gamma-ray sources and those at greater distances.
"It would be like the difference between using a backyard telescope to view faint objects and one located in a major observatory," says Dr. Saull. "The goal is to have the detector measure a moderately strong source of gamma rays at a distance of several tens of metres in under a minute."
Eventually, an even more sensitive instrument could track radioactivity on the move in real time, he adds. "For example, the detector could be put in a stadium during a football game to see whether sources of radioactivity are moving among the crowd."
A final prototype of the detector should be completed by 2012. Once commercialized, it may be used by law-enforcement officials in major Canadian cities and at border crossings.
Developing nuclear forensic tools
NRC is involved in another CRTI-funded initiative on nuclear forensics in collaboration with Health Canada, Royal Military College and Laval University. The four-year project, which began in 2009 and involves the RCMP and FBI, aims to develop radiochronometer standards and associated procedures for analyzing them.
A radiochronometer is any radioactive material from which useful dating information may be extracted, such as the carbon-14 (C-14) isotope. While alive, an organism's carbon content is roughly at equilibrium with its surroundings. But when carbon intake ceases upon death, the abundance of C-14 begins to decline through radioactive decay. As a result, a measurement of C-14 levels relative to the more stable C-12 and C-13 isotopes helps determine the age of an organism.
In this project, radiochronometers based on the radioactive isotopes Cobalt-60, Cesium-137 and Strontium-90 - all of which could be used in a dirty bomb (one containing radioactive material) - will be developed with well-defined apparent ages.
Radiochronometer standards will then be disseminated as reference materials to labs across Canada and the U. S. to help develop procedures for determining the age of arbitrary radioactive samples - such as those taken from the scene of an exploded dirty bomb.
"The aim is to be able to determine when and where the radioactive source was made, and have those results upheld in a court of law during a criminal prosecution," says Dr. Saull, who works on the project with fellow NRC-INMS colleague Dr. Raphael Galea.
"The Americans are working on developing similar technology, but they wanted NRC involved because we have the expertise to alleviate some of their workload as well as to provide new insights into this research."
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