Gamma-Ray Imaging Spectrometer (GRIS)
The gamma-ray imaging spectrometer (GRIS), can take gamma-ray pictures of the high-energy radiation emitted by nuclear materials. This instrument is useful for a variety of applications, including treaty inspections, mapping radioactive contamination, and determining what is inside a suspect object. Because gamma radiation is so difficult to focus, the instrument uses an imaging technique originally developed for high-energy astrophysics. The images are encoded on the detector by placing a sheet of material opaque to the radiation in front of the detector. The sheet is pierced with a carefully selected hole pattern that allows researchers to mathematically recover the image with a simple computer program. The system is about half the size of a personal computer.
GRIS was first developed for use in treaty inspections to monitor the location of nuclear missile warheads in a nonintrusive manner. In addition to its use in counterterrorism applications, GRIS is also expected to be useful in space to search for distant black holes and in hospitals to better detect, diagnose, and treat cancer.
Another version of GRIS being developed by Lawrence Livermore, the large-area imager, will be suited for longer-range searches. The large-area imager—approximately the size of a sofa—will be mounted in a small truck and capable of picking out weak radioactive sources from as far away as 100 meters.
The Compton camera is yet another type of gamma-ray imager under development. In addition to taking gamma-ray pictures, this imager should be able to identify very weak and typically invisible gamma-ray sources. The Compton camera operates without a mask or collimator, which can block many of the gamma rays emitted from a source. Instead, gamma rays coming from all directions at once are tracked as they scatter inside the detector. The camera's omnidirectional sensitivity is significantly higher than that of other imaging systems. Mathematical algorithms are used to retrace the paths of the gamma rays within the detector, and the results reveal the direction of the source.
Livermore's work on the Compton camera was originally funded through the Laboratory Directed Research and Development Program and later by DOE. Today, DHS is funding a Livermore effort to develop a compact, potentially portable Compton camera. The main goal for the camera is to detect clandestine nuclear materials. However, the instrument could also be used to detect cancer early by using radiolabeled tracers to target unique molecular characteristics of the disease. A field-deployable prototype of the Compton camera is still a few years away. Laboratory researchers continue to test detector materials and determine the best size for the instrument.
In a number of trials, GRIS technology has proven effective in imaging nuclear materials. With its ability to "see through walls," GRIS allows sophisticated remote data collection without the need for direct access to storage or contamination sites. Advantages of using this system include faster data acquisition, and reductions in worker exposure to radiation and contamination hazards. For example, a GRIS image, taken through the heat shielding in the Portsmouth gaseous diffusion plant, revealed a material deposit in one length of pipe. Worker access to this area of the plant would be difficult because of the high temperatures around the operating equipment and the possibility of contamination.
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