A week ago, (11/29) I attended a fascinating talk by Dr. Kai Vetter, a researcher at Lawrence Livermore National Lab. He is pioneering a new technique to detect not only the energy of gamma ray photons, but also determine their direction of arrival upon the detector which conventional gamma ray detectors are incapable of. His talk was titled "High-Sensitivity Gamma Ray Imaging for Homeland Security Applications."
Gamma rays are interesting since they are emitted by radioactive materials and therefore can be used to detect the presence of dirty-bombs, improvised nuclear devices, or stolen nuclear weapons. The amount of gamma rays emitted from radioactive materials like highly enriched uranium (HEU) can also be tiny, and these gamma rays are also emitted in small quantities from the environment and from outer space (background). Therefore it is desirable to find better ways of detecting where the gamma rays come from in order to distinguish them from the background and pinpoint their source.
The technique Vetter described is very cool and also very hard but explain and I'm not sure I understand it completely. But I'll try. The gamma rays are photons range in energy from 150keV to 3MeV. The technique involves tracking the position of the photon as it scatters within the detector (Compton scattering) until it is finally absorbed within the detector material (Photoelectric effect). Hence Vetter's technique is called "Compton imaging" and the detector is known as a Compton camera. The distance between scatters is of the order of millimeters and the time measured in picoseconds. Such high-speed tracking of photon scattering has only become feasible due to advances in digital signal processing over the last five years. The photon's location is measured by a rows of detectors where each detector produces different signal signatures based on the particular position in which that the scatter took place. Using this location information, they can "triangulate" the photon's angle of arrival and generate an image (picture) of where the gamma rays come from.
In comparison, an older technique that tries the to accomplish the same thing is "collimation" in which shields are used to reflect/absorb gamma rays that come from within a small predetermined direction (2-3 degrees). The drawback with collimation is that all the other photons are lost. Compton imaging removes this drawback making the detector more efficient.
Ultimately, Vetter and colleagues are shooting for detection of nuclear materials at 100 meters distance. Since this is still in research, this technique costs several hundred thousand dollars.
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