Camera Pinpoints Gamma Ray Sources

ARGONNE, Ill., Nov. 11 -- A device originally developed for nuclear physics research may find applications in homeland security and medicine. A Compton camera being developed at the Department of Energy's Argonne National Laboratory could be used to create detailed images of radioactive materials, from smuggled weapons to "tracers" used in nuclear medicine.

The camera uses four-inch-square sheets of germanium to detect gamma rays, a high-energy form of light produced by nuclear reactions. By using two such counters -- arranged much like optical lenses -- and sophisticated electronics, researchers are creating a camera that can pinpoint the origin of gamma rays to within five millimeters. This would allow scanning of shipping containers, efficient mapping of radiation in the body, or scanning of nuclear warheads to verify the presence or absence of fissile material.

GAMMA CAMERA: At the heart of the Compton camera are sheets of germanium about four inches square, divided into strips and set into boxes about one inch deep. Gamma rays striking the sheets induce a shower of electrons, which are sensed by the wires crossing each sheet. Placing two such arrays a certain distance apart creates the camera (right). Measuring the angle at which the gamma ray is refracted after passing through the first sheet can reveal the ray’s origin, energy and polarization.

Using germanium crystals to detect gamma rays has been a staple for physicists studying the atomic nucleus. At present, the world’s most sensitive gamma-ray spectrometer is Gammasphere, a $20 million, 12-ton gamma-ray "microscope" located at Argonne. It uses an array of 100 coffee-cup-sized germanium crystals to detect the faint signals from rotating atomic nuclei.

Argonne physicists and engineers, working with their counterparts at Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory, have working prototypes of the germanium strip detectors and hope the Compton camera will be in operation by the end of the year. After that, the camera will be field-tested with radioactive sources.

For more information, visit: www.anl.gov