Gamma-ray imaging exploiting the Compton effect

Novel gamma ray imaging methods relying on the Compton effect are described. Ge tracking detectors, e.g. of AGATA-type, employing two-dimensional segmentation, pulse shape analysis and tracking of Compton scattered gamma rays show omni-directional position sensitivity. The obtainable angular resolution paired with the high energy resolution of Ge allows precise location of specific target isotopes over some ten to hundred meters in the presence of strong natural background radiation. At shorter distances the method is orders of magnitude more sensitive than conventional collimator-based imaging. Another basic method uses directional information from a point-like positron source placed in front of an object to be examined. A position detector behind the source measures 511 keV gamma rays from positron annihilation to determine the direction of the correlated 5 11 keV gamma rays used to probe the object. Detecting the probing gamma ray after Compton scattering off the object in time coincidence to the position determining gamma ray enables to map the matter distribution in the object. Among other applications the method can be employed for the detection of land mines.