A &ggr;-ray detector that comprises a dislocation-free single crystal. Typical crystals include silicon and germanium, for example. The crystal is surrounded by an active shield that functions as an anticoincidence counter and a &ggr;-ray detector is disposed adjacent the crystal in order to detect received -65 -rays. The &ggr;-ray detector comprises an imaging proportional counter, scintillation counter, or a &ggr;-ray detecting charge coupled device, for example. The . gamma.-ray detector of the present invention is based on the Bormann effect, which is the anomalous transmission of &ggr;-rays through dislocation free single crystals at the Bragg angle. For extended sources, data processing is provided to separate the directional and wavelength dependence of the intensity variation. The &ggr;-ray detector has angular resolution of at least one arc second and may have an effective aperture of several square meters. Reduction in background radiation, which is essentially determined by the crystal thickness, may be made quite large, greater than 10.sup.12. Consequently, the use of the present invention makes it possible to detect a weak &ggr;-ray source even in the presence of a large isotropic background.
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